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IODP Expedition 385

Guaymas Basin Tectonics and Biosphere

Daily Science Report for 31 October 2019

Location:
• Hole U1545C (27°38.2420′N, 111°53.3290′W; water depth 1594.9 mbsl)
• Underway to Site U1546 (proposed Site GUAYM-02B)
• Hole U1546D (27°37.8948′N, 111°52.7818′W; water depth 1585.9 mbsl)

Science Update: We continued coring in Hole U1545C, alternating between the HLAPC and XCB coring tools. Cores 30F to 63F penetrated from 198.0 to 329.0 mbsf and recovered 129.6 m (99%). We deployed the XCB coring system whenever we had to core through hard carbonate layers, which happened on Cores 48X, 50X, 52X, 56X, and 60X. We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on all cores. Upon accomplishing our microbiology sampling objectives for Hole U1545C, we terminated coring at a final depth of 329.0 mbsf and started to pull the drill string out of the hole. The bit cleared the seafloor at 2025 h, ending Hole U1545C and Site U1545. While cutting and reheading the core line, we moved the vessel in dynamic positioning mode to the coordinates of Hole U1546D to recover an additional set of microbial and biogeochemical samples from Site U1546. We began to move at 2041 h and arrived at the Hole U1546D coordinates at 2214 h. The sinker bars and APC core barrel were installed, and Hole U1546D was spudded at 2330 h. Mudline Core U1546D-1H recovered 3.9 m. This established a seafloor depth of 1585.9 mbsl. We deployed PFTs for drilling fluid contamination monitoring.

Cores U1550B-21X to 23X have been split and described. Core 21X consists of clayey siltstone that represents part of a contact zone to an underlying sill. Core 22X contains slightly altered, nonvesicular dolerite with subophitic texture. Core 23X contains both basaltic and doleritic rocks. These doleritic intervals are similar to the dolerite observed in Core 22X. The basaltic rock is slightly to moderately altered and highly plagioclase-phyric. It occasionally shows xenoliths of dolerite. Multiple calcite veinlets are present in both basalt and dolerite.

Cores U1545C-7H to 40F have been split and described. They consist of light olive to olive-gray micrite-rich diatom ooze and diatom clay with laminated and homogenous textures. Subordinate intercalated carbonate layers and nodules are common. Starting with Core 22F, a decrease in both authigenic micrite content and abundance of carbonate concretions has been observed in comparison with shallower cores.

Daily Science Report for 30 October 2019

Location:
• Underway to Site U1545 (proposed Site GUAYM-01B)
• Hole U1545C (27°38.2420′N, 111°53.3290′W; water depth 1594.9 mbsl)

Science Update: We continued our sea passage to Site U1545 (proposed Site GUAYM-01B) and arrived on site at 0206 h. Upon lowering the thrusters over the coordinates of Hole U1545C and switching from cruise to dynamic positioning mode at 0228 h, we made up an advanced piston corer/extended core barrel (APC/XCB) bottom-hole assembly and started lowering the drill string to the seafloor. At 0730 h, we picked up the top drive and positioned the bit at a water depth of 1589.0 mbsl for spudding Hole U1545C. The sinker bars and core barrel were installed, and Hole U1545C was spudded at 0745 h. Mudline Core 1H recovered 3.6 m. This determined a seafloor depth of 1594.9 mbsl. Cores 1H to 29F penetrated from the seafloor to 198.0 mbsf and recovered 194.9 m (98%). After encountering a partial stroke on Core 16H, we switched to the half-length APC (HLAPC) coring tool at 140.6 mbsf. We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on all cores. The pace of coring was adjusted on some cores to accommodate the complex microbiology sampling program on the core receiving platform.

Cores U1550A-30X to 32X have been split and described. They consist of slightly to moderately altered, nonvesicular to sparsely vesicular, inequigranular to equigranular doleritic basalt with subophitic texture. Vesicles are present at the top margin of the recovered sill in Section 30X-CC. They are filled with chlorite and carbonate. For the biostratigraphic assessment of the overlying sediments, scanning electron microscopy confirmed the occurrence of the age-diagnostic calacareous nannofossil Emiliania huxleyi. This points to a sedimentation age of <0.29 Ma in Hole U1550A.

Cores U1550B-8H to 20X have been split and described. Cores 8H to 18X contain an alternation of clay-rich diatom ooze and nannofossil-rich diatom clay and diatom-rich silty clay or clayey silt. Several sandy beds occur as a minor component. Cores 19X and 20X consist of silt-rich siliceous claystone that is rich in organic carbon.

Cores U1545C-1H to 6H have been split and described. They consist of olive-gray laminated diatom ooze with variable proportions of clay, silt, authigenic carbonate, and other biosiliceous and calcareous microfossils.

Scientists held the second science summary meeting for Ringvent Sites U1547 and U1548, with half of the laboratory groups presenting their results to the science party.

Daily Science Report for 29 October 2019

Location:
• Hole U1550B (27°15.1592′N, 111°30.4161′W; water depth 2001.4 mbsl)
• Underway to Site U1545 (proposed Site GUAYM-01B)

Science Update: We continued coring in Hole U1550B, alternating between the APC, half-length APC (HLAPC), and XCB coring systems. Cores 9H to 23X penetrated from 71.9 to 174.2 mbsf and recovered 85.5 m (84%). We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on all cores. After encountering strong overpull force on Core 12H, we had to drill over the core barrel in order to retrieve it. We thus switched to the HLAPC coring tool from 109.9 mbsf (Core 13F) downward. Upon encountering a partial stroke on Core 17F (131.7 mbsf), we deployed the XCB coring tool until refusal at a final depth of 174.2 mbsf. We then terminated coring in Hole U1550B and started pulling the drill string out of the hole. The bit cleared the seafloor at 1815 h, ending Hole U1550B and Site U1550. The decision was made to return to Site U1545 to recover another complete set of microbiology and biogeochemistry samples from the northwestern part of Guaymas Basin. We thus secured the vessel for transit. At 2233 h, we switched from dynamic positioning to cruise mode and started our ~31 nmi sea passage at 2300 h.

Cores U1550A-20X to 30X have been split and described. Cores 20X to 25X consist of alternations between diatom clay and graded, silty sand beds. These cores include several deformed intervals characterized by tilting, folding, and disruption of sedimentary beds. Biscuiting and other drilling disturbance features are common. Carbonates—either occurring as disseminated micrite or distinct concretionary beds—disappear in Core 21X. Cores 25X to 29X have alternating deformed intervals of nannofossil-bearing siliceous claystone and silty sand. In Core 29X, the siliceous claystone is darker and appears more recrystallized. Core 30X contains altered igneous rock. Biostratigraphic analysis down to Core 29X suggests the possible occurrence of calcareous nannofossil age marker Emiliania huxleyi. No diatom age marker taxa were found in Hole U1550A.

Cores U1550B-1H to 7H have been split and described. The first two cores include serval thick gravity flows composed of a mixture of silty diatom clay and sand. Core 2H also marks the beginning of a deformed interval characterized by tilting and folding, which includes Cores 2H to 4H. The interval from Cores 5H to 6H is relatively undisturbed and includes several graded, sandy to silty beds. Evidence of soft sediment deformation appears again in the sandy and silty sediment of Core 7H together with abundant gravel-sized scoria clasts.

Scientists held the first science summary meeting for Ringvent Sites U1547 and U1548, with half of the laboratory groups presenting their results to the science party.

Daily Science Report for 28 October 2019

Location:
• Hole U1550A (27°15.1602′N, 111°30.4163′W; water depth 2001.0 mbsl)
• Hole U1550B (27°15.1592′N, 111°30.4161′W; water depth 2001.4 mbsl)

Science Update: We continued XCB coring in Hole U1550A. Cores U1550A-24X to 32X penetrated from 151.4 to 207.0 mbsf and recovered 37.6 m (68%). After the XCB cutting shoe was destroyed on Core 32X, we terminated coring in Hole U1550A at 1200 h at a final depth of 207.0 mbsf. We then pulled the drill string out of the hole and the bit cleared the seafloor at 1320 h, ending Hole U1550A. The vessel then moved over to the Hole U1550B coordinates 55 m to the west of Hole U1550A. Upon cutting and reheading the core line, we loaded the sinker bars, deployed the core barrel of the advanced piston corer (APC) system, and positioned the bit above the seafloor at a water depth of 1997.3 mbsl. We then spudded Hole U1550B at 1445 h. Mudline Core U1550B-1H recovered 5.5 m. This determined a seafloor depth of 2001.4 mbsl. Cores U1550B-1H to 8H penetrated from the seafloor to 71.9 mbsf and recovered 75.3 m (105%). We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on all cores. The pacing of coring was adjusted to the complex high-resolution microbial and geochemical sampling program conducted on the core receiving platform.

Cores U1550A-6H to 19F have been split and described. Core 6H is characterized by undeformed alternations between laminated diatom clay and graded silty beds. Cores 7H to 10H include several deformed intervals characterized by tilting, folding, and disruption of beds and laminae that make up diatom clay and minor silty sand and sandy silt. Cores 11H and 12H consist of homogenous diatom clay with no evidence of sediment deformation. Deformation, folds, and tilting reappear in Cores 13F to 16F, which contain laminated diatom clay that alternates with multiple sandy beds. Starting with Core 17F, a significant sedimentological change is observed through the occurrence of disseminated micrite. Also, gray sand and gravel layers become the dominant lithologic component, while silt- and clay-rich diatom ooze is only a subordinate component. Biostratigraphic analysis revealed the presence of calcareous nannofossil age marker Emiliania huxleyi in Cores 4H to 15F, indicating a Holocene to Middle Pleistocene sedimentation age (<0.29 Ma). No age-diagnostic diatom taxa were found.

Scientists submitted the first draft of the combined Site U1547 and Site U1548 report.

Daily Science Report for 27 October 2019

Location: Hole U1550A (27°15.1602′N, 111°30.4163′W; water depth 2001.0 mbsl)

Science Update: We continued lowering the drill string to the seafloor to start coring in Hole U1550A with the advanced piston corer (APC) system. At 0315 h, we picked up the top drive and positioned the bit above the seafloor at a water depth of 1997.3 mbsl. We then spudded Hole U1550A at 0405 h. Mudline Core U1550A-1H recovered 5.8 m. This established a seafloor depth of 2001.0 mbsl. Cores 1H to 23X penetrated from the seafloor to 151.4 mbsf and recovered 153.3 m (101%). After encountering strong overpull force on Core 12H, we had to drill over the core barrel in order to retrieve it. We thus switched to the half-length APC (HLAPC) coring tool from 110.3 mbsf (Core 13F) onward. While deploying the HLAPC tool, we used the extended core barrel (XCB) coring system to core through infrequently appearing hard layers that were penetrated on Cores 18X and 20X. Starting with Core 22X at a depth of 135.6 mbsf, we continuously deployed the XCB coring tool. We made formation temperature measurements with the APCT-3 tool on Cores 4H, 6H, 8H, and 10H, as well as the SET2 tool following Core 15F.

Cores U1549B-9H to 18H have been split and described. They consist of laminated to homogenous nannofossil-bearing, clay-rich diatom ooze and diatom clay with intercalating siliciclastic-rich layers. Preliminary observations suggest that Hole U1549B correlates very well with Hole U1549A. Scanning electron microscopy assessment of biostratigraphic samples from the bottom of Hole U1549A confirmed the occurrence of calcareous nannofossil species Emiliania huxleyi. This points to a Holocene to Middle Pleistocene deposition age (<0.29 Ma) for the entire sedimentary sequence in Hole U1549A and thus at Site U1549.

Cores U1550A-1H to 5H have been split and described. Core 1H is characterized by two successive ~3 m thick gravity flows, each composed of silty diatom clay with sandy bottoms. The top of Core 2H marks the beginning of a deformed interval characterized by tilting, folding, and disruption of beds and laminae that continue through the top of Core 5H, corresponding to a total thickness of about 30 m. The bottom of Core 5H is characterized by undeformed alternations between more or less laminated diatom clay and distinctively gray-colored, graded silty beds that are capped by light-colored diatom ooze bands.

Daily Science Report for 26 October 2019

Location:
• Hole U1549B (27°28.3383′N, 111°28.7927′W; water depth 1841.2 mbsl)
• Hole U1550A (27°15.1592′N, 111°30.4161′W; water depth 2002.3 mbsl)

Science Update: We continued APC coring in Hole U1549B. Cores U1549B-7H to 18H penetrated from 52.9 to 166.9 mbsf and recovered 110.3 m (97%). We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on all cores. The pacing of coring was adjusted to the complex high-resolution microbial and geochemical sampling program conducted on the core receiving platform. Upon accomplishing the sampling and target depth objectives for Hole U1549B, we terminated coring at a final depth of 166.9 mbsf. We started pulling the drill string out of the hole and the bit cleared the seafloor at 1550 h, ending Hole U1549B and Site U1549. Upon implementing a routine cut and slip of 115 ft of the drilling line, we pulled the drill string up to the surface and reassembled the rig floor. At 2025 h, we secured the vessel for transit to Site U1550 (proposed Site GUAYM-06B), located in the northern axial graben of Guaymas Basin. We switched from dynamic positioning (DP) to cruise mode at 2034 h, raised the thrusters, and started the ~13 nmi sea passage. We arrived at the coordinates of Site U1550 at 2209 h, lowered the thrusters, and switched back to DP mode. At 2230 h, we started lowering the drill string with the APC/XCB bottom-hole assembly to the seafloor to start coring in Hole U1550A. The bit reached a water depth of 124.9 mbsl by the end of the day.

Cores U1547D-18R to 21R have been split and described. They consist of moderately to highly altered, moderately to highly vesicular, aphyric to sparsely plagioclase-phyric basalt. Vesicles are either partially or fully filled with calcite and pyrite.

All cores that recovered igneous rocks from Site U1548 have been split and described. Cores U1548A-19X to 20X contain slightly to moderately altered non-vesicular aphyric basalt that occasionally shows glassy margins and mingling with sediment. Cores U1548B-11X to 12X mainly contain slightly to moderately altered non-vesicular aphyric basalt. Vesicles are sporadically present in association with glassy margins in Core U1548B-11X.

Cores U1549A-15H to 18H have been split and described. They consist of laminated to homogenous nannofossil-bearing clay-rich diatom ooze that alternates with gray siliciclastic layers. The latter occur about twice per core and often have abrupt to scoured bases overlain by normally graded to laminated sand that fines upward to silt and diatom-rich silty clay. Biostratigraphic examination of Hole U1549A samples suggests a Middle Pleistocene age (<0.29 Ma). This is based on the potential presence of poorly preserved calcareous nannofossil species Emiliania huxleyi at the bottom of Hole U1549A, which is awaiting confirmation by scanning electron microscopy. No age-diagnostic diatom taxa were found in Hole U1549A.

Cores U1549B-1H to 8H have been split and described. They consist of laminated to homogenous nannofossil-bearing clay-rich diatom ooze and diatom clay. Abundant siliciclastic layers are up to 6 m thick. Preliminary observations indicate that Hole U1549B correlates very well with Hole U1549A.

Daily Science Report for 25 October 2019

Location:
• Hole U1549A (27°28.3315′N, 111°28.7848′W; water depth 1840.1 mbsl)
• Hole U1549B (27°28.3383′N, 111°28.7927′W; water depth 1841.2 mbsl)

Science Update: We started advanced piston coring (APC) in Hole U1549A. Cores U1549A-1H to 18H penetrated from the seafloor to 168.0 mbsf and recovered 166.9 m (99%). We made formation temperature measurements on Cores 4H, 7H, 10H, and 13H. Upon accomplishing the target depth objective in Hole U1549A, we pulled the drill string out of the hole and the bit cleared the seafloor at 1515 h, ending Hole U1549A at a final depth of 168.0 mbsf. We then moved the vessel 20 m to the northwest to spud Hole U1549B. We positioned the bit above the seafloor and spudded Hole U1549B at 1640 h. Mudline Core U1549B-1H recovered 5.48 m. This determined a seafloor depth of 1841.2 mbsl. Cores U1549B-1H to 6H penetrated from the seafloor to 52.9 mbsf and recovered 54.2 m (102%). The pacing of coring was adjusted to the complex high-resolution microbial and geochemical sampling program conducted on the core receiving platform.

Cores U1547D-14R to 17R have been split and described. They consist of moderately altered, moderately to highly vesicular plagioclase-phyric basalt. Vesicles are either partially or fully filled with calcite and pyrite.

Cores U1549A-1H to 14H have been split and described. These contain intervals of laminated to homogenous nannofossil-bearing, clay-rich diatom ooze with gray layers that are more terrigenous in composition, ranging in thickness from mm-thick laminae to meter-scale beds. The latter often have abrupt to scoured bases overlain by normally graded to laminated sand that fines upward into silt and diatom-rich silty clay.

Scientists continued to collect and analyze data from Holes U1547C, U1547D, U1548A, and U1548B.

Daily Science Report for 24 October 2019

Location:
• Hole U1547D (27°30.3947′N, 111°40.7477′W; water depth 1732.2 mbsl)
• Hole U1549A (27°28.3315′N, 111°28.7848′W; water depth 1840.1 mbsl)

Science Update: We continued RCB coring in Hole U1547D. Cores 18R to 21R penetrated from 173.6 to 293.0 mbsf and recovered 9.1 m (47%). We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on all cores. Upon accomplishing the microbiology sampling objectives for Hole U1547D, we terminated coring at a final depth of 193.0 mbsf. At 0815 h, we deployed the Elevated Temperature Borehole Sensor (ETBS) tool to make a temperature measurement at the bottom of the hole. We retrieved the ETBS tool at 0945 h and started pulling the drill string out of the hole. The bit cleared the seafloor at 1130 h, ending Hole U1547D and Site U1547. The bit arrived on the rig floor at 1445 h and we secured the vessel for transit to Site U1549 (proposed Site GUAYM-16A), located ~9.5 km northwest of the northern axial graben of Guaymas Basin. While in transit, we performed a precision depth recorder (PDR) survey of the transect seismic line between Sites U1547 and U1548 across Ringvent and during the ~11 nmi sea passage to Site U1549. We switched from dynamic positioning (DP) to cruise mode at 1519 h, raised the thrusters, began the PDR survey at 1528 h, and started the sea passage at 1554 h. We arrived at the coordinates of Site U1549 at 1706 h, lowered the thrusters, and switched back to DP mode. We then made up the advanced piston corer/extended core barrel (APC/XCB) bottom-hole assembly and started lowering the drill string to the seafloor for coring in Hole U1549A. We picked up the top drive, positioned the bit at 1837.1 mbsl, and spudded Hole U1549A at 2355 h. This established a seafloor depth of 1840.1 mbsl.

Cores U1547D-8R to 13R have been split and described. They consist of moderately altered, nonvesicular to highly vesicular, plagioclase-phyric basalt. Mingling with sediment has been observed in Core 12R.

Scientists continue to collect and analyze data from Holes U1548B, U1547C, and U1547D.

Daily Science Report for 23 October 2019

Location: Hole U1547D (27°30.3947′N, 111°40.7477′W; water depth 1732.2 mbsl)

Science Update: We continued RCB coring in Hole U1547D. Cores 4R to 17R penetrated from 100.8 to 173.6 mbsf and recovered 24.6 m (34%). We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on all cores.

Cores U1547D-2R to 7R have been split and described. Cores 2R to 3R recovered limestone in core catchers. Core 4R contains silt-rich siliceous claystone. No age-diagnostic taxa were observed. Cores 5R to 7R consist of moderately to highly altered, sparsely to moderately vesicular, aphyric to sparsely plagioclase-phyric basalt. Vesicles are mainly filled with calcite and pyrite.

Daily Science Report for 22 October 2019

Location:
• Hole U1547C (27°30.4455′N, 111°40.7064′W; water depth 1732.2 mbsl)
• Hole U1547D (27°30.3947′N, 111°40.7477′W; water depth 1732.2 mbsl)

Science Update: In preparation for spudding Hole U1547C, we continued lowering the drill string to the seafloor with the rotary core barrel (RCB) coring system. Upon reaching a water depth of 1706.9 mbsl, we picked up the top drive, dropped the center bit, and the end of the drill string tagged the seafloor at 1732.2 mbsl. We then spudded Hole U1547C at 0300 h and drilled without core recovery from the seafloor to 81.3 mbsf. Upon retrieving the center bit, we began RCB coring at 0545 h. Cores U1547C-2R to 9R penetrated from 81.3 to 159.2 mbsf and recovered 9.0 m (12%). We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on Cores 3R to 9R. Hole problems led to strong overpull force from 139.7 to 149.5 mbsf. While recovering Core 9R, we determined that our coring location should be shifted to the southwest of Hole U1547B. We thus terminated coring in Hole U1547C at 1830 h at a final depth of 149.5 mbsf. We pulled the drill string out the hole and the bit cleared the seafloor at 1927 h, ending Hole U1547C. We then offset the vessel to the coordinates for Hole U1547D. At 1945 h, we dropped the center bit and positioned the drill string above the seafloor. A tagged seafloor depth of 1732.2 mbsl was determined. We spudded Hole U1547D at 2005 h and drilled without core recovery from the seafloor to 81.3 mbsf. After retrieving the center bit, we began RCB coring at 2130 h. Cores U1547D-2R to 3R penetrated from 81.3 to 100.8 mbsf and recovered 0.2 m (1%).

Cores U1547C-2R to 9R have been split and described. Core 2R consists of diatom-rich silty clay. Cores 3R to 5R recovered dolostone and limestone in core catchers. Core 6R contains nannofossil-rich diatom clay. No age-diagnostic taxa were observed. Cores 7R to 9R consist of moderately to highly altered, sparsely to highly vesicular aphyric basalt.

Daily Science Report for 21 October 2019

Location:
• Hole U1548B (27°30.2540′N, 111°40.8601′W; water depth 1738.9 mbsl)
• Hole U1547C (27°30.4453′N, 111°40.7055′W; water depth 1732.2 mbsl)

Science Update: We continued APC and XCB coring in Hole U1548B. Cores 3H to 12X penetrated from 15.4 to 95.1 mbsf and recovered 71.9 m (90%). Upon encountering a partial stroke on Core 9H, we switched to the XCB coring system on Core 10X at a depth of 80.4 mbsf. We then continued coring until XCB refusal at a final depth of 95.2 mbsf in Hole U1548B. At 1345 h, we raised the end of the drill string to 58 mbsf and rigged up the Kuster Flow Through Sampler (FTS) tool. Upon lowering the bit to a depth of 70 mbsf, we deployed the Kuster FTS tool and recovered a borehole fluid sample from the same depth. We then pulled the drill string out of the hole and the bit cleared the seafloor at 1622 h, ending Hole U1548B and Site U1548. The vessel then moved back to Site U1547, and it arrived at the coordinates of Hole U1547C at 1806 h. We made up the rotary core barrel (RCB) bottom-hole assembly and had lowered the drill string to a water depth of 590 mbsl by the end of the day.

Cores U1548A-15X to 20X have been split and described. Cores 15X to 18H recovered the bottom of the sediments in Hole U1548A and mainly consist of diatom clay with hard carbonate layers. Below this depth interval, Cores 19X and 20X recovered the top margin (at ~99 mbsf) and succeeding interval of an igneous body (in total 1.6 m), consisting of volcanic glass mingled with sediment and sparsely vesicular to nonvesicular, aphyric basalt that is moderately altered.

Cores U1548B-1H to 11X have been split and described. They consist of alternations between laminated and homogenous clay-rich diatom ooze. Starting with Core 5H, the micrite component becomes a significant part of the sediment. Micrite and carbonate layers become more abundant in Cores 6H to 9H, whereas the micrite becomes an accessory component in Core 11X.​

Daily Science Report for 20 October 2019

Location:
• Hole U1548A (27°30.2460′N, 111°40.8660′W; water depth 1739.9 mbsl)
• Hole U1548B (27°30.2540′N, 111°40.8601′W; water depth 1738.9 mbsl)

Science Update: Upon completing the seafloor survey over Site U1548, we retrieved and secured the subsea camera system by 0230 h. We then picked up the top drive, positioned the bit above the seafloor, and spudded Hole U1548A at 0350 h. Mudline Core U1548A-1H recovered 1.9 m. This established a seafloor depth of 1739.9 mbsl. While coring with the APC tool, we had to deploy the XCB tool to penetrate hard layers at several depths (Cores 11X, 13X, 15X, and 17X). After recovering Core 18H from a depth of 99.4 mbsf, we switched to XCB coring permanently. Cores 1H to 20X penetrated from the seafloor to 103.4 mbsf and recovered 114.0 m (110%). We terminated coring upon recovering Core 20X due to slow penetration rates with the XCB coring system. We then pulled the drill string out of the hole and the bit cleared the seafloor at 2140 h, ending Hole U1548A at a final depth of 103.4 mbsf. We moved the vessel 20 m to the northwest and arrived at the coordinates of Hole U1548B at 2200 h. We positioned the bit above the seafloor to start APC coring and spudded Hole U1548B at 2215 h. Mudline Core U1548B-1H recovered 5.9 m. This determined a seafloor depth of 1738.9 mbsl. By the end of the day, Cores 1H to 2H penetrated from the seafloor to 15.4 mbsf and recovered 15.8 m (103%).

Cores U1548A-1H to 14H have been split and described. Cores 1H and 2H consist of homogenous diatom clay with shell fragments and open burrows. Below Core 2H, the sediment is mainly composed of alternations between laminated and homogenous diatom clay and diatom ooze with occasional gray silt beds that contain subordinate diatoms and clay. Starting with Core 6H, authigenic carbonate precipitates are present as both dispersed micrite and concretions. Sand- and silt-rich layers of a few centimeters in thickness also become more frequent. The abundance of authigenic carbonates peaks in Core 8H. Below Core 9H, the micritic sediment component virtually disappears. ​Biostratigraphic analyses have been completed on Cores 1H to 9H and did not yield any age-diagnostic taxa for either nannofossils or diatoms.

Scientists continued to collect and analyze data from Hole U1547B.

Daily Science Report for 19 October 2019

Location:
• Hole U1547B (27°30.4128′N, 111°40.7341′W; water depth 1732.2 mbsl)
• Hole U1548A (27°30.2460′N, 111°40.8660′W; water depth 1734.4 mbsl)

Science Update: We continued XCB coring in Hole U1547B. Cores 45X to 50X penetrated from 205.2 to 209.8 mbsf and recovered 3.4 m (74%). We pumped 20 barrels of high-viscosity mud after every core for hole cleaning. At 1200 h, we terminated coring at a final depth of 209.8 mbsf in Hole U1547B due to the slow advancement and hole stability issues encountered at the bottom of the hole. We picked up the top drive and raised the drill string to 103.7 mbsf. We then deployed the Kuster Flow Through Sampler (FTS) tool twice to recover borehole fluid samples successfully from 109.7 and 135.7 mbsf, respectively, by 1600 h. In preparation for wireline logging, we raised the end of the drill string to a depth of 69.7 mbsf, and the rig floor prepared for deploying the triple combination logging tool string (natural gamma ray, porosity, density sondes). We lowered the logging tool string into the hole until we encountered a ledge at 168.7 mbsf. After a short calibration pass, we implemented a full logging pass up to the seafloor. The intended deployment of the Formation MicroScanner (FMS)-sonic (resistivity imaging) logging tool string was canceled due to deteriorating hole conditions. We pulled the drill string out of the hole and the bit cleared the seafloor at 2205 h, ending Hole U1547B and Site U1547. We then deployed the subsea camera system to observe the seafloor while moving ~500 m southwest to the coordinates of Site U1548 (proposed Site GUAYM-03B). The seafloor survey did not find any vent communities at the location of Site U1548. At midnight, we started preparing the rig floor for coring in Hole U1548A.

Cores U1547B-41X to 50X have been split and described. They consist of moderately to highly altered, fine-grained, nonvesicular to highly vesicular basalt. The alteration is mostly expressed by vesicle and vein filling, and comprises mainly calcite associated with minor pyrite and zeolite as alteration minerals. Hydrothermal fracturing and halos are occasionally present. Core 44X revealed mingling with clay-rich sediment.

Daily Science Report for 18 October 2019

Location: Hole U1547B (27°30.4128′N, 111°40.7341′W; water depth 1732.2 mbsl)

Science Update: We continued XCB coring in Hole U1547B. Cores 33X to 44X penetrated from 158.8 to 205.2 mbsf and recovered 14.7 m (32%). We pumped 20 barrels of high-viscosity mud after every core for hole cleaning. A request for extending the target depth of Hole U1547B from 200 to 240 mbsf was approved by the IODP Environmental Protection and Safety Panel (EPSP).

Cores U1547B-29F to 40X have been split and described. Core 29F consists of gray-brown siliceous claystone that is highly disturbed due to drilling-induced brecciation. This core did not reveal any age-diagnostic microfossil taxa so that the deposition age remains <0.29 Ma. Core 30X recovered the top margin of an igneous body from ~151 mbsf, characterized by a glassy margin. Below this margin, Cores 30X to 40X consist of moderately altered, fine-grained, sparsely to highly vesicular basalt. The alteration is generally expressed by vesicle and vein filling, showing mainly pyrite and calcite, and minor zeolite as alteration minerals. Hydrothermal fracturing is occasionally present.

Scientists submitted the first draft of their Site U1546 reports.

Daily Science Report for 17 October 2019

Location: Hole U1547B (27°30.4128′N, 111°40.7341′W; water depth 1732.2 mbsl)

Science Update: We continued coring in Hole U1547B, deploying the APC/HLAPC/XCB coring systems. Cores 8H to 32F penetrated from 63.2 to 158.8 mbsf and recovered 77.6 m (81%). Upon refusal of the APC coring tool, we switched to the HLAPC tool after Core 10H at 90.5 mbsf. While coring with the HLAPC tool, we had to deploy the XCB tool to penetrate hard layers at several depths (Cores 11X, 13X, 15X, 17X, 20X, 23X, 30X, and 31X). At the end of the day, we permanently switched to XCB coring after Core 32F indicated refusal at a depth of 158.8 mbsf. We made formation temperature measurements with the APCT-3 tool on Cores 8H, 10H, and 12F, and with the SET2 tool following Cores 20X and 26F. We pumped perfluorocarbon tracers (PFTs) downhole on all cores for monitoring drilling fluid (seawater) contamination.

Cores U1547B-5H to 28F have been split and described. Core 5H marks the end of abundant micrometer-sized authigenic carbonate particles (micrite) and carbonate nodules. The recovered material consists of mainly laminated, diatom clay and scattered silt-sized, gray beds. The latter are often characterized by a more coarse-grained, sand-sized bottom layer. These beds become more frequent and coarser (up to granule size) from Core 18F to 21F. The drilling deformation is extensive. ​The sedimentary sequence is interrupted by a thin sill layer that was recovered in Core 23X from a depth of ~120 mbsf. This 0.5 m thick interval consists of moderately altered, sparsely to highly vesicular, sparsely plagioclase-phyric basalt. Pyrite and zeolite dominate the alteration mineral assemblage that fills veins and vesicles. Biostratigraphic analysis confirmed the occurrence of calcareous nannofossil Emiliania huxleyi down to Core 27F (144.2 mbsf), suggesting a deposition age younger than 0.29 Ma. Diatoms are partially barren and no diatom age marker has been found in Hole U1547B to date.

Scientists continued to analyze data from Hole U1546C and prepare their Site U1546 reports. The laboratory groups presented their results at the Site U1546 science summary meeting.

Daily Science Report for 16 October 2019

Location:
• Hole U1547A (27°30.4561′N, 111°40.6980′W; water depth 1733.7 mbsl)
• Hole U1547B (27°30.4124′N, 111°40.7345′W; water depth 1732.2 mbsl)

Science Update: We continued HLAPC/XCB coring in Hole U1547A, deploying the XCB coring system whenever we had to core through hard layers. This took place on Core 20X and continuously from Core 25X onward. Cores 20X to 27X penetrated from 108.1 to 141.3 mbsf and recovered 26.5 m (80%). We made a formation temperature measurement with the SET2 tool following Core 23F. While coring and recovering Core 27X, hole conditions deteriorated and continued to worsen below 136.3 mbsf so that we decided to terminate coring at a final depth of 141.3 mbsf. We then raised the drill string ~10 m and deployed the Kuster Flow Through Sampler (FTS) tool to recover borehole fluid from 131.3 mbsf. We retrieved the tool at 1330 h and sampled 600 mL of fluid. We then started pulling the drill string out of the hole, and the bit cleared the seafloor at 1425 h, ending Hole U1547A.

After offsetting the vessel 100 m to the southwest, we positioned the bit at a water depth of 1728.9 mbsl, installed the sinker bars and APC coring system, and spudded Hole U1547B at 1545 h. Mudline Core U1547B-1H recovered 6.3 m. This determined a seafloor depth of 1732.2 mbsl. Cores 1H to 7H penetrated from the seafloor to 65.6 m (104%). We deployed the APCT-3 tool to make formation temperature measurements on Cores 4H and 6H. We pumped perfluorocarbon tracers (PFTs) downhole on all cores for monitoring drilling fluid (seawater) contamination. The pacing of coring was adjusted to the complex microbiology and biogeochemistry sampling on the core receiving platform.

Cores U1547A-14H to 27X have been split and described. Cores 14H to 24X consist of carbonate (micrite)-rich diatom clay and carbonate concretions. Starting with Core 25X, the sediment is rich in silt- and sand-size layers that show evidence of resedimentation by mass-gravity deposition. This type of lithology occurs both above and below an igneous layer recovered in Core 26X and at the top of Core 27X. We recovered ~70 cm of this sill interval from a top depth of 132.8 mbsf. The sill consists of moderately altered, pyrite-bearing basalt with a glassy margin on top. There are abundant subrounded to angular vesicles in the uppermost 25 cm of the recovered interval.

Cores U1547B-1H to 4H have been split and described. They are composed of more or less laminated olive-green clay-rich diatom ooze with several grayish silt-rich layers.

Scientists continued collecting and analyzing data from Hole U1546C.

Daily Science Report for 15 October 2019

Location: Hole U1547A (27°30.4561′N, 111°40.6980′W; water depth 1733.7 mbsl)

Science Update: We continued lowering the drill string to the seafloor to start coring with the advanced piston corer (APC) in Hole U1547A. After the drill string had reached a water depth of 1700.9 mbsl at 0300 h, we deployed the subsea camera to conduct a survey of the seafloor at this site, which is a hydrothermally active area about 30 km northwest of the northern axial graben of Guaymas Basin. The seafloor survey did not find any vent communities. At 0530 h, we picked up the top drive and positioned the drill string above the seafloor to start APC coring. We spudded Hole U1547A at 0835 h. Mudline Core 1H recovered 5.4 m. This established a seafloor depth of 1733.7 mbsl. Cores 1H to 19F penetrated from the seafloor to 108.1 mbsf and recovered 118.8 m (110%). We switched to the half-length APC coring tool after encountering a partial stroke on Core 11H. Deeper in the hole, we needed to deploy the extended core barrel (XCB) coring system to break through hard carbonate layers. This took place on Cores 12X, 14X, 16X, and 18X. We made formation temperature measurements by using the APCT-3 tool on Cores 4H, 6H, 8H, and 10H, as well as the SET2 tool following Core 16X.

Cores U1547A-1H to 13H have been split and described. Core 1H consists of homogenous diatom clay. Starting with Core 2H, the sediments are mainly composed of alternations between laminated and homogenous diatom clay and diatom ooze. In Core 5H (from ~40 mbsf onward), authigenic carbonate precipitates (micrite and carbonate nodules) become abundant, and occasionally they are the dominant lithologic component. Up to 1.2 m thick silty beds, often with erosional bottom contacts, are also frequent in that depth range. The presence of the calcareous nannofossil Emiliania huxleyi suggests a Middle Pleistocene deposition age of <0.29 Ma. No age-diagnostic diatom species were found.

Daily Science Report for 14 October 2019

Location:
• Hole U1546C (27°37.8724′N, 111°52.7568′W; water depth 1585.6 mbsl)
• Hole U1547A (27°30.4560′N, 111°40.6980′W; water depth 1733.5 mbsl)

Science Update: We made up the triple combo logging tool string (natural gamma ray, porosity, and density sondes) to collect downhole measurements in Hole U1546C. We lowered the logging tool string into the hole to the bottom of the hole at 540.2 mbsf. After a short calibration pass, we implemented a full logging pass up to the seafloor. After the triple combo tool string returned to the rig floor at 0600 h, we rigged up the Formation MicroScanner (FMS)-sonic (resistivity imaging) logging tool string. The FMS-sonic tool string was deployed in the hole until we reached fill 15 m above the bottom of the hole. Upon implementing a single upward pass to the seafloor, the tool string returned to the rig floor at 1140 h and was disassembled by 1230 h. Next we deployed the subsea camera system to the seafloor to survey Hole U1546C and did not observe any gas release. The subsea camera system was recovered and secured by 1445 h. We then pulled the drill string out of the hole. The end of the drill string cleared the seafloor at 1500 h, ending Hole U1546C, and reached the rig floor at 1800 h. We then secured the vessel for transit and started the 13 nmi sea passage to Site U1547 (proposed Site GUAYM-12A) at 1822 h. After a 2 h transit, we arrived at the site coordinates, lowered the thrusters, and switched to dynamic positioning mode at 2047 h. We then made up the advanced piston corer (APC) bottom-hole assembly and started lowering the drill string to the seafloor, reaching a water depth of 408.9 mbsl by the end of the day.

Cores U1546C-32R to 42R have been split and described. They consist of more or less laminated, brownish gray, siliceous claystone with regular occurrence of dolostone layers. The calcareous nannofossil Pseudoemiliania lacunosa was identified in Core 42R. This suggests that the sediments at the bottom of Hole U1546C are older than 0.44 Ma, which corresponds to the Middle Pleistocene nannofossil Zone NN19. Cores 32R to 42R were barren of marine diatoms.

Daily Science Report for 13 October 2019

Location: Hole U1546C (27°37.8724′N, 111°52.7568′W; water depth 1585.6 mbsl)

Science Update: We started the day continuing RCB coring in Hole U1546C. Cores 29R to 42R penetrated from 473.6 to 540.2 mbsf and recovered 44.4 m (67%). The safety monitoring for hydrocarbon gases then obtained an anomalously low C1/C2 value from Core 42R, which led to termination of coring in Hole U1546C for safety reasons at a final depth of 540.2 mbsf at 1530 h. After circulating 50 barrels of high-viscosity mud for hole cleaning twice, we prepared the release of the RCB bit to enable downhole measurements with logging tool strings through the end of the drill pipe into the open hole. At 1830 h, we deployed the rotary shifting tool (RST) to trigger the mechanical bit release (MBR). Upon dropping the bit at the bottom of the hole, we deployed the reverse RST to shift the MBR sleeve back into the circulation position. We then raised the end of the drill string at a logging depth of 80.9 mbsf. At 2145 h, we started making up the triple combination logging tool string (triple combo: natural gamma ray, porosity, and density sondes) on the rig floor.

Cores U1546C-17R to 31R have been split and described. Cores 17R to 20R mainly consist of medium-grained mafic igneous rock representing the lower part of the intersected sill. This moderately to highly phyric dolerite contains phenocrysts of mainly plagioclase and subordinate pyroxene. A slight alteration is present. Sericite, chlorite, and epidote are the predominant alteration minerals. No vesicles or veins have been observed. Core 20R shows a transition from slightly altered dolerite to highly altered basalt that appears to be mingled with some sediment, making up the lowermost 0.4 m of the recovered sill, with contact to the underlying sediment at 431.7 mbsf. Core 21R recovered sediment from the contact zone with the sill, consisting of altered fine-grained silty clay with chert. Cores 22R to 31R contain well-laminated brownish black siliceous claystone with variable amounts of nannofossils and benthic foraminifers. No new age-diagnostic species have been observed through Core 31R, so the examined interval is still within nannofossil Zone NN20 (>0.29 Ma).

Daily Science Report for 12 October 2019

Location: Hole U1546C (27°37.8724′N, 111°52.7568′W; water depth 1585.6 mbsl)

Science Update: We continued RCB coring in Hole U1546C. Cores 15R to 28R penetrated from 405.5 to 473.6 mbsf and recovered 47.5 m (70%). Core 20R intersected the bottom sill/sediment contact at 431.7 mbsf. We pumped perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring on Cores 21R and 22R, the first two sediment cores below the sill.

Cores U1546C-9R to 16R have been split and described. They consist of slightly to moderately altered, medium- to coarse-grained mafic igneous rocks. From Sections 9R-2 through 10R-6, the recovered sill material shows a change in texture from porphyritic dolerite to equigranular gabbro. The gabbroic layer is ~15 m thick. At the bottom of Core 10R, the doleritic texture resumes. Plagioclase phenocrysts are present throughout the doleritic intervals, while pyroxene is a minor phenocryst phase below the gabbro layer only. Occasionally occurring veins are mainly filled with zeolite minerals.

Daily Science Report for 11 October 2019

Location: Hole U1546C (27°37.8724′N, 111°52.7568′W; water depth 1585.6 mbsl)

Science Update: We continued RCB coring in Hole U1546C. Cores 8R to 14R penetrated from 360.4 to 405.5 mbsf and recovered 37.6 m (83%). Deployment of perfluorocarbon tracers (PFTs) for drilling fluid (seawater) contamination monitoring was stopped after Core 8R at a depth of 366.6 mbsf.

Cores U1546C-6R to 8R have been split and described. Section 6R-1 recovered the sediment/sill contact at ~348 mbsf. Black to dark gray claystone and brown dolomite are present above the contact with the sill. The top margin of the sill consists of highly altered, plagioclase-phyric subvolcanic rock mingled with some sediment. Core 7R consists of sparsely to highly vesicular, sparsely plagioclase phyric dolerite. Core 8R consists of sparsely vesicular to nonvesicular, highly plagioclase phyric dolerite. The material in both cores is moderately altered, occasionally showing veins filled with calcite and zeolite. Generally, these minerals also fill the observed vesicles.

The absence of E. huxleyi from Cores U1546C-1R to 6R suggests that the sediments above the sill are younger than 0.29 Ma. This would place the examined interval within nannofossil Zone NN20. No age-diagnostic diatoms were observed.

Daily Science Report for 10 October 2019

Location: Hole U1546C (27°37.8724′N, 111°52.7568′W; water depth 1585.6 mbsl)

Science Update: We continued lowering the drill string to the seafloor. After the bit had reached a water depth of 1564 mbsl, we picked up the top drive, deployed the center bit, and positioned the end of the drill string for spudding Hole U1546C. At 0305 h, we spudded Hole U1546C (40 m south and 20 m east of Hole U1546B). We then drilled without core recovery from the seafloor to 308.2 mbsf, while pumping 20 barrels of high-viscosity mud in total. Upon recovering the center bit at 1330 h, we began coring with the rotary core barrel (RCB) system. Cores U1546C-2R to 7R penetrated from 308.2 to 360.4 mbsf and recovered 10.4 m (20%). The recovery increased substantially after a formation change from the sedimentary overburden (16%) to the underlying sill (69%) at the bottom of Core 6R at ~357 mbsf.

Cores U1546B-44F to 59X have been split and described. Cores 44F to 56X consist of greenish brown diatom clay. Starting with Core 57X, an increase in sediment stiffness and a decrease in diatom preservation was observed. Dolostone layers occur regularly once per core on average, and laminated intervals are present, although the majority of the sediment is homogeneous.

Cores U1546C-2R to 5R have been split and described. They consist of brown siliceous claystone that contains several dolostone layers.

Daily Science Report for 9 October 2019

Location:
• Hole U1546B (27°37.8840′N, 111°52.7809′W; water depth 1585.6 mbsl)
• Underway to Guaymas, Mexico
• Guaymas pilot station
• Underway to Site U1546
• Hole U1546C (27°37.8732′N, 111°52.7575′W; water depth 1585.6 mbsl)

Science Update: We started the day continuing XCB coring in Hole U1546B. Core 59X penetrated from 331.3 to 333.8 mbsf and recovered 2.5 m (77%). We then terminated coring at 0045 h and pulled the drill string out of the hole to prepare for transit to Guaymas, Mexico, to conduct a medical repatriation of a crew member. The bit cleared the seafloor at 0215 h, ending Hole U1546B at a final depth of 333.8 mbsf. After the bit had arrived on the rig floor at 0530 h, we secured the vessel for transit. At 0600 h, we began our transit to Guaymas. We arrived at the pilot station at 1215 h, and the patient disembarked the vessel at 1225 h. After the pilot boat had left, we began the sea passage for returning to Site U1546. We arrived on site at 1730 h and switched to dynamic positioning mode. At 1815 h, we started preparing to spud Hole U1546C. We assembled the rotary core barrel (RCB) coring tool and made up the RCB bottom-hole assembly. We then began lowering the drill string to the seafloor, reaching a water depth of 590 mbsl by the end of the day.

Cores U1546B-7H to 43F have been split and described. They consist of diatom ooze and clay, showing overall decreasing lamination and increasing bioturbation with predominance of diatom clay from Core 20H onwards. Carbonate-rich indurated intervals and dolostones occur regularly.

Daily Science Report for 8 October 2019

Location: Hole U1546B (27°37.8840′N, 111°52.7809′W; water depth 1585.6 mbsl)

Science Update: We continued HLAPC/XCB coring in Hole U1546B throughout the day. Cores 24X to 58X penetrated from 202.5 to 331.3 mbsf and recovered 136.3 m (106%). We mainly deployed the HLAPC coring tool to a depth of 293.3 mbsf (Core 54F) and switched to the XCB coring tool whenever we had to core through hard carbonate layers. This happened at several depths, corresponding to Cores 24X, 27X, 31X, 34X, 36X, 40X, 42X, 46X, 48X, 53X. Starting with Core 55X, we deployed the XCB system permanently. We pumped perfluorocarbon tracers (PFTs) down the hole on all cores for monitoring drilling fluid (seawater) contamination.

The last cores of Hole U1546A, i.e., Cores U1546A-63X and 64X, have been split. They consist of medium-grained dolerite that will be described in detail once processing and description of Hole U1546B sediment cores have been completed. Biostratigraphic analysis of sediment cores from Hole U1546A has been completed through the bottom of the sediment succession in Core 62X. No older age-diagnostic flora has been observed in either calcareous nannofossils or marine diatoms. The bottom of Hole U1546A thus remains in nannofossil Zone NN20 with an age between 290 and 440 ka.

Cores U1546B-1H to 6H have been split and described. They consist of alternating finely laminated and homogenous diatom clay and diatom ooze. Soft sediment deformation has occasionally been observed. There are no samples taken for micropaleontology in Hole U1546B.

Daily Science Report for 7 October 2019

Location: Hole U1546B (27°37.8840′N, 111°52.7809′W; water depth 1585.6 mbsl)

Science Update: The day started with preparing for coring operations in Hole U1546B. We installed the sinker bars and lowered the bit to the seafloor to spud Hole U1546B. We then started APC coring in Hole U1546B at 0050 h. Mudline Core U1546B-1H recovered 4.3 m. This established a seafloor depth of 1585.6 mbsl. Cores 1H to 23H penetrated from the seafloor to 202.5 mbsf and recovered 213.0 m (105%). We switched to the XCB coring tool to core through a hard layer at 134.8 mbsf (Core 16X). We pumped perfluorocarbon tracers (PFTs) down the hole on all cores for monitoring drilling fluid (seawater) contamination. As at the previous site, Hole U1546B is dedicated to extensive collection of whole-round cores for microbiology and biogeochemistry research. The pacing of coring was adjusted to the complex microbial sampling program conducted on the core-receiving platform.

Cores U1546A-44F to 62X have been split and described. Cores 44F to 56X consist of greenish brown diatom clay. A change in color from olive to dusky yellowish brown, starting with Core 57X, corresponds to an increase in sediment stiffness and a decrease in diatom preservation. This siliceous claystone continues until the bottom of the sedimentary overburden in Core 62X, above the contact with the sill. In Core 62X, darker siliceous claystone is present. Dolostone layers occur regularly once per core on average and laminated intervals are present, although the majority of the sediment is homogeneous. At the bottom of Section 62X-CC we recovered the top chilled margin of the underlying basalt sill in a ~20 cm thick interval. Biostratigraphic examination through Core 60X has not yielded any new age-diagnostic taxa for either calcareous nannoplankton or marine diatoms. Thus, a Late to Middle Pleistonce biostratigraphic age (<290 ka) remains through Core 39F based on the NN21 nannofossil biozone, which is the base of identified species E. huxleyi.

Daily Science Report for 6 October 2019

Location:
• Hole U1546A (27°37.8851′N, 111°52.7939′W; water depth 1586.1 mbsl)
• Hole U1546B (27°37.8840′N, 111°52.7818′W; water depth 1585.6 mbsl)

Science Update: We started the day continuing HLAPC/XCB coring in Hole U1546A. Cores 51F to 64X penetrated from 283.0 to 361.2 mbsf and recovered 61.5 m (79%). Formation temperature measurements were made through deployment of the SET-2 tool after Cores 51F and 57X. We switched to XCB coring to penetrate through a hard carbonate layer on Core 53X. After Core U1546A-54F, we permanently deployed the XCB coring tool in stiffer sediment formation. Our goal was to deepen the hole to the top contact of a seismically imaged sill at around 350 mbsf. We eventually intersected the sill contact on Core U1546A-63X at ~358.5 mbsf. Upon recovering another interval of the same sill in Core 64X at 2120 h, we started pulling the drill string out of Hole U1546A from a final depth of 361.2 mbsf. The bit cleared the seafloor at 2320 h, ending Hole U1546A. We then moved the vessel 20 m to the east to position the bit for spudding Hole U1546B.

Cores U1546A-19H to 43H have been split and described. They mainly consist of diatom clay. At the bottom of Core 22F, the micrite component that was significant in shallower cores becomes less abundant. However, carbonate-rich indurated intervals and dolostones continue to occur regularly. Unlike the shallower part of the hole, the majority of the sediments are bioturbated and lamination is rare. Biostratigraphic examination through Core U1546A-35F has not yielded any new age-diagnostic taxa for either calcareous nannoplankton or marine diatoms, so a Late to Middle Pleistonce biostratigraphic age (<290 ka) remains.

Scientists continued to collect and analyze Hole U1545B data and submitted the first draft of their Site U1545 reports. We held the second science summary meeting for Site U1545, with the other half of the laboratory groups presenting their Site U1545 results to the entire science party.

Daily Science Report for 5 October 2019

Location: Hole U1546A (27°37.8851′N, 111°52.7939′W; water depth 1586.1 mbsl)

Science Update: We continued coring in Hole U1546A throughout the day, using the APC, HLAPC, and XCB coring tools. Cores 18H to 50F penetrated from 155.8 to 283.0 mbsf and recovered 139.1 m (109%). We recorded strong overpull force upon retrieval of Core U1546A-21H, so we switched to HLAPC. While using the HLAPC coring tool, we occasionally switched to the XCB coring tool to penetrate through hard layers. This happened on Cores 26X, 31X, 34X, 37X, 41X, 45X, and 47X. All APC cores were oriented. At several depths, we made formation temperature measurements using both the APCT-3 tool (on Cores 19H, 22F, and 28F) and the SET-2 tool (after Cores 28F, 35F, and 43F).

Cores U1546A-7H to 18H have been split and described. Cores 7H to 10H mainly represent alternations between laminated and homogenous diatom clay and diatom ooze. ​From Cores 11H to 18H, tilting and folding of laminae and beds occur regularly, and diatom ooze and clay are mixed with micrite. On average, two carbonate layers/nodules with different levels of induration are present in each core. Biostratigraphic analysis through Core U1546A-16H has not yielded any new age-diagnostic taxa for either calcareous nannoplankton or marine diatoms, so a Late to Middle Pleistonce biostratigraphic age (<290 ka) remains.

Scientists continued to collect and analyze Hole U1545B data and prepare their Site U1545 reports. We held the first science summary meeting for Site U1545, with half of the laboratory groups presenting their Site U1545 results to the entire science party.

Daily Science Report for 4 October 2019

Location:
• Hole U1545B (27°38.2301′N, 111°53.3295′W; water depth 1594.2 mbsl)
• Hole U1546A (27°37.8851′N, 111°52.7939′W; water depth 1586.1 mbsl)

Science Update: We started the day continuing to XCB core in Hole U1545B. Cores 63X to 67X penetrated from 347.9 to 387.3 mbsf and recovered 7.0 m (18%). We made a formation temperature measurement following Core 63X using the SET2 tool. After the XCB bit was destroyed with Core 66X and we advanced only 0.5 m in 45 min of coring Core 67X, we decided to terminate coring in Hole U1545B. At 0815 h we started pulling the drill string out of the hole, and the bit cleared the seafloor at 0945 h, ending Hole U1545B and Site U1545. At 1005 h, we started moving the vessel to Site U1546 (proposed primary Site GUAYM-02B). We arrived on the site coordinates at 1045 h and prepared for APC coring operations in Hole U1546A. We lowered the drill string to a water depth of 1558 mbsl and picked up the top drive. The bit was then positioned at 1581.4 mbsl—5 m above the seafloor depth obtained from the precision depth recorder—for taking the first core. At 1230 h, we spudded Hole U1546A. Mudline Core U1546A-1H arrived on deck at 1250 h, recovering 3.8 m. This established a seafloor depth of 1586.1 mbsl. APC Cores 1H to 17H advanced from the seafloor to 155.8 mbsf and recovered 165.2 m (106%). We made temperatures measurements on every third core with the APCT-3 tool, starting with Core 4H.

Cores U1545B-46F to 67X, the last from Hole U1545B, have been split and described. They consist of diatom clay and silty clay. We also confirmed a basaltic modal composition for the microcrystalline hard rock layer recovered in Core U1545A-71X.

Cores U1546A-1H to 6H have been split and described. They consist of alternations between finely laminated and homogenous diatom clay and diatom ooze. Cores 2H and 3H show evidence of soft sediment deformation. Calcareous biostratigraphic markers indicate a Late to Middle Pleistocene age (<290 ka) at the base of Core 3H. No age diagnostic diatoms have been observed.

Scientists continued to collect and analyze Hole U1545B data, and prepare their Site U1545 reports and presentations for the science summary meeting.

Daily Science Report for 3 October 2019

Location: Hole U1545B (27°38.2301′N, 111°53.3295′W; water depth 1594.2 mbsl)

Science Update: We continued coring in Hole U1545B, alternating between the HLAPC and XCB coring tools. Cores 40F to 62X penetrated from 245.8 to 347.9 mbsf and recovered 84.8 m (83%). We mainly used HLAPC coring to a depth of 328.5 mbsf (Core 60F) and switched to XCB coring on Cores 48X, 54X, and 57X after we recorded partial strokes due to encountering hard carbonate layers. Starting with Core 61X, we deployed the XCB coring tool permanently after we had to drill over the core barrel on Core 60F. For formation temperature measurements, we deployed the APCT-3 tool on Cores 45F and 49F, and the SET-2 tool following Cores 45F, 49F, and 60F. We continuously deployed perfluorocarbon tracers (PFTs) on all cores for monitoring drilling fluid (seawater) contamination.

Cores U1545B-21F to 45F have been split and described. They consist of light olive to olive gray micrite-rich diatom clay, revealing a decrease in both authigenic micrite content and abundance of carbonate concretions compared to shallower cores.

Daily Science Report for 2 October 2019

Location: Hole U1545B (27°38.2301′N, 111°53.3295′W; water depth 1594.2 mbsl)

Science Update: We continued coring in Hole U1545B, starting the day with switching from APC to HLAPC coring. Cores 18F to 39F penetrated from 147.6 to 245.8 mbsf and recovered 94.6 m (96%). For formation temperature measurements, we deployed the APCT-3 tool on Cores 20F, 27F, 33F, and 39F, and the SET2 tool following Cores 27F, 33F, and 39F. Partial strokes were recorded on Cores 18F, 23F, 24F, 26F, 27F, 35F, and 36F. We pumped perfluorocarbon tracers (PFTs) downhole on all cores for monitoring drilling fluid (seawater) contamination. The pacing of coring continued to be adjusted to the microbial sampling program implemented on the catwalk.

Cores U1545B-8H to 20F have been split and described. They mainly consist of light olive-gray diatom ooze with laminated and homogenous textures. Subordinate intercalated carbonate layers and nodules are common.

Daily Science Report for 1 October 2019

Location: Hole U1545B (27°38.2301′N, 111°53.3295′W; water depth 1594.2 mbsl)

Science Update: We started the day deploying the subsea camera system to the seafloor to survey Hole U1545A. We did not observe any gas release from the subseafloor through the hole. The subsea camera system was recovered by 0200 h. We then started pulling the drill string out of the hole and the bit cleared the seafloor at 0225 h, ending Hole U1545A. After the drill string was raised to a water depth of 1567.1 mbsl, we picked up the top drive, and lowered the bit to the seafloor to spud Hole U1545B. We then installed the sinker bars and started APC coring Hole U1545B at 0410 h. Mudline Core U1545A-1H recovered 3.7 m. This established a seafloor depth of 1594.2 mbsl. Cores 1H to 17H penetrated from the seafloor to 147.6 mbsf and recovered 154 m (104%). We made formation temperature measurements (APCT-3 tool) on every third core starting with Core 4H. A partial stroke was encountered on Core U1545B-16H. We pumped perfluorocarbon tracers (PFTs) down the hole on all cores for monitoring drilling fluid (seawater) contamination. The pacing of coring was adjusted to the complex microbial sampling program conducted on the core-receiving platform.

Hole U1545B is dedicated to extensive collection of whole-round cores for microbiology and biogeochemistry research. The selected core sections are processed on board for (1) few shipboard microbial analyses (e.g., cell counting, microbial rate measurement) and (2) shipboard sampling for personal postexpedition research. This approach implies that the majority of the recovered material from each core is not subjected to Core Laboratory analyses and description. Hole U1545B core description is thus based on observations obtained from the top and bottom section of each core. There are no core catcher samples taken for micropaleontology in this hole.

Cores U1545B-1H to 7H have been split and described. They consist of olive-gray laminated diatom ooze with variable proportions of clay, silt, authigenic carbonate, and other biosiliceous and calcareous microfossils.