Site U1301 | Site 1027
IODP Expedition 327:
Juan de Fuca Ridge-Flank Hydrogeology
Site U1363 Summary
PDF file is available for download.
5 September 2010
The three basaltic outcrops near the IODP Juan de Fuca
drill sites are locations where altered seawater from the basaltic basement
vents to the overlying ocean (Wheat
et al., 2004). In contrast, Grizzly Bare outcrop is a large basaltic
feature 52 km to the south, where heat flow, seismic, and modeling studies
indicate that cold seawater enters the crust (Fisher et al., 2003; Hutnak et
al., 2006). As at other locations where ridge-flank hydrothermal circulation is
guided by basement outcrops, seawater is hypothesized to enter basaltic
basement at Grizzly Bare, flow laterally within the crust and become warm,
react with basalt, exchange solutes with overlying pore waters in the sediment,
and support microbial processes. Thermal and chemical changes are dramatic and
clearly defined with temperatures that reach 64¡C and a fluid composition that
has lost all of its dissolved oxygen, nitrate and magnesium while gaining Ca by
the time it reaches Baby Bare. This transition from oxic seawater to a
hydrothermal fluid takes tens to hundreds of years in this setting.
Site U1363 (Scientific Prospectus Sites GRB-1A, 2A and
3A; Fisher et al., 2010) was placed adjacent to the northeastern edge of
Grizzly Bare outcrop. If Grizzly Bare is a significant hydrothermal recharge
site, as hypothesized, fluid and microbiological composition at the base of the
sediment column should look much like that near the seafloor, whereas the
chemical composition of pore fluids and microbiological communities from the
middle of the sediment section will show the influence of diffusion and
digenetic reactions as a function of distance from the outcrop, depth, and
Resolution departed Site U1362 at 1300 hr
on 30 August 2010 and the 31 nmi transit to Grizzly Bare outcrop took 3 hr at
an average speed of 10.0 kt.
Hole U1363A (proposed
site GRB-1A) was spudded at 2300 hr on 30 August at a depth of 2689 m. Drilling
without coring continued for 2-1/2 hr to the basement contact at 58 m below
seafloor (mbsf). The sole purpose of drilling Hole U1363A was to determine the
depth of basement to avoid a possible impact with an APC core barrel or
temperature shoe at the next hole.
The ship was offset 10
m on a bearing of 135¡ and Hole U1363B was spudded at 0530 hr on 31 August at a
depth of 2690 m. APC Cores 1H through 6H advanced to 42.5 mbsf by 1300 hr, at
which point the APC was not able to penetrate the sandy turbidite formation.
APCT3 temperature measurements were taken with Cores 3H through 6H. Cores 7X
and 8X advanced to 55.0 mbsf by 1610 hr. The first SET temperature measurement
was taken at ~56 mbsf and coring continued with Core 9X, which advanced through
the sediment/basement interface to 57 mbsf. Core 10X advanced to 61 mbsf and
was on deck by 2100 hr. The drill string was pulled clear of the seafloor at
2140 hr on 31 August.
The ship was offset in
DP mode to the coordinates for Hole U1363C (GRB-3A), the deepest of the 3-hole
transect at Grizzly Bare. Hole U1363C was spudded at 2255 hr on 31 August at a
depth of 2689 m. Hole U1363C was drilled without recovering cores to 170 mbsf.
A SET temperature measurement was taken at ~171 mbsf, followed by Cores 2X and
3X to a depth of 183.2 mbsf. A second SET temperature was taken at ~184 mbsf,
followed by Cores 4X and 5X to a depth of 202.4 mbsf. A third SET temperature
measurement was taken at ~204. At 1445 hr on 1 September, the SET tool became
stuck inside the outer core barrel due to the sandy formation and the drill
string had to be recovered back to the ship. The SET tool was retrieved at 0145
hr on 2 September and the ship was offset 10 m on a bearing of 135¡.
Hole U1363D was
spudded at 0745 hr on 2 September at a depth of 2689 m. Drilling without coring
continued using an XCB bit to 198 mbsf. Cores 2X-5X advanced to 231 mbsf by
0035 hr on 3 September and the drill string was pulled out of the seafloor at
0700 hr, ending Hole U1363D.
Hole U1363E was
spudded at 0840 hr on 3 September and was drilled without coring to establish
the depth to basement. The sediment/basement interface was confirmed at 36 mbsf
at 1000 hr. The drill string was pulled out of the seafloor at 1125 hr, ending
Hole U1363F was
spudded at 1200 hr on 3 September. Cores 1H to 4H advanced to 35 mbsf by 1600
hr. APCT3 temperature measurements were taken with Cores 3H and 4H. The drill
string was pulled out of the seafloor at 1735 hr, ending Hole U1363F.
The ship was offset
and basement contact was again established at 17 mbsf by washing down. Hole
U1363G was spudded at 2000 hr on 3 September. Cores 1H to 3H advanced to 24.9
mbsf by 2250 hr. The true advance is closer to 17 mbsf as the last core
recovered mostly flow-in material. An APCT3 temperature measurement was taken
with Core 2H.
Transit to Victoria, British Columbia, Canada
The rig floor was secured for transit and the ship was underway at 1015 hr on 4 September. The first line ashore was at 0836 hr on 5 September 2010.
Site U1363 sediments are
composed of turbidite sequences interspersed with hemipelagic mud. Several
lithologic units can be distinguished.
Very few, small pieces of
basalt were recovered from Site U1363. The basalt is cryptocrystalline and
plagioclase phyric, with glomeroporphyritic texture visible in hand specimen.
Phenocrysts are large (up to 8 mm) and are anhedral to euhedral in shape. The basalt
is sparsely vesicular with highly variable vesicle size and shape. Secondary
minerals are present as background groundmass replacement, alteration halos,
filling vesicles, and lining hydrothermal veins.
All cores were run
through the whole-round multisensor logger (WRMSL), yielding magnetic
susceptibility values from <500 x 10–6 SI in clay sections
to ~1400 x 10–6 SI in sandy turbidites. Point susceptibility
data run on the section half multisensor logger (SHMSL) are similar, with data tending to be slightly lower than
whole round values, except in the case of turbidite sequences where SHMSL
values are consistently higher. Gamma ray attenuation density averages 1.8 to
2.0 g/cm3, depending on lithology, with some compaction evident with
depth in clay data.
including moisture and density (MAD), P-wave,
and thermal conductivity, were measured on most cores from Holes U1363B,
U1363C, U1363D, and U1363F, with insufficient time to run cores from Hole
U1363G. Thermal conductivity in Hole U1363B averaged 1.30 W/mK, while MAD bulk
densities averaged 1.77 g/cm3, both showing bimodal distributions
corresponding to clay and sand lithologies. MAD porosity in Hole U1363B
averages 60.6%. P-wave velocities
in Hole U1363B average 1.52 km/s, with marked differences in both average and
variability across lithologies. Velocities also show some anisotropy between
vertical and horizontal directions. Insufficient time was available to analyze
data gathered in holes other than Hole U1363B.
water samples were recovered from 5 holes at 4 locations, providing systematic
trends to gauge the composition of the underlying basaltic formation fluid at
these locations. Pore waters were
extracted in a nitrogen atmosphere and some analyses (alkalinity and ion
chromatography) were conducted immediately to guide future drilling operations.
We collected a total of 57 pore water samples: 15 from Hole U1363G, 14 from
Hole U1363F, 14 from Hole U1363B, and 14 from Holes U1363C/U1363D, with
basement depths of 25, 33, 54, and 223 mbsf, respectively. In the upper portion
of the sediment, biogenic processes release dissolved Mn and Fe near the
sediment/water interface and consume sulfate with a sulfate minimum of 22 mM at
20 mbsf. There is a corresponding increase in alkalinity, phosphate, and
ammonium and an initial decrease in Ca resulting from carbonate formation given
the high alkalinity values. Similar trends for sulfate, Mn, and Fe exist near
the sediment/basement interface. However, phosphate and ammonium are more
influenced by diffusion and reaction within the upper basaltic basement. The
cations Ca, Mg, and K show gradients near the sediment/basement interface,
indicative of a formation fluid that is slightly altered relative to sea water.
Minor and trace elements in sea water also show gradients in the basal sediment
section, projecting to a formation fluid that is slightly altered relative to
sea water and more altered than locations closer to Grizzly Bare outcrop.
shows a progression from higher to lower values at the sediment/basement
interface and a progression of increasing mid-sediment column concentrations
with increasing depth to the basement for all the sites sampled. Also measured
in these samples was pH, with consistent values near 7.3 from the seafloor to
the sediment/basement interface. At Hole U1363G, alkalinity increased from 3.5
meq at 1.5 mbsf up to 4.6 at 10 mbsf and then decreased to 2.9 meq at 25 mbsf
at the basement. Hole U1363F had a similar profile. At this site, alkalinity
rose from 6.7 meq at 4.5 mbsf up to 10.5 meq at 12 mbsf and then decreased to
1.8 meq as the sediment/basement interface of 33 mbsf. The highest alkalinity
measured was 13.9 meq at 16.5 mbsf in Hole U1363B, whereas alkalinity was 2.1
meq at the sediment/basement interface. Coring at Holes U1363C/U1363D sampled
only the lowermost 50 m, so the alkalinity maximum was missed in the upper
sediments. The lowest alkalinity measured was 1.3 meq at 233 mbsf at the Hole
U1363D sediment/basement interface. These findings are consistent with a
progressively altered basement fluid with increasing distance from the Grizzly
Microbiologists collected whole round
core and pore water samples from sediments and basement pieces recovered at
Site U1363. Eleven sediment intervals were targeted for microbiology
sampling at Hole U1363B. Most samples were taken from hemipelagic clay
layers, although some sandy turbidite layers were also sampled. The
deepest sediment sample was taken from a carbonate-rich layer near the
sediment/basement interface. Thirteen sediment intervals and one basement
basalt were sampled from the combined coring at Holes U1363C and U1363D.
Again, sediment samples were mostly from clay-rich layers, although some
samples contained sand. The basement sample from Core U1363D-6X was a
relatively unfractured basalt with spots of light green and orange alteration
At each sampling location,
whole round core samples were collected for shore-based DNA analysis,
shore-based characterization of halogenated organic matter, and shore-based
incubation experiments to examine dehalogenation reaction activities. Syringe samples were also collected for headspace gas analysis and microsphere contamination checks from the interior and exterior of the cores.
Headspace samples were analyzed shipboard for safety purposes and only a
few samples had quantifiable levels of methane or higher hydrocarbon gases,
which is expected in the high sulfate sediments samples. Microsphere samples will be returned to
the shore-based lab for postcruise analysis, due to time limitations at the end
of the cruise. Those samples will also be used for shore-based cell counting analysis and fluorescence in situ hybridization analysis. A subset
of samples were also collected for analyses of DOC/DN, POC/PN, amino acids, low
molecular weight organic acids and lipid biomarkers.
measurements were made on ~2/3 of core sections from Hole U1363B. Samples were
demagnetized at 10 mT steps from 0 to 40 mT using the cryogenic magnetometerÕs
inline alternating field coils. Although the majority of samples have positive
inclinations there is a large scatter of positive and negative inclinations.
This is probably the result of core deformation during the drilling process and
the alternating sequences of hemipelagic mud and turbidite deposits.
Temperature measurements were
collected with the APCT-3 and SET tools at Holes U1363B–U1363E. Several
good measurements were obtained with both tools.
Fisher, A. T., E.
E. Davis, and K. Becker (2008), Borehole-to-borehole hydrologic response across
2.4 km in the upper oceanic crust: implications for crustal-scale properties, J. Geophys. Res., 113, doi:10.1029/2007JB005447.
Fisher, A.T., Tsuji, T., and
Gamage, K., 2010. Juan de Fuca Ridge-Flank Hydrogeology: the hydrogeologic
architecture of basaltic oceanic crust: compartmentalization, anisotropy,
microbiology, and crustal-scale properties on the eastern flank of Juan de Fuca
Ridge, eastern Pacific Ocean. IODP Sci. Prosp., 327. doi:10.2204/iodp.sp.327.2010
Hutnak, M., A. T.
Fisher, L. Zhlsdorff, V. Spiess, P. Stauffer, and C. W. Gable (2006),
Hydrothermal recharge and discharge guided by basement outcrops on 0.7-3.6 Ma
seafloor east of the Juan de Fuca Ridge: observations and numerical models, Geochemistry
Geophysics Geosystems (G3), 7, doi:10.1029/2006GC001242.
Wheat, C. G., M.
J. Mottl, A. T. Fisher, D. Kadko, E. E. Davis, and E. Baker (2004), Heat and
fluid flow through a basaltic outcrop on a ridge flank, Geochemistry
Geophysics Geosystems (G3), 5(12), doi:10.1029/2004GC000700.