Site U1366 | Site U1367 | Site U1368 | Site U1369 | Site U1370 | Site U1371
IODP Expedition 329:
Subseafloor Life in the South Pacific Gyre
Site U1365 Summary
PDF file is available for download.
The scientific objectives at Site U1365 are (1) to document
the nature of subseafloor life in very old (>100 Ma) and slowly accumulating
organic-poor sediments; (2) to test how oceanographic factors (such as surface
ocean productivity, sedimentation rate and distance from shore) control
variation in sedimentary habitats, activities and communities from gyre center
to gyre margin; (3) to quantify the extent to which subseafloor communities in
organic-poor sediment are sustained by H2 from radiolysis of water;
and (4) to determine how basement habitats, potential activities and, if
measurable, communities vary with basalt age and hydrologic regime (from ridge
crest to abyssal plain).
Site U1365, at 5708 m water depth, is centrally
located in ocean crust formed during the Cretaceous Normal Superchron (CNS).
The tectonic history of this Australia-sized area is poorly constrained because
correlatable magnetic seafloor anomalies are not present. Consequently,
radiometric dating of the recovered basalt will provide important constraints
on the tectonic and volcanic history of this region.
The complete sedimentary succession was recovered by
APC in Hole A. Excluding a drilled-over chert interval in the lower sediment
column, complete successions were also recovered from Holes B and C. Core
recovery in the underlying basalt was unusually high (75%). However, slow
penetration (<1 m/hour) allowed us to drill only about 50 m of basalt,
preventing us from reaching sufficient hole depth to deploy downhole logging
The sedimentary succession at Site U1365 is composed of
three units. Unit I consists of medium brown zeolitic metalliferous pelagic
clay (0-44 mbsf); Unit II of porcellanite and chert (44-65 mbsf); and Unit III
of dark brown metalliferous clay (65-75 mbsf).
Cell counts are much lower than at the same sediment
depths in all sites previously cored by scientific ocean drilling. Total
organic carbon and total nitrogen disappear in the lower portion of Unit I.
Dissolved oxygen and dissolved nitrate are present to great depth.
The drilled sequence of basement rock is a series of
massive basalt flows. Secondary mineralization and wall-rock interaction is
primarily restricted to regions between lava flows.
Downhole temperature was measured using the APCT-3
(Advanced Piston Coring Temperature tool). The result closely agrees with the
thermal gradient observed by the site survey expedition. The heat flow is
typical for this age crust. Sediment temperatures are within the range
inhabited by psychrophilic microbes.
A wide range of microbiology experiments was initiated
shipboard. Experiments on major microbial processes and experiments for
enumeration of viable microbes were initiated at selected depths ranging from
near the sediment/water interface to nearly 50 m into the basaltic basement.
Subsamples for postcruise biomolecular assays and microbiological experiments
were routinely taken from all of the distinct lithologic units.