166 Preliminary Report


The primary objective of Sites 1008/1009 was to obtain heat flow and interstitial-water geochemistry measurements from a second area of the margin of Great Bahama Bank to compare with data from Sites 1003-1007. This approach was necessary to help support our findings regarding fluid flow and diagenetic reactions in the margin of the platform. A secondary objective was to retrieve a high-resolution section of Holocene sediment for the study of recent climate change.

Sites 1008 and 1009 penetrated thick Pleistocene sections. At Site 1008, the age at the base of the hole (134.5 mbsf) is 1.44 Ma, with sedimentation rates varying between 4.5 and 16 cm/k.y. At Site 1009 a similar age was attained at a depth of 226.1 mbsf, with sedimentation rates between 5 and 55 cm/k.y. Eight seismic sequences can be recognized in the drilled section that are separated from each other by seven seismic boundaries. The strata at Sites 1008 and 1009 consist of lithified to partially lithified peloidal and bioclastic mudstones, wackestones, packstones, and grainstones with interbedded foraminifer nannofossil ooze. Most of the SSBs correlate with distinct layers in the cores, that are dark in color, coarse grained, and show signs of submarine cementation. Adara and WSTP temperature measurements revealed an irregular heat flux in the upper 40 mbsf and a much lower heat flux (20 mW vs. 40 mW) in the lower portion of the profile, as compared to Sites 1003 1007. Pore-water geochemistry profiles showed little variation in the upper 30 mbsf. Below this zone sulfate reduction and other diagenetic reactions were prevalent.

Two major lithologic units were distinguished at both Sites 1008 and 1009.

Unit I (0-78.2 mbsf, Site 1008; 0-147.15 mbsf, Site 1009) is latest Pleistocene to Holocene in age. Subunit IA (0-27.1 mbsf, Site 1008; 0-22.02 mbsf, Site 1009) consists of a coarsening-upward, pale-yellow to white, unlithified, peloidal wackestone grading into wackestone and mudstone, with minor to moderate bioturbation. The base of this subunit is marked in both sites by a bored and encrusted hardground. Subunit IB (27.1-78.2 mbsf, Site 1008, 22.02-98.71 mbsf, Site 1009) consists of multiple coarsening-upward sequences separated by harder layers. The sequences are composed of unlithified peloidal mudstones at the base and peloidal packstones at the top, which also correspond to cycles in physical properties. Characteristically they have relatively low velocities, densities, and gamma-ray values at their base that increase toward the top. The floatstones contain large gray lithoclasts composed of pteropod and planktonic foraminifer wackestone to packstone. The clasts are cemented and bored and are interpreted as being the remnant of a marine hardground. Subunit IC (98.71-147.15, Site 1009) was not recognized at Site 1008. It contains four coarsening-upward intervals and is separated from Unit IB by a fragmented hard layer consisting of pteropod biopackstone. Subunit IA at Site 1008 has a sedimentation rate of 4.5 cm/k.y., whereas subunit IB has a rate of 16 cm/k.y. At Site 1009, the sedimentation rates are 18 cm/k.y. throughout Subunit IA, up to 35 cm/k.y. in Subunit IB and 7 cm/k.y. in Subunit IC.

Unit II (78.2-134.5, Site 1008; 147.15-226.1, Site 1009) consists of unlithified lithoclastic and bioclastic floatstone. At Site 1009 two hardgrounds are recognized in this sequence. At Site 1008, the sedimentation rate in Unit II is 8 cm/k.y. At Site 1009 sedimentation rates are between 22 and 55 cm/k.y. throughout Unit II.

At both sites a number of alternations in sediment composition, color, and mineralogy can be recognized, which probably correspond to sea-level changes. Lowstands are characterized by higher concentrations of low-Mg calcite that have a darker color compared to highstands which contain abundant aragonite and high-Mg calcite. Dolomite becomes a minor component of the sediment below 60 mbsf in Site 1008 and 120 mbsf at Site 1009, and percent carbonate was generally higher than 95% throughout.

Sites 1008 and 1009 recovered extremely expanded sections of Pleistocene and Holocene sediments. The nannofossil biostratigraphy indicates the presence of zones NN19 to NN21. The planktonic foraminifer record is restricted to N22. Sediments were substantially reworked in the lower part of the record.

The youngest seismic sequence s appears at Site 1009 at 29 mbsf and at Site 1008 at 9.6 mbsf. The age of the SSB (0.15-0.12 Ma) probably corresponds to the last sea-level lowstand at the Pleistocene/Holocene boundary. SSB T corresponds to 48 mbsf at Site 1009 and 14.5 at Site 1008 and has an age of 0.20 to 0.25 Ma. These two boundaries, as well as SSB U, appear in both holes slightly deeper than high-velocity layers, allowing for the possibility that too high a velocity was assumed for these sediments in the depth/time conversions. Sequence w has its lower boundary at 126 and 53.2 mbsf at Sites 1009 and 1008, respectively. The associated age of SSB W which might correlate to a high-velocity layer 13 m higher in the core, is 0.7-0.062 Ma. The rest of the holes show two prominent velocity deviations coinciding with SSBs X and Y.

Based on the pore-water geochemistry, two distinct geochemical zones were identified within Sites 1008 and 1009. An upper zone, extending from the seafloor to 40 mbsf, is characterized by an absence of significant changes in the interstitial pore-water geochemistry. This flushed zone is similar to those observed at Sites 1006 and 1007, in that there was slight evidence of carbonate recrystallization reactions in the form of a small, but nevertheless significant, increase in the Sr concentration. Sites 1003, 1004, and 1005 showed no evidence of such an increase. The upper zone gradually merges into a region with elevated chlorinity, up to 730 mM at Site 1009 and 670 mM at Site 1008. This zone shows a small reduction in sulfate and large increases (up to 1600 µM) in Sr. Calcium and Mg are reduced relative to their seawater values in this interval, indicating carbonate mineral precipitation. Methane and hydrogen sulfide reached concentrations of 100 and 10,000 ppm, respectively, within the zone of low sulfate.

Heat flow at both sites is significantly lower than at Sites 1003-1007. At Sites 1008 and 1009 the geothermal gradient is approximately 27°C/km and 17°C/km respectively. This compares to approximately 35°C/km for the northern sites. The low heat flow may indicate regional cooling as a result of the inflow of bottom seawater, perhaps deeper in the sedimentary section.

166 Conclusions

166 Table of Contents

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