Sampling Strategy | Table of Contents


Alternate sites are listed as discussed below, not necessarily in priority order except for Site PRB-1A, which remains the top priority alternate site.

Proposed SitePrevious SiteLatitudeLongitudeWater Depth (m)
PRB-1A DSDP Site 320 9°00.40'S83°31.80'W4487
ODP Site 67911°03.76'S78°16.25'W450
ODP Site 68211°15.99'S79°03.73'W3789
ODP Site 68613°28.81'S76°53.49'W446
ODP Site 68712°51.78'S76°59.43'W307
ODP Site 68811°32.28'S78°56.65'W3826
ODP Site 683 9°01.69'S80°24.40'W3072
ODP Site 845 9°34.95'N94°35.45'W3704
ODP Site 849 0°10.98'N110°31.18'W3839
ODP Site 850 1°17.83'N110°31.29'W3786
ODP Site 844 7°55.28'N90°28.85'W3414
ODP Site 847 0°11.59'N95°19.23'W3334
ODP Site 848 2°59.63'S110°28.79'W3853
ODP Site 852 5°17.55'N110°04.58'W3861
ODP Site 853 7°12.66'N109°45.08'W3715

Because time constraints appear to limit our operations strategy to a less ambitious schedule than outlined in the proposal for this expedition, we have relegated one of the original primary sites to alternate status. However, recognizing that unforeseen circumstances may require us to alter our operations schedule once we are at sea, we have included several other alternate sites in our planning as well. In each case, as with our primary sites, these alternate sites are all previously occupied ODP sites. Since we envision the opportunity to occupy these sites as being vanishingly small, we have not accorded them new prospectus site numbers, but refer instead to the original ODP nomenclature. None of these alternate sites are intended to be occupied in lieu of primary sites, but only in case of unexpected circumstances; whereby, the primary scientific objectives outlined in this prospectus cannot be achieved at our primary sites, or in the unlikely event we find ourselves with additional time at the conclusion of operations at our primary sites. A brief summary of the scientific objectives that might be addressed at each of these sites follows. Depth of penetration requested for all sites is the same depth as approved for previous expedition.

Site PRB-1A (DSDP Site 320)
Coring at proposed Site PRB-1A will allow Leg 201 scientists to (1) determine conditions under which subsurface microbes may be active, inactive but capable of resuscitation, or dead; (2) begin to assess the specialized metabolic strategies, if any, that are required to survive in deeply buried marine sediments; and (3) explore phylogenetic affinities and differences between microbes of the organic-poor Peru Basin sediments and nearby organic-rich Peru Margin sites.

ODP Site 679
ODP Site 679 has a very deeply buried anaerobic-methanotrophy zone (~170 mbsf) and would consequently provide an interesting record for comparing to the primary proposed sites. For example, it would allow us to test whether or not there is any significant difference between the microbial communities in relatively high-activity methanogenic zones that lie ~20 mbsf, as at our primary sites, and the microbial community in the much lower-activity methanogenic zone of Site 679. Heat flow data suggest that there may be flow in the ~250-mbsf interval at this site, but pore water chemistry data is not available in that depth interval. If there is flow at depth, sulfate-based methanotrophy along the flow boundary could be what is keeping the anaerobic-methanotrophy zone deep at this site (because less methane can diffuse upward if it's being oxidized at a deep flow boundary).

ODP Sites 682, 686, and 687
These sites appear to be fairly standard continental-margin ODP sites (e.g., they have abundant methane and no sulfate beginning a little below seafloor). Because they do not contain the intervals of subsurface flow that we targeted as a coring objective, recoring the positions of one or more of these sites could provide a control site for comparison to Sites PRU-1A, PRU-2A, PRU-3A, and PRU-4A.

ODP Site 688
This site would be interesting for recoring because it is probably about as high in microbial activity as we are likely to see on an ODP cruise (its sediments contain 2% to 9% organic carbon and very high levels of dissolved ammonium down to 500 mbsf). However, this site does not optimally fulfill our flow and hydrate-focused objectives, and the anaerobic-methanotrophy zone is very near to the sediment surface.

ODP Site 683
Cores recovered from ODP Site 683 (and Site 682) might contain hydrates (as inferred by the shipboard Scientific Party from ODP Leg 112). However, no hydrates were recovered from any cores at these sites and pore water chemical evidence for hydrates at these sites is weak (for example, chloride varies smoothly downhole instead of the erratic behavior commonly observed in sediments recovered from hydrate-bearing sites).

ODP Site 845—Potential Alternate for Site EQP-2A
This site exhibits clear pore water geochemical evidence of basement flow. Although pore water Sr concentrations indicate less carbonate diagenesis at this site than at ODP Site 851 (proposed Site EQP-2A), the geochemical records of the two sites otherwise resemble each other closely.

Nonideal Alternates for Site EQP-1A
ODP Sites 849 and 850 are nonideal alternates for Site EQP-1A (ODP Site 846). Although both Site 849 and Site 850 contain co-occurring methane and sulfate and geochemical evidence of basement hydrologic flow, their downhole methane concentrations are a factor of two to three lower than at Site 846 (Site EQP-1A). Pore water communication between their upper sediment columns and lowermost sediment columns is more greatly hindered by chert than at Site 846.

ODP Site 849
Methane and high sulfate co-occur here (methane approaches 50 µL/L). Pore water profiles indicate basement flow. A pronounced chert layer at 237 mbsf greatly hinders pore water geochemical (and microbiological?) communication between the shallower sediments in diffusional contact with the sediment/water interface and the deeper sediments in diffusional contact with the basement.

ODP Site 850
Methane and high sulfate co-occur here also (methane approaches 40 µL/L). Pore water profiles indicate basement flow. Three chert layers in the Site 850 interval from Core 138-850C-36X to 41X (~360 to 390 mbsf) limit pore water geochemical communication between the shallower sediments in diffusional contact with the sediment/water interface and the deeper sediments in diffusional contact with the basement.

Other Equatorial Pacific Sites
ODP Sites 844 and 847

Sites 844 and 847 are normal open-ocean sites. Their sulfate concentrations decline 1 to 2 mmol downhole and they contain no geochemical evidence of subsurface flow. They could provide appropriate control sites for comparison to Sites EQP-1A and EQP-2A.

ODP Sites 848, 852, and 853
ODP Sites 848, 852, and 853 are open-ocean sites with very low accumulation rates. All exhibit little or no downhole change in mean sulfate concentrations. They might provide an equatorial Pacific counterpart to the low-activity Peru Basin Sites PRB-1A and PRB-2A. ODP Site 848 is a fairly normal open-ocean site, with only 100 m of sediment overlying basement and very little sulfate reduction (sulfate concentrations vary slightly but do not decline significantly downhole). At ODP Site 852, only ~125 m of sediment overlies basement. Leg 138 scientists inferred the Site 852 sediments to be oxidized throughout, with early diagenesis limited to the top few meters of the sediment. Although sulfate concentrations do not decline significantly downcore, there is some pore water geochemical evidence of basement interaction at Site 852. ODP Site 853 is similar to Site 852, but with only ~70 m of sediment thickness and even less downhole variation in pore water geochemical composition.

Sampling Strategy | Table of Contents