DRILLING STRATEGYDrilling Operations
Downhole chemical transitions are sharp in sediments that are rich in organic matter and microbial activity. Under such conditions, critical transitions in microbial activity and community composition are also expected to be sharp and difficult to identify, anaerobically sample, and geochemically and microbiologically analyze, particularly considering the requirement of rapid processing prior to equilibration of the cores with shipboard temperature resulting in degradation of their microbiological community and its environment. These difficulties will be further complicated by the rapid pace of core recovery that is typical of relatively shallow-water environments, such as the Peru Margin (at the site nearest to shore, a 10-m core will probably be recovered every 20 to 30 min). If a third hole is necessary at any site to sample a microbiologically critical transition interval (e.g., an anaerobic methanotrophy zone or a hydrate interface), we will drill a third hole to the shallow target depth (mbsf) of that critical interval.
In the first hole at each site, we will undertake paired microbiological and geochemical analyses. The sampling frequency will be dictated by recovery rate and core handling and processing time. The resultant data will be immediately used to (1) define broad downhole microbiological and geochemical trends (on 10-m scales), (2) geochemically focus on stratigraphic horizons of special interest, and (3) identify diagnostic sedimentological or physical properties of those horizons. The second (and possibly third) hole will be used for high-resolution microbiological, geochemical, and geophysical studies of special stratigraphic horizons identified in the previous holes.
Special Downhole Operations
Downhole temperature gradients will be measured with the APC temperature shoe when piston coring, and with the Davis-Villinger temperature probe when coring with the XCB assembly. Recent improvements to the ODP pressure coring system (PCS) cutting shoe will be tested in a series of operations currently set for three sites, allowing sufficient time between deployments to allow evaluation of tool performance. In the event the PCS performs well, we may chose to deploy it more often and at more than three sites. We also plan to deploy the APC methane tool. This device will be used to monitor the effects of gas loss in cores from the time the core is cut until it reaches the deck by recording temperature, pressure, and conductivity with sensors mounted in the APC piston. Testing of hydrate autoclave coring equipment (HYACE, HYACINTH) has been endorsed by the ODP advisory structure. As much as one day of operations may include HYACE/HYACINTH testing.
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