PALEOCLIMATOLOGICAL ANALYSIS OF OCEANIC SEDIMENT FROM THE CHILEAN MARGIN ODP SITE 1234 TO DETERMINE PAST CLIMATIC CONDITIONS AND EVENTS

Ocean Drilling Program (ODP) Site 1234 (36°13.153´S, 73°40.902´W; 1015 m water depth), drilled shoreward of the Peru-Chile Trench ~65 km offshore of Chile, represents an ultra high-resolution record of terrigenous deposition with the potential to provide information about the erosion and transport of sediment from the continent to the Chilean margin at glacial/interglacial scales. The characterization of oceanic sediment, including the calculation of terrigenous mass accumulation rates (MAR), grain size analyses, and the determination of magnetic fabrics, can help provide clues to past climactic shifts and conditions on land, such as changes in rainfall and aridity, as well as in the ocean, including variations in depositional current velocities.

In this preliminary study, 150 paired samples of hemipelagic sediment were collected from the 239-mcd-thick composite core section of ODP Site 1234, which exhibited a fairly homogenous lithology of diatom nannofossil silty clay and clay. With a basal age of less than 0.26 Ma (Quaternary), the sampling interval provided an average resolution of ~1.8 ky/sample. One set of samples were subject to a five-step chemical extraction process to remove biogenically derived carbonate and silicate components, oxy-hydroxy coatings, and grains larger than 63µ, from which the terrigenous mass accumulation rates (MAR) were then calculated. Terrigenous MARs for this core, which can reflect erosion rates, show a fairly consistent terrigenous MAR value between 50 to 80 g/cm²/kyr, with no discernable trends as of yet.

In addition to the calculation of MARs, terrigenous grain size will be analyzed on these same extracted samples and compared to the grain alignment found in the paired samples. Grain aligment was determined by analyzing the magnetic fabric using anisotropy of magnetic susceptibility (AMS). As both these parameters (grain size and magnetic fabric) can be influenced by changes in the velocity of ocean currents and oceanic circulation patterns, they can be used, in combination, as additional climatic indicators. Preliminary magnetic fabric results indicate that the energy of the depositional environment at Site 1234 fluctuated through time, possibly substantively; completion of AMS and grain size analyses on all samples will help analyze for trends.