LATE QUATERNARY CARBON AND OXYGEN ISOTOPES USING ORBULINA UNIVERSA, OCEAN DRILLING PROGRAM LEG 165 SITE 1002C, CARIACO BASIN, CARIBBEAN SEA

Ocean freshening, along with ocean acidification, has become a recent threat to the ocean ecosystems. With increasing loss of sea ice and runoff, freshwater lenses will become more prevalent, resulting in biological and chemical shifts in the ocean stratification. The Cariaco Basin is a well-known semi-enclosed basin (maximum depth of 1400 m) and is the second-largest anoxic marine body in the world. This basin is located on the northern continental shelf of Venezuela and is considered a structural, pull-apart depression basin with restricting bottom water circulation into the open ocean. Late Quaternary (Marine Isotope Stages 1-9) sediment from Ocean Drilling Program (ODP) Leg 165 Site 1002c, located on the saddle between two smaller sub-basins, consists of one formal lithologic unit and eight sub-units. The formal unit consists of primarily olive gray to greenish gray nannofossil silty clays with intervals of abundant clays, diatoms, and foraminifers. Site 1002c was selected because of (1) the presence of laminated sediments noted in the core, (2) high sedimentation rates (~350 m/m.y), and (3) its location in a climatically sensitive region of the Caribbean Sea.

Extensive stable isotope work has been done at Site 1002c with oxygen isotopes (δ¹⁸O), using both benthic and planktonic foraminifers. Studies of carbon isotopes (δ¹³C) obtained from planktonic foraminifers remain incomplete. The mixed layer dwelling planktonic foraminifer, Orbulina universa, has been selected for the stable isotope measurements in this study.

Fresh water runoff from continental sources is enriched in ¹⁶O. Previous studies of δ¹⁸O at ODP site 1002 indicate a strong local influence on the isotopic record when compared to global ice volume records of δ¹⁸O from outside the Cariaco Basin. By utilizing the δ¹³C record of Orbulina universa, an understanding of local productivity in response to cycles of localized increased ocean freshening within the basin from river runoff can be made. It is expected that the cycles of freshwater fluctuations will be detected in the δ¹³C and δ¹⁸O records at ODP site 1002c. This will provide an understanding of the interactions between local biotic productivity and ocean basin freshening of wet and dry periods in the Cariaco Basin and be used as a modern analogue for future deep time studies.