SECULAR CHANGES OF MARINE PRODUCTIVITY IN THE WESTERN CARIBBEAN SEA DURING THE LATE QUATERNARY AS INDICATED BY GEOCHEMICAL DATA OF ODP 999A SEDIMENTS

A correlation exists between the waxing and waning of higher-latitude continental glaciers and tropical marine productivity throughout the Quaternary, though this cyclic correspondence is at times affected by secular trends that result from local or regional re-arrangement of surface- or deep-water currents. Here, we report the presence of a secular trend affecting the Caribbean region for the past 180 kyr, which is observed in the geochemical data of sediment samples from ODP Site 999A. This site located on a bathymetric high in the western Colombia Basin.

The studied sedimentary sequence consists of calcareous muds, whose chronology is based on the oxygen isotopic composition of the planktonic foraminifer G. sacculifer. To unravel changes in marine conditions affecting marine productivity, we measured organic carbon (OC) and nitrogen (N) contents, and carbon and nitrogen isotope ratios of organic matter. Geochemical results indicate overall low abundances of OC (0.12-0.54 wt%), low mass-accumulation rates of OC (0.07-0.47 g/m²yr), and low OC/N ratios (8-12.3). The low OC/N ratios suggest a marine origin for the organic matter, which is supported by the carbon isotope ratios of OC that range from –24 to –20.2 per mil. An increasing trend in the carbon isotope ratios occurs during the last 180 kyr. A similar trend occurs in the nitrogen isotope record, with values increasing from +1.3 to +5.2 per mil. We interpret that this trend results from a greater utilization of dissolved nitrate by phytoplankton in the western Caribbean Sea. The increased presence of nitrate probably resulted in an increase in primary productivity and the consequent decrease in the carbon-isotope fractionation effect during photosynthesis, thereby producing the increasing trend of carbon isotope ratios of OC. Based on the nitrogen isotope signature, we hypothesize that the secular trend observed in the geochemical record of ODP 999A sediments was caused by an increased delivery of nutrients from northern South America via the Magdalena River.