Precession Control on Sedimentary Burial of Organic Matter and Redox Sensitive Metals in the Tropical Indian Ocean

Abstract

The total organic carbon (TOC) content of oceanic sediments and the total concentrations of several biomarkers are commonly used as qualitative palaeoproductivity proxies. However, fluctuations of these proxies may also be influenced by the variability of secondary processes taking place in the water column and in surface sediments. Past redox conditions of bottom and sediment pore waters are important parameters that can be studied by measuring the concentrations of redox-sensitive trace metals in the authigenic phases of the sediment. Here, we present a 330 kyr record of organic carbon, total alkenone and authigenic trace metal (U, Cd and Mo) in order to better understand how primary productivity and secondary physico-chemical processes are linked to each other and could explain the TOC variability. Core MD900963 is a 54 m long piston core recovered in the eastern part of the tropical Indian Ocean near the Maldives platform (05~ 73~ 2450 m water depth). At this location the present day bottom water oxygen is about 130 lamol/1 and the primary productivity is about 160 gCm-Zyr -1 (Antoine et al., 1996). Core MD900963 is well dated by orbital tuning of a high resolution 51So record measured on planktonic foraminifera (Bassinot et al., 1994). The studied first 16 meters of the core span the last 330 kyr which translates to an average s e d i m e n t a t i o n ra te o f a b o u t 5 c m / k y r . Palaeoproductivity was estimated from independent biological indices based on statistical transfer functions using coccoliths or planctonic foraminifera distributions (Beaufort e t al., 1997; Cayre et al., 1998). These two records suggest that during a typical 22 kyr cycle the primary productivity varied by roughly a factor of two: between c. 200-300 gCm-2yr -1 during glacial stages (high 5180) and c. 100-150 gCm-2yr 1 warm interstadials (low 8180). The bulk sediment concentrations (in ppm) of U, Mo, Cd and Th were measured by isotope dilution ICP-MS following the methods of Zheng et al. (1998). During the course of our measurements we estimate the reproducibility to about 3 -5% based on replicate analyses of a sediment standard. Authigenic U concentrations were obtained by subtracting a terrigenous component based on the Th concentrations and an aragonitic component based on X-ray analyses of the sediment fine fraction (Haddad, 1994). In general both corrections are small, i.e. less than 10% of the total U concentrations. On the same samples we measured the concentrations of TOC (wt.%) and of C37 alkenones (Tot-C37alk in lag/g), which are synthesized by several species of coccolithophorids. Both proxies were measured by gas chromatographic techniques (see Verardo et al., 1990 for TOC and Sonzogni et al. , 1997 for alkenones). By replicating measurements on sediment standards we estimate that reproducibilities for TOC and Tot-C37alk are on the order of 5 and 15%, respectively. The TOC values are rather low and vary between 0.25 and 0.85% while the Tot-C37alk exhibit larger variations between 0.2 and 7 lag@ Both records are very well correlated and are dominated by a clear cyclicity on the order of 22 kyr, with high concentrations during glacial stages (high 8180) and low values during warm interstadials (low 8180). Authigenic U concentrations range between 0.5 and 5.5 ppm, those of Cd vary between 0.3 and 1.9 ppm and those of Mo between 0.2 and 1.2 ppm which are comparable to the range of values reported by Rosenthal et aL (1995) and Crusius et al. (1996). Authigenic metals concentrations are also characterized by large and systematic fluctuations with a clear 22 kyr cyclicity and are positively correlated with records of TOC and Tot-C37alk. The clearest correlation with biomarkers and the strongest 22 kyr cyclicity are observed for the U record. This is partly due to its rather high values (about 2.5 ppm on