HR: 1340h
AN: PP43B-1273    [Abstracts]
TI: Pleistocene Variations in Delivery and Deposition of Organic Matter Under the Benguela Current Upwelling System - Biomarker Isotopic Evidence From Sediment Light-Dark Color Cycles
AU: * Meyers, P A
EM: pameyers@umich.edu
AF: Marine Geology and Geochemistry Program, Department of Geological Sciences, The University of Michigan, Ann Arbor, MI 48109-1005, United States
AU: Bouloubassi, I
EM: ibou@ccr.jussieu.fr
AF: Laboratoire de Physique et Chimie Marines, Universite Pierre et Marie Curie, Paris, Cedex 05, France
AU: Pancost, R D
EM: r.d.pancost@bristol.ac.uk
AF: Organic Geochemistry Unit, School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom
AU: Robinson, R S
EM: rebeccar@gso.uri.edu
AF: Graduate School of Oceanography, University of Rhode Island, Narraganset, RI 02882, United States
AB: The light-dark color cycles that are distinctive features of sediment beneath the Benguela Current Upwelling System imply repetitive alternations in organic matter delivery and deposition. Organic geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in two sediment sequences from ODP Site 1084 corresponding to 0.7 and 1.1 Mya. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1 wt percent, and those of calcium carbonate fluctuate inversely between 68 and 1 percent, suggesting that carbonate dissolution is involved with the light-dark cycles. Bulk organic del 13C and del 15N values that remain constant across the two light-dark sediment intervals indicate that the extent of nutrient utilization did not change in each cycle. Biomarker compositions in both sequences reflect a range of organic matter sources. Abundant n-alkanes and n-alkanols with odd-over-even and even-over-odd distributions, respectively, record land-plant inputs. Other terrestrial biomakers (e.g triterpenoid acids and alcohols) are present but in very low abundances, suggesting that the n- alkyl components derive predominantly from eolian inputs. Carbon isotopic values of n-alkanes range from –25 to –28 permil, suggesting a mixture of C3 and C4 sources. In contrast, n-alkanol isotopic compositions range from –28 to –34 permil, suggesting that they derive solely from C3 plants. Algal biomarkers are abundant and diverse, represented by 1,15-C30 diols (eustigmatophytes), 4-desmethyl and –methylsterols (diatoms, dinoflagellates), and alkenones (haptophytes). These compounds all have del 13C values ranging from ca. –22 to –24 permil, consistent with a marine origin. Systematic differences in isotopic values imply that marine productivity at 1.1 Mya was higher than at 0.7 Mya, but alkenone-based sea-surface temperatures are higher at 1.1 Mya (21 deg) than at 0.7 Mya (15 deg), which indicates that changes in water-mass properties were also involved.
DE: 0428 Carbon cycling (4806)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 1055 Organic and biogenic geochemistry
DE: 4279 Upwelling and convergences (4964)
DE: 4850 Marine organic chemistry (0470, 1050)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting