Geochemical evidence for variations in delivery and deposition of sediment in Pleistocene light-dark color cycles under the Benguela Current upwelling system

Author(s): Robinson, Rebecca S.; Meyers, Philip A.; Murray, Richard W.
Author Affiliation(s): Primary:
University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States
Universite Bordeaux I, France
Boston University, United States
Volume Title: Neogene and Quaternary evolution of the Benguela coastal upwelling system
Volume Author(s): Christensen, Beth A., editor; Giraudeau, Jacques
Source: Marine Geology, 180(1-4), p.249-270; Evolution of major upwelling systems , American Geophysical Union 1999 fall meeting, San Francisco, CA, Dec. 13-17, 1999, edited by Beth A. Christensen and Jacques Giraudeau. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0025-3227 CODEN: MAGEA6
Note: In English. 69 refs.; illus., incl. 3 tables, sketch map
Summary: Distinctive light-dark color cycles in sediment beneath the Benguela Current Upwelling System indicate repetitive alternations in sediment delivery and deposition. Geochemical proxies for paleoproductivity and for depositional conditions were employed to investigate the paleoceanographic processes involved in creating these cycles in three mid-Pleistocene intervals from ODP Sites 1082 and 1084. Concentrations of total organic carbon (TOC) vary between 3.5 and 17.1%. Concentrations of CaCO3 vary inversely to TOC and Al, which suggests that both carbonate dissolution and terrigenous dilution contribute to the light-dark cycles. Opal concentrations are independent of both TOC and CaCO3, therefore eliminating diatom production and lateral transport of shelf material as causes of the light-dark cycles. δ13Corg and δ15Ntot values do not vary across light-dark sediment intervals, implying that the extent of relative nutrient utilization did not change. The stable δ15Ntot values represent a balanced change in nitrate supply and export production and therefore indicate that productivity was elevated during deposition of the TOC-rich layers. Parallel changes in concentrations of indicator trace elements and TOC imply that changes in organic matter delivery influenced geochemical processes on the seafloor by controlling consumption of pore water oxygen. Cu, Ni, and Zn are enriched in the darker sediment as a consequence of greater organic matter delivery. Redox-sensitive metals vary due to loss (Mn and Ba) or enrichment (Mo) under reducing conditions created by TOC oxidation. Organic matter delivery impacts subsequent geochemical changes such as carbonate dissolution, sulfate reduction and the concentration of metals. Thus, export production is considered ultimately responsible for the generation of the color cycles. Abstract Copyright (2002) Elsevier, B.V.
Year of Publication: 2002
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 24 Surficial Geology, Quaternary Geology; Africa; Aluminum; Atlantic Ocean; Benguela Current; C-13/C-12; Carbon; Carbonate sediments; Cenozoic; Color; Concentration; Continental margin sedimentation; Currents; Cycles; Diagenesis; Geochemistry; Isotope ratios; Isotopes; Leg 175; Marine sediments; Metals; N-15/N-14; Nitrogen; ODP Site 1082; ODP Site 1084; Ocean Drilling Program; Ocean circulation; Ocean currents; Optical properties; Organic carbon; Organic compounds; Paleo-oceanography; Paleocirculation; Paleocurrents; Paleoecology; Pleistocene; Productivity; Quaternary; Sedimentation; Sediments; South Atlantic; Southeast Atlantic; Southern Africa; Stable isotopes; Total organic carbon; Trace elements; Upwelling
Coordinates: S210539 S210539 E0114914 E0114914
S253049 S253049 E0130140 E0130140
Record ID: 2002026695
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands