A carbon,nitrogen, and sulfur elemental and isotopic study in dated sediment cores from the Louisiana Shelf |
| |
| Authors: | Robert J Rosenbauer Peter W Swarzenski Carol Kendall William H Orem Frances D Hostettler Mark E Rollog |
| |
| Institution: | (1) U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, USA;(2) U.S. Geological Survey, 956 National Center, Reston, VA 20192, USA;(3) U.S. Geological Survey, 400 Natural Bridges Dr., Santa Cruz, CA 95065, USA |
| |
| Abstract: | Three sediment cores were collected off the Mississippi River delta on the Louisiana Shelf at sites that are variably influenced
by recurring, summer-time water-column hypoxia and fluvial loadings. The cores, with established chronology, were analyzed
for their respective carbon, nitrogen, and sulfur elemental and isotopic composition to examine variable organic matter inputs,
and to assess the sediment record for possible evidence of hypoxic events. Sediment from site MRJ03-3, which is located close
to the Mississippi Canyon and generally not influenced by summer-time hypoxia, is typical of marine sediment in that it contains
mostly marine algae and fine-grained material from the erosion of terrestrial C4 plants. Sediment from site MRJ03-2, located
closer to the mouth of the Mississippi River and at the periphery of the hypoxic zone (annual recurrence of summer-time hypoxia
>50%), is similar in composition to core MRJ03-3, but exhibits more isotopic and elemental variability down-core, suggesting
that this site is more directly influenced by river discharge. Site MRJ03-5 is located in an area of recurring hypoxia (annual
recurrence >75%), and is isotopically and elementally distinct from the other two cores. The carbon and nitrogen isotopic
composition of this core prior to 1960 is similar to average particulate organic matter from the lower Mississippi River,
and approaches the composition of C3 plants. This site likely receives a greater input of local terrestrial organic matter
to the sediment. After 1960 and to the present, a gradual shift to higher values of δ13C and δ15N and lower C:N ratios suggests that algal input to these shelf sediments increased as a result of increased productivity
and hypoxia. The values of C:S and δ34S reflect site-specific processes that may be influenced by the higher likelihood of recurring seasonal hypoxia. In particular,
the temporal variations in the C:S and δ34S down-core are likely caused by changes in the rate of sulfate reduction, and hence the degree of hypoxia in the overlying
water column. Based principally on the down-core C:N and C:S ratios and δ13C and δ34S profiles, sites MRJ03-3 and MRJ03-2 generally reflect more marine organic matter inputs, while site MRJ03-5 appears to be
more influenced by terrestrial deposition. |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
