Abstract
We report the mechanism controlling changes of δ15Norg and δ13Corg values of marine organic matter, based on the change of calcareous nannoplankton assemblage during the last 500,000 years in core samples from ODP Hole 846B off Peru in the equatorial Pacific Ocean. The δ15Norg values fluctuate in a range from 2.1‰ to 6.7‰, giving an abrupt increase since about 250 ka with the averages of 3.8‰ and 5.0‰ during the older and younger periods, respectively. The δ13Corg values change in a range from ?23.5‰ to ?20.1‰ in an inverse correlation with δ15Norg values, describing an increase of average values at 250 ka as well as those of δ15Norg values. The total organic carbon content also shows averages of 0.6% and 0.3% during the younger and older periods, respectively. The numerical increase of deep dwelling species (Florisphaera profunda) of calcareous nannoplankton during the younger period in comparison with the older period indicates that these chemical and isotopic jumps synchronized with nannoplankton assemblage changes in marine sediments are caused by activity of deep dwellers in photic zone more stratified by an abrupt decline of trade wind strength on this sea area since about 250 ka. A study coupling δ15Norg and δ13Corg values and nannoplankton assemblage can be a useful method for evaluating the extent of stratification of photic zone and the roles of surface and deep dwellers of phytoplankton in producing primary organic matter.
Highlights
The stratification and its collapse of marine photic zone can be taken place by climatic change in strength of the wind on the sea giving the changes of nutrient content and temperature in it
Based on core samples from Indian Ocean, Beaufort et al [11] reported that the abundance of F. profunda in fossil assemblages can be used to monitor the depth of the nutricline for paleoproductivity study, because its relative abundance increases in stratified photic water where the upper photic zone is impoverished in nutrients and the nutricline is deep
The remains (10 30 mg) wrapped up in a tin foil were set into the elemental analyzer (ThermoQuest, NC2500) with Cr2O3 and silvered cobaltous-cobaltic oxide mixed with Ag (Co3O4/ Ag) and metallic cupper, at heated at 1000 ̊C and 650 ̊C, respectively, to measure their total organic carbon (TOC) and total nitrogen (TN)
Summary
The stratification and its collapse of marine photic zone can be taken place by climatic change in strength of the wind on the sea giving the changes of nutrient content and temperature in it. The structural change of photic zone acts the most important and basic role on biological productivity and marine chemistry. Calcareous nannoplankton can be both surface and deep dwelling species. Based on core samples from Indian Ocean, Beaufort et al [11] reported that the abundance of F. profunda in fossil assemblages can be used to monitor the depth of the nutricline for paleoproductivity study, because its relative abundance increases in stratified photic water where the upper photic zone is impoverished in nutrients and the nutricline is deep.
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