Synchronous Changes of Nitrogen and Carbon Isotopic Ratios and Nannoplankton Assemblage in Marine Sediments off Peru at 250 ka: A Role of Phytoplankton in Primary Ocean Productivity

Hidenao Hasegawa, Itsuro Kita, Shingo Tsukamoto, Shun Chiyonobu, Yoshihiro Kuwahara

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Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, Fukuoka, Japan.

DOI: 10.4236/ojg.2013.32015                

Abstract

We report the mechanism controlling changes of δ¹⁵N_org and δ¹³C_org 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 δ¹⁵N_org 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 δ¹³C_org values change in a range from ?23.5‰ to ?20.1‰ in an inverse correlation with δ¹⁵N_org values, describing an increase of average values at 250 ka as well as those of δ¹⁵N_org 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 δ¹⁵N_org and δ¹³C_org 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.

Keywords

Nannoplankton Assemblage; Nitrogen and Carbon Isotopes; Organic Matter; Biological Productivity; Se-diment off Peru

Conflicts of Interest

The authors declare no conflicts of interest.

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