TITLE:
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
AUTHORS:
Hidenao Hasegawa, Itsuro Kita, Shingo Tsukamoto, Shun Chiyonobu, Yoshihiro Kuwahara
KEYWORDS:
Nannoplankton Assemblage; Nitrogen and Carbon Isotopes; Organic Matter; Biological Productivity; Se-diment off Peru
JOURNAL NAME:
Open Journal of Geology,
Vol.3 No.2,
April
29,
2013
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.