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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 09 Apr 2018

Research article | 09 Apr 2018

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal Biogeosciences (BG).

Latitudinal variations of δ30Si and δ15N signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years

Kristin Doering1, Claudia Ehlert2, Philippe Martinez3, Martin Frank1, and Ralph Schneider4 Kristin Doering et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, 24148, Germany
  • 2Max Planck Research Group – Marine Isotope Geochemistry, Carl von Ossietzky University, 26129 Oldenburg, Germany
  • 3EPOC, UMR CNRS 5805, Université de Bordeaux 1, 33615 Pessac Cedex, France
  • 4Institute of Geosciences, University of Kiel, Kiel, 24118, Germany

Abstract. The sedimentary stable nitrogen isotope compositions of bulk organic matter (δ15Nbulk) and silicon isotope composition of diatoms (δ30SiBSi) both mainly reflect the degree of past nutrient utilization by primary producers. However, in ocean areas where anoxic and suboxic conditions prevail, the δ15Nbulk signal ultimately recorded within the sediments is also influenced by water column denitrification causing an increase in the subsurface δ15N signature of dissolved nitrate (δ15NO3) upwelled to the surface. Such conditions are found in the oxygen minimum zone off Peru, where at present an increase in subsurface δ15NO3 from North to South along the shelf is observed due to ongoing denitrification within the pole-ward flowing subsurface waters, while the δ30Si signature of silicic acid (δ30Si(OH)4) at the same time remains unchanged.

Here, we present three new δ30SiBSi records between 11°S and 15°S and compare these to previously published δ30SiBSi and δ15Nbulk records from Peru covering the past 600 years. We present a new approach to calculate past subsurface δ15NO3 signatures based on the correlation of δ30SiBSi and δ15Nbulk signatures at a latitudinal resolution for different time periods. Our results show source water δ15NO3 compositions during the last 200 years, the Current Warm Period (CWP) and during short-term arid events prior to that, which are close to modern values increasing southward from 7 to 10‰ (between 11°S and 15°S). In contrast, humid conditions during the Little Ice Age (LIA) reflect consistently low δ15NO3 values between 6 and 7.5‰. Furthermore, we are able to relate the short-term variability in both isotope compositions to changes in the ratio of nutrients (NO3:Si(OH)4) taken up by different dominating phytoplankton groups (diatoms and non-siliceous phytoplankton) under the variable climatic conditions of the past 600 years.

Kristin Doering et al.
Interactive discussion
Status: final response (author comments only)
Status: final response (author comments only)
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Kristin Doering et al.
Kristin Doering et al.
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