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

Doering, Kristin, Ehlert, Claudia, Martinez, Philippe, Frank, Martin and Schneider, Ralph (2019) Latitudinal variations in δ30Si and δ15N signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years. Open Access Biogeosciences (BG), 16 (10). pp. 2163-2180. DOI 10.5194/bg-16-2163-2019.

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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.

Document Type: Article
Keywords: nitrogen isotopes, silicon isotopes; denitrification; Current Warm Period; Little Ice Age
Research affiliation: OceanRep > SFB 754 > A6
OceanRep > SFB 754 > A5
OceanRep > SFB 754
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-P-OZ Paleo-Oceanography
Kiel University
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.5194/bg-16-2163-2019
ISSN: 1726-4170
Projects: SFB754
Date Deposited: 10 Apr 2018 06:32
Last Modified: 05 Jun 2019 13:23
URI: http://oceanrep.geomar.de/id/eprint/42651

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