Abstract
Abstract. The stable sedimentary nitrogen isotope compositions of bulk organic matter (δ15Nbulk) and the 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 the coast of Peru, where, at present, an increase in subsurface δ15NO3- from north to south along the shelf is observed due to ongoing denitrification within the poleward-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 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 direct comparison of δ30SiBSi and δ15Nbulk signatures at a latitudinal resolution for different time periods. Our results show that, during the Current Warm Period (CWP, since 1800 CE) and prior short-term arid events, source water δ15NO3- compositions have been close to modern values, increasing southward from 7 to 10 ‰ (between 11 and 15∘ S). In contrast, during the Little Ice Age (LIA) we calculate low δ15NO3- values between 6 ‰ and 7.5 ‰. Furthermore, the direct δ30SiBSi versus δ15Nbulk comparison also enables us 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. Accordingly, we estimate a shift from a 1:1 (or 1:2) ratio during the CWP and a 2:1 (up to 15:1) ratio during the LIA, associated with a shift from overall high nutrient utilization to NO3--dominated (and thus non-siliceous phytoplankton) utilization.
Highlights
Investigations of the isotopic compositions of the macronutrients, such as silicic acid (Si(OH)4) and nitrate (NO−3 ), have been used to infer changes of biogeochemical cycles in the past (Brunelle et al, 2007; Horn et al, 2011; Robinson et al, 2014)
Based on a compilation of new and previously published δ30SiBSi and δ15Nbulk records of several short sediment cores from the southern Peruvian shelf (11–15◦ S), we present a new evaluation of the impact of denitrification on the isotopic source signature of NO−3 and its subsequent utilization
As denitrification increases southward along the shelf today, we applied a latitudinal comparison between δ30SiBSi and δ15Nbulk signatures in modern surface and latest Holocene sediments
Summary
Investigations of the isotopic compositions of the macronutrients, such as silicic acid (Si(OH)4) and nitrate (NO−3 ), have been used to infer changes of biogeochemical cycles in the past (Brunelle et al, 2007; Horn et al, 2011; Robinson et al, 2014). K. Doering et al.: Latitudinal variations in δ30Si and δ15N signatures nutrient pools (Altabet et al, 1991; De La Rocha et al, 1997; Wada and Hattori, 1978). Doering et al.: Latitudinal variations in δ30Si and δ15N signatures nutrient pools (Altabet et al, 1991; De La Rocha et al, 1997; Wada and Hattori, 1978) This preferential incorporation is associated with an approximate enrichment factor of −5 ‰ for NO−3 (Waser et al, 1998) and −1.1 ‰ for Si(OH) (De La Rocha et al, 1997), which agree well with estimates for the Peruvian shelf (Ehlert et al, 2012; Mollier-Vogel et al., 2012; Grasse et al, 2016). While a potential fractionation of δ30Si signatures of biogenic opal during dissolution of
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