Latitudinal variations in <i>δ</i><sup>30</sup>Si and <i>δ</i><sup>15</sup>N signatures along the Peruvian shelf: quantifying the effects of nutrient utilization versus denitrification over the past 600 years

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.

Controls on the Silicon Isotope Composition of Diatoms in the Peruvian Upwelling

“…Overall, we find that the commonly used fractionation models (Rayleigh-type vs. steady state) for the temporal evolution of δ 30 Si dSi are oversimplified and need to include other processes such as admixture with additional sources (vertical and horizontal nutrient supply, as well as multiple intrusion of nutrients) and should integrate dissolution processes in order to properly reconstruct dSi utilization in the present. This is especially important, when these models are used to reconstruct dSi utilization in the past (e.g., Doering et al, 2019). The deviation from the assumed fractionation factor of −1.1 in highly productive areas might have to be taken into account in regard to Si modeling studies.…”

“…The isotopic signal preserved in diatom frustules (δ 30 Si bSi_sed. ) in the underlying sediments can therefore serve as a proxy to reconstruct the utilization of dSi in surface waters in the past (e.g., De La Rocha et al, 1998;Ehlert et al, 2013Ehlert et al, , 2015Doering et al, 2019).…”

Toward a Solution of the “Peruvian Puzzle”: Pelagic Food-Web Structure and Trophic Interactions in the Northern Humboldt Current Upwelling System Off Peru

“…OMZs like the one in the northern HCS are sites of intense nitrogen loss, which leads to an enrichment of δ 15 N baseline values (Sigman et al, 1999;Graham et al, 2010). Denitrification within the OMZ causes isotopic fractionation where isotopically lighter nitrate (δ 14 N) is respired by bacteria leaving behind subsurface nitrate enriched in δ 15 N. This nitrate with higher δ 15 N values is upwelled to the surface, where it is taken up by phytoplankton and passed on through the food web to higher trophic levels (Mollier-Vogel et al, 2012;Doering et al, 2019). The intensified oxygen depletion from North to South in the northern HCS provides evidence for the strongest denitrification rates that have been previously reported from the south (Fuenzalida et al, 2009;Bertrand et al, 2010;Espinoza et al, 2017).…”