Differences between mono-generic and mixed diatom silicon isotope compositions trace present and past nutrient utilisation off Peru

Abstract

In this study we combine for the first time silicon (Si) isotope compositions of small mixed diatom species (δ³⁰Si_bSiO2) and of large handpicked mono-generic (i.e. genus=Coscinodiscus) diatom samples (δ³⁰Si_Coscino) with diatom assemblages extracted from marine sediments in the Peruvian upwelling region in order to constrain present and past silicate utilisation. The extension of a previous core-top data set from the Peruvian shelf demonstrates that δ³⁰Si_Coscino values record near-complete Si utilisation, as these are similar to the isotopic composition of the subsurface source waters feeding the upwelling. In contrast, the δ³⁰Si_bSiO2 of small mixed diatom species increase southward along the shelf as well as towards the shore. We attribute highest δ³⁰Si_bSiO2 values partly to transient iron limitation but primarily to the gradual increase of Si isotope fractionation within the seasonal diatom succession, which are mainly recorded by small diatom species during intense bloom events. In contrast, lower δ³⁰Si_bSiO2 values are related to initial Si isotope utilisation during periods of weak upwelling, when low Si(OH)₄ concentrations do not permit intense blooms and small diatom species record substantially lower δ³⁰Si signatures. As such, we propose that the intensity of the upwelling can be deduced from the offset between δ³⁰Si_bSiO2 and δ³⁰Si_Coscino (δ³⁰Si_{coscino-bSiO2}), which is low for strong upwelling conditions and high for prevailing weak upwelling. We apply the information extracted from surface sediments to generate a record of the present-day main upwelling region covering the past 17,700years and find that this location has also been characterized by a persistent offset (δ³⁰Si_{coscino-bSiO2}). By comparison with the diatom assemblages we show that the coastal upwelling system changed markedly between weak and strong upwelling conditions. In addition, our model calculations to quantify species-specific Si isotope fractionation effects based on the diatom assemblages indicate an overall minor influence that cannot explain the high amplitude in the measured δ³⁰Si_bSiO2 record.

• author: Doering, Kristin ; Ehlert, Claudia ; Grasse, Patricia ; Crosta, Xavier ; Fleury, Sophie ; Frank, Martin and Schneider, Ralph
• publishing date: 2016-03-15
• type: Contribution to journal
• publication status: published
• subject: Oceanography, Hydrology, Water Resources
• keywords: silicon isotopes, Diatoms
• in: Geochimica et Cosmochimica Acta
• volume: 177
• pages: 18 pages
• publisher: Elsevier
• external identifiers: scopus:84957309957
• ISSN: 0016-7037
• DOI: 10.1016/j.gca.2015.12.029
• language: English
• LU publication?: no
• additional info: Funding Information: The data presented in this study were obtained from surface sediments from along the Peruvian shelf and upper slope between 0°N and 15°S ( Fig. 1 ; Table 1 ) and from latest Pleistocene sediments of piston core M77/2-003-2 (003-2) recovered from 271 m water depth at 15°06′S, 75°41′W ( Figs. 1 and 2 , Tables 1 and 2 ). Sediments were retrieved during the German R/V Meteor cruises M77/1 and M77/2 in 2008 in the framework of the Collaborative Research Centre (Sonderforschungsbereich/SFB) 754 funded by the German Science Foundation (DFG). Surface sediment samples were collected with a multicorer (MUC) and only the uppermost cm was used for sample preparation. Particulate organic carbon (C org ) and bSiO 2 concentrations of the surface sediments were published previously by Mollier-Vogel et al. (2012) and Ehlert et al. (2012) ( Supplementary Material, Table S1 and Fig. S1 ). Publisher Copyright: © 2016 Elsevier Ltd.
• id: d30aad06-b315-4526-8563-49442153bf10
• date added to LUP: 2022-09-05 12:11:09
• date last changed: 2022-09-15 21:53:24