Assessment of C, N and Si isotopes as tracers of past ocean nutrient and carbon cycling

Abstract

Review of oceanic distribution, controlling processes, and sedimentary archives of C, N, and Si isotopes • Late Quaternary C, N, and Si sedimentary isotope records demonstrate coupling between the biological pump and atmospheric pCO2 • Cenozoic C, N, and Si sedimentary isotope records indicate large-scale changes in nutrient sources, concentrations, and the carbon cycle ESSOAr |

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES‐Era Data

“…If considering only the euphotic zone, NEP is equivalent to export production, which is an important component of the biological carbon pump (Boyd et al., 2019); indeed, changes in export production have been implicated as a major driver of variations in glacial–interglacial p CO 2 and hence climate (e.g., Berger et al., 1989; Boyle, 1988b; Broecker, 1982; Paytan, 2009). Many of these changes in past export production were inferred on the basis of variations in the cycling of major nutrients and their isotopes (e.g., N, Si; Farmer et al., 2021). The net consumption of nutrients can be interpreted in terms of export production by assuming a fixed proportionality between the major nutrients and C. This proportionality provides the connection linking major nutrient cycles to the sequestration of atmospheric CO 2 via biological productivity.…”

“…Indeed, culture studies consistently showed that phytoplankton assimilated isotopically light Zn from their environment (e.g., John et al., 2007; Köbberich & Vance, 2017; Samanta et al., 2018). It was therefore thought that dissolved profiles of δ 66 Zn would exhibit profiles similar to those of the macronutrients (e.g., δ 13 C, δ 15 N, δ 30 Si; Farmer et al., 2021) and certain micronutrient metals (e.g., δ 114 Cd, δ 138 Ba; Sections 6 and 8, respectively), whereby Zn‐depleted surface waters exhibit isotopically heavy compositions relative to deeper waters. In contrast, the first Zn isotope profile for seawater, obtained by Bermin et al.…”

“…Sponges are primarily deep‐sea organisms. Hence, if the controls on diatom Zn:Si and δ 66 Zn opal can be understood, a combination of Zn:Si and δ 66 Zn measurements in diatoms and sponges (as used for Si isotopes; Farmer et al., 2021) could provide a vertically resolved view of the past ocean global Zn cycle, including the role of Southern Ocean processes in the biological carbon pump.…”

“…Taking a general view, the review of each element highlights that our knowledge of bioactive trace metal isotope systematics lags behind those of trace element abundances and far behind those of macronutrient isotopes (e.g., Farmer et al., 2021). The bioactive metal isotope field is nascent; excepting Mo, the very first isotope data for seawater for all of the elements reviewed here were published within the last 15 years, or—in the case of Ag—are yet to be reported.…”

“…For those bioactive metals where biological processes are dominant, the implication of Reactions (1) and (2) is that many of the elements listed in (3) may, in turn, be used as proxies of POC cycling and, potentially, paleoproductivity. Indeed, aspects of past ocean productivity that impact carbon and nutrient cycling are routinely reconstructed using the stable isotope compositions of carbon and macronutrient elements (N and Si; see Farmer et al., 2021). It thus follows that the abundance and isotopic compositions of bioactive trace metals cycled along with POC could also provide valuable information on past ocean productivity.…”