Biological and Climate Controls on North Atlantic Marine Carbon Dynamics Over the Last Millennium: Insights From an Absolutely Dated Shell‐Based Record From the North Icelandic Shelf

Abstract

Given the rapid increase in atmospheric carbon dioxide concentrations (pCO2) over the industrial era, there is a pressing need to construct long‐term records of natural carbon cycling prior to this perturbation and to develop a more robust understanding of the role the oceans play in the sequestration of atmospheric carbon. Here we reconstruct the past biological and climate controls on the carbon isotopic (δ13Cshell) composition of the North Icelandic shelf waters over the last millennium, derived from the shells of the long‐lived marine bivalve mollusk Arctica islandica. Variability in the annually resolved δ13Cshell record is dominated by multidecadal variability with a negative trend (−0.003 ± 0.002‰ yr−1) over the industrial era (1800–2000 Common Era). This trend is consistent with the marine Suess effect brought about by the sequestration of isotopically light carbon (δ13C of CO2) derived from the burning of fossil fuels. Comparison of the δ13Cshell record with Contemporaneous proxy archives, over the last millennium, and instrumental data over the twentieth century, highlights that both biological (primary production) and physical environmental factors, such as relative shifts in the proportion of Subpolar Mode Waters and Arctic Intermediate Waters entrained onto the North Icelandic shelf, atmospheric circulation patterns associated with the winter North Atlantic Oscillation, and sea surface temperature and salinity of the subpolar gyre, are the likely mechanisms that contribute to natural variations in seawater δ13C variability on the North Icelandic shelf. Contrasting δ13C fractionation processes associated with these biological and physical mechanisms likely cause the attenuated marine Suess effect signal at this locality.

Unexpected isotopic variability in biogenic aragonite: A user issue or proxy problem?

“…Analysis of the GoM_C isotopic measurements support the hypotheses that A. islandica shells growing in the same environment record consistent oxygen isotopic information in their carbonate structures (e.g., Mette et al, 2016) and that, between individuals, carbon isotope values are more variable at young ages (Butler et al, 2011;Reynolds et al, 2017). The range of temperatures derived from the  18 O values (13.52 to 15.97 °C) is larger than the range of temperatures in which the shells were grown (14.95 ±0.06 °C, last nine days).…”

“…Inter-shell  13 C variability for the GoM_C shells measured 0.65 ‰ (2, mean between splits; Table 2), compared to 2 analytical uncertainty of 0.17 ‰ for  13 C. The unexpected and relatively large variability in  13 C values between individuals, in this case, is hypothesized to arise from vital effects during early ontogeny (for example, see Butler et al, 2011 andReynolds et al, 2017). Beirne et al (2012) found that both juvenile and adult A. islandica clams incorporated ~10% of metabolic carbon into their shells, however, a positive relationship between juvenile (2-3 year old) growth rates and  13 C values was found during the spring bloom period (March-May; r 2 = 0.23; p < 0.012).…”

“…Beirne et al (2012) found that both juvenile and adult A. islandica clams incorporated ~10% of metabolic carbon into their shells, however, a positive relationship between juvenile (2-3 year old) growth rates and  13 C values was found during the spring bloom period (March-May; r 2 = 0.23; p < 0.012). Reynolds et al (2017) suggest younger individuals, like the ~3 year-old GoM_C shells, tend to record inconsistent  13 C values potentially due to variable incorporation of metabolic carbon induced by highly variable growth rates. Variability of inter-shell  18 O measurements (0.27 ‰), however, was within analytical precision for  18 O (0.30 ‰).…”

A multi-decadal record of oceanographic changes of the past ~165 years (1850-2015 AD) from Northwest of Iceland

“…Based on these considerations, we chose to not correct the planktic δ 13 C record for the Marine Suess effect as done elsewhere [e.g. 75].…”

The revolution of crossdating in marine palaeoecology and palaeoclimatology

“…For example, regionally crossdated bivalve series demonstrate highly synchronous Ba/Ca ratios in shell aragonite potentially related to productivity dynamics [39]. Stable carbon ( 13 C) isotope values [40] from exactly dated increments provide constraints on carbon cycling and the socalled Suess effect [41,42] through space and time. Moreover, radiocarbon measurements from exactly dated increments can be used to assess changes in circulation and provide tight constraints for the marine reservoir effect [10,25,37].…”