Carbon geochemistry of plankton-dominated samples in the Laptev and East Siberian shelves: contrasts in suspended particle composition

Jorien E Vonk,Igor Semiletov,Christoph Humborg,Emma Karlsson,Elena Panova,Martin Kruså,Joan A Salvado,Lisa Bröder,Christof Pearce,Marc C Geibel,Örjan Gustafsson,Tommaso Tesi

doi:10.5194/os-13-735-2017

Abstract

Abstract. Recent Arctic studies suggest that sea ice decline and permafrost thawing will affect phytoplankton dynamics and stimulate heterotrophic communities. However, in what way the plankton composition will change as the warming proceeds remains elusive. Here we investigate the chemical signature of the plankton-dominated fraction of particulate organic matter (POM) collected along the Siberian Shelf. POM (> 10 µm) samples were analysed using molecular biomarkers (CuO oxidation and IP25) and dual-carbon isotopes (δ13C and Δ14C). In addition, surface water chemical properties were integrated with the POM (> 10 µm) dataset to understand the link between plankton composition and environmental conditions. δ13C and Δ14C exhibited a large variability in the POM (> 10 µm) distribution while the content of terrestrial biomarkers in the POM was negligible. In the Laptev Sea (LS), δ13C and Δ14C of POM (> 10 µm) suggested a heterotrophic environment in which dissolved organic carbon (DOC) from the Lena River was the primary source of metabolisable carbon. Within the Lena plume, terrestrial DOC probably became part of the food web via bacteria uptake and subsequently transferred to relatively other heterotrophic communities (e.g. dinoflagellates). Moving eastwards toward the sea-ice-dominated East Siberian Sea (ESS), the system became progressively more autotrophic. Comparison between δ13C of POM (> 10 µm) samples and CO2aq concentrations revealed that the carbon isotope fractionation increased moving towards the easternmost and most productive stations. In a warming scenario characterised by enhanced terrestrial DOC release (thawing permafrost) and progressive sea ice decline, heterotrophic conditions might persist in the LS while the nutrient-rich Pacific inflow will likely stimulate greater primary productivity in the ESS. The contrasting trophic conditions will result in a sharp gradient in δ13C between the LS and ESS, similar to what is documented in our semi-synoptic study.

Highlights

The progressive reduction of sea ice extent in the Arctic Ocean is indisputable evidence of modern global warming (Comiso et al, 2008; Ding et al, 2017; Kwok and Rothrock, 2009)
The land-derived material that does not settle in the coastal zone further travels across the continental margin reaching out to the outer-shelf region resuspended within the benwww.ocean-sci.net/13/735/2017/
Analyses of large-volume filtrations of plankton-dominated > 10 μm particle samples revealed a high degree of heterogeneity in the dual-carbon isotope signature (δ13C and 14C) between ice-free waters (Laptev Sea) and the ice-covered region (East Siberian Sea)

Summary

Introduction

The progressive reduction of sea ice extent in the Arctic Ocean is indisputable evidence of modern global warming (Comiso et al, 2008; Ding et al, 2017; Kwok and Rothrock, 2009). This study seeks a better understanding of the chemical composition of plankton that dominates regions of the Arctic Ocean characterised by different sea ice coverages, nutrient availability and riverine influence. The motivation behind investigating the chemical fingerprint of plankton from different regimes is to provide a better understanding of the carbon signature for direct applications to carbon studies of both modern systems and palaeo-reconstructions. Dual-carbon isotope mixing models (δ13C and 14C) are commonly used to quantify the relative proportion of marine and various allochthonous sources (e.g. permafrost soil) in both contemporary and palaeo-reconstructed carbon cycling of the Arctic (Karlsson et al, 2016; Tesi et al, 2016; Vonk et al, 2012, 2014)

Methods

Results

Conclusion