Radiocarbon in dissolved organic and inorganic carbon of the Arctic Ocean

PUBLICATIONS Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL072138 Key Points: • DOC in the deep Eurasian Basin appears to contain at least 8% bomb C • Modern DOC appears to be selectively lost in the Beaufort Sea and slope water • A relationship between DOC Δ C and total hydrolysable amino acids in the Beaufort suggests that chemical composition controls DOC cycling Supporting Information: • Supporting Information S1 Correspondence to: E. R. M. Druffel, edruffel@uci.edu Citation: Druffel, E. R. M., S. Grifﬁn, C. S. Glynn, R. Benner, and B. D. Walker (2017), Radiocarbon in dissolved organic and inorganic carbon of the Arctic Ocean, Geophys. Res. Lett., 44, doi:10.1002/ 2016GL072138. Received 28 NOV 2016 Accepted 13 FEB 2017 Accepted article online 15 FEB 2017 Radiocarbon in dissolved organic and inorganic carbon of the Arctic Ocean E. R. M. Druffel 1 , S. Grifﬁn 1 , C. S. Glynn 1 , R. Benner 2 , and B. D. Walker 1 Department of Earth System Science, University of California, Irvine, California, USA, 2 Marine Science Program, University of South Carolina, Columbia, South Carolina, USA Abstract Dissolved organic carbon (DOC) in the ocean is thousands of 14 C years old, yet a portion of the DOC cycles on much shorter time scales (days to decades). We present 14 C measurements of DOC in the Arctic Ocean and estimate that ≥8% of the DOC in the deep Eurasian Basin contains bomb 14 C. While this is a limited data set, there appears to be selective loss of modern DOC in the surface and halocline waters of the open Beaufort Sea versus the Beaufort slope. At one of the Beaufort Sea stations, there is a linear relationship between DOC Δ 14 C values and previously measured total hydrolysable amino acid concentrations as reported by Shen et al. (2012), indicating that deep DOC contains small amounts of bioavailable DOC. The 14 C data show that not all of the deep DOC is recalcitrant. 1. Introduction The Arctic Ocean has a large continental shelf region and the largest input of freshwater per unit volume of any ocean. Dissolved organic carbon (DOC) in surface waters of the Arctic is the highest of any ocean, due to the large ﬂux of terrigenous DOC from Arctic rivers [Amon et al., 2012; Benner et al., 2005; Hansell et al., 2004]. This ﬂux of DOC is expected to increase as the Earth continues to warm, caus- ing release of organic C from expansive Arctic permafrost watersheds [Anderson and Amon, 2015; Benner et al., 2004]. There are four main water masses in the Arctic Ocean: (1) polar surface water, (2) halocline water (30–200 m), (3) Atlantic derived water (200–800 m), and (4) deep Atlantic water. Polar surface water is cold and fresh, having been derived from Arctic rivers and sea ice melt. The halocline water is formed on the continental shelves from freezing and brine release and is laterally advected into the Arctic Basin [Aagaard et al., 1981]. Water from the Paciﬁc Ocean ﬂows into the Beaufort Sea between 75 and 100 m depth (Upper Halocline Water), while water from the Atlantic Ocean enters from the Fram Strait and Barents Sea (Lower Halocline Water). The Beaufort Sea (in the southern Canadian Basin) and Eurasian Basin are separated by the Lomonosov Ridge whose sill depth is about 1500 m, keeping the deep waters of the Canada Basin and Beaufort Sea relatively isolated [Ostlund et al., 1987] (Figure 1). Radiocarbon provides quantitative information on the time scale of DOC cycling in the global ocean. Though the existing data are sparse, the available 14 C ages of DOC reveal that the difference between the average, prebomb ( 1000 m depth) and slope (bottom depth from 100 to 1000 m) and the Eurasian Basin to evaluate the distribution and cycling of DOC in the Arctic Ocean. We estimate that ≥8% of the DOC in the Eurasian Basin is of postbomb origin. We ﬁnd that DOC concentrations and Δ 14 C values in the upper Beaufort Sea and slope are linearly correlated with salinity. Samples from one station in the Beaufort Sea has total hydrolysable amino acid measurements [Shen et al., 2012] that are strongly correlated with DOC Δ 14 C values, which indicates that deep DOC contains little bioavailable DOC. RADIOCARBON IN THE ARCTIC OCEAN