Introduction!The Arctic Ocean receives a disproportionate amount of global river runoff (~10%), which delivers 25 to 36 Tg C yr-1 of terrigenous dissolved organic carbon (tDOC) to the Arctic Basin (Raymond et al., 2007). This large riverine influence combined with wide continental shelves and a stable water column make the Arctic a desirable region to study the fate of tDOC within the marine system. Recent studies have observed significant removal of tDOC within the Beaufort gyre of the western Arctic system (Hansell et al., 2004; Cooper et al., 2005) contradicting the view that Arctic tDOC was largely refractory (Kohler et al., 2003; Amon and Meon, 2004). With recent field campaigns that have captured the spring freshet period of highest river flow, new lines of evidence have emerged showing a more dynamic tDOC pool in terms of composition (Spencer et al., 2008), lability (Holmes et al., 2008), and age (Raymond et al., 2007). The impact of these new insights must be incorporated into a new picture of the Arctic carbon cycle.

Data Collection Field sampling was carried out on the circumpolar expedition ARKXXIII/3 aboard the FS Polarstern during the summer of 2008. Surface water samples were collected using the underway seawater intake aboard the ship.

Dataset includes: Salinity DOC δ18O 228-,226-Ra

Acknowledgments!We thank C. Farmer and W. Chen for DOC analysis, the University of Miami Stable Isotope Laboratory for δ18O analysis, and M. Stephens for 228-,226-Ra analysis. Thanks to the crew and science party aboard FS Polarstern for inclusion and support of our water sampling program during expedition ARKXXIII/3. This work was supported by NSF grant OPP 0822429.

Results! Conclusions!1)  Eastern Arctic river runoff plume was located over Makarov Basin during the summer of 2008.

2) tDOC removal occurs in eastern Arctic system, largely over shelf residence time

>50% of tDOC input is removed within 2-5 years

3) Eastern Arctic tDOC decay constant, λ = 0.24 ± 0.07 yr-1

Likely captures rapid tDOC loss processes in estuaries and shelf seas

4) Western Arctic tDOC decay constant, λ = 0.06–0.097 yr-1

Likely captures slower tDOC decay over the open ocean

Robert T. Letscher1, Dennis A. Hansell1, David Kadko1 1Rosenstiel School of Marine & Atmospheric Science, University of Miami, Miami, FL 33149"

Literature cited!Amon, R. M. W., and B. Meon (2004), The biogeochemistry of dissolved organic matter and nutrients in two large Arctic

estuaries and potential implications for our understanding of the Arctic Ocean system, Mar. Chem., 92, 311–330. Cooper, L. W., R. Benner, J. W. McClelland, B. J. Peterson, R. M. Holmes,P. A. Raymond, D. A. Hansell, J. M.

Grebmeier, and L. A. Codispoti(2005), Linkages among runoff, dissolved organic carbon, and the stable oxygen isotope composition of seawater and other water mass indicators in the Arctic Ocean, J. Geophys. Res., 110, G02013, doi:10.1029/ 2005JG000031.

Ekwurzel, B., P. Schlosser, J. H. Swift, R. A. Mortlock, and R. G. Fairbanks (2001), River runoff, sea ice meltwater and Pacific water distribution and mean residence times in the Arctic Ocean, J. Geophys. Res., 106, 9075 – 9092, doi:10.1029/1999JC000024.

Hansell, D. A., D. Kadko, and N. R. Bates (2004), Degradation of terrige- nous dissolved organic carbon in the western Arctic Ocean, Science, 304, 858–861, doi:10.1126/science.1096175.

Hanzlick, D. and K. Aagaard (1980), Freshwater and Atlantic Water in the Kara Sea. Journal of GeophysicalResearch, 85, 4,937-4,942.

Holmes, R. M., J. W. McClelland, P. A. Raymond, B. B. Frazer, B. J. Peterson, and M. Stieglitz (2008), Liability of DOC transported by Alaskan rivers to the Arctic Ocean, Geophys. Res. Lett., 35, L03402, doi:10.1029/2007GL032837.

Kadko, D., Muench, R., 2005. Evaluation of shelf-basin interaction in the western Arctic by use of short-lived radium isotopes: the importance of mesoscale processes. Deep-Sea Research II 52, 3227–3244.

Karcher, M.J. and J.M. Oberhuber (2002), Pathways and modification of the upper and intermediate waters of the Arctic Ocean. Journal of Geophysical Research, 107, 3049-3061, doi: 10.1029/2000JC000530, 2002.

Ko ̈hler, H., Meon, B., Gordeev, V.V., Spitzy, A., Amon, R.M.W., 2003. Dissolved organic matter (DOM) in the estuaries of Ob and Yenisei and the adjacent Kara-Sea, Russia. In: Stein, R., Fahl, K., Fu ̈tterer, D.K., Galimov, E.M., Stepanets, O.V. (Eds.), Siberian river run-off in the Kara Sea. Proc. Mar. Sci. 6, 281–308.

Raymond, P. A., J. W. McClelland, R. M. Holmes, A. V. Zhulidov, K. Mull, B. J. Peterson, R. G. Striegl, G. R. Aiken, and T. Y. Gurtovaya (2007), Flux and age of dissolved organic carbon exported to the Arctic Ocean: A carbon isotopic study of the five largest arctic rivers, Global Biogeochem. Cycles, 21, GB4011, doi:10.1029/2007GB002934.

Schlosser, P., D. Bauch, R. Fairbanks, and G. Bo ̈nisch, Arctic river runoff: mean residence time on the shelves and in the halocline, Deep Sea Res., Part I, 41, 1053–1068, 1994.

Spencer, R. G. M., G. R. Aiken, K. P. Wickland, R. G. Striegl, and P. J. Hernes (2008), Seasonal and spatial variability in dissolved organic mat- ter quantity and composition from the Yukon River basin, Alaska, Global Biogeochem. Cycles, 22, GB4002, doi:10.1029/2008GB003231.

Figure 1. Sea surface salinity and circulation for the Arctic Ocean (data from WOA, 2005) along with location of major river deltas. Removal of tDOC delivered to the Beaufort gyre (BG), has been observed previously, while the fate of tDOC entering the transpolar drift (TPD) is examined in this study."

Figure 2. Station locations (black dots) along cruise track of FS Polarstern expedition ARKXXIII/3 shown in reference to sea ice cover (color) during September 2008."

Figure 3. Surface distribution of (a) DOC, (b) salinity, (c) river water fraction, (d) sea ice melt fraction. Note the high DOC, low salinity, high river water fraction, low sea ice melt fraction located offshore over the Makarov Basin (black arrows). "

For further information!Please contact rletscher@rsmas.miami.edu. More information about the work at our lab can be found at www.rsmas.miami.edu/groups/biogeochem/index.html.

Figure 7. Source of dissolved radium is in sediments such that waters overlying the shelf become enriched in 228Ra/226Ra. As shelf and open ocean waters mix, waters offshore can be “young” owing to rapid mixing (straight line) or “aged” (curved line)."

Distribution of dissolved organic carbon in the source waters of the transpolar drift:! Inferences on terrigenous DOC dynamics in the eastern Arctic Ocean"

Objective!In light of the new evidence for a more dynamic tDOC pool, we investigated the fate of tDOC in the eastern Arctic system in the region of the transpolar drift.

Eurasian shelf residence time 2.5 years Hanzlick and Aagaard, 1980 3.5 years Pavlov et al., 1993 3.5 ± 2 years Schlosser et al., 1994 2-5 years Ekwurzel et al., 2001 2-3 years Karcher and Oberhuber, 2002

Figure 4. DOC vs. Salinity plot showing mixing between 3 end members present in the polar surface layer (PSL): high DOC, low salinity river water; inter-"mediate DOC, high salinity marine water, and low DOC, low salinity sea ice melt water."

Data falling on conservative mixing line between river and marine end members coincide with the offshore stations in the eastern Arctic system, which were used for subsequent interpretations

Figure 5. (a) The case for net removal of DOC across an estuarine salinity gradient"(b) Net removal of DOC found for the eastern Arctic (this study) and western Arctic (Hansell et al., 2004) in relation to the conservative DOC-salinity mixing line. The apparent river water end member [DOC] is given by the y-intercept."

Figure 6. Schematic showing net removal of ~55% of tDOC input to eastern Arctic as river runoff transits the Eurasian shelves and enters the trans polar drift.!

What is the quantity of tDOC removal?

What is the timescale of tDOC removal in the eastern Arctic?

Figure 8. Plot of 228Ra/226Ra vs. salinity. Data fall largely between the “aged” line found previously for the western Arctic (Kadko & Muench, 2005) and conservative mixing line describing rapid mixing in eastern Arctic (this study).!

Rapid mixing between Eurasian shelf and offshore waters determined from radium model means that tDOC removal occurs during residence time over the shelf.

Figure 9. Estimates of river water residence time on Eurasian shelves. A removal of 393 ± 55 µM tDOC occurring over 3.5 ± 1.5 yr yields a tDOC decay constant of 0.24±0.07 yr-1 for the eastern Arctic. "

Figure 10. Arctic tDOC is likely comprised of differing fractions based on lability with consequently differing removal rates across the shelf, open ocean, and deep ocean environments.!

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