A comparison between multi-proxy and historical data of drift-ice conditions on the East Greenland shelf

The reduction in sea ice along the SE Greenland coast during the last century has severely impacted ice-rafting to this area. In order to reconstruct ice-rafting and oceanographic conditions in the area of Denmark Strait during the last ~150 years, we conducted a multiproxy study on three short (20 cm) sediment cores from outer Kangerdlugssuaq Trough (~300 m water depth). The proxy-based data obtained have been compared with historical and instrumental data to gain a better understanding of the ice sheet-ocean interactions in the area. A robust chronology has been developed based on 210Pb and 137Cs measurements on core PO175GKC#9 (~66.2°N, 32°W) and expanded to the two adjacent cores based on correlations between calcite weight percent records. Our proxy records include sea-ice and phytoplankton biomarkers, and a variety of mineralogical determinations based on the <2 mm sediment fraction, including identification with quantitative x-ray diffraction, ice-rafted debris counts on the 63-150 µm sand fraction, and source identifications based on the composition of Fe oxides in the 45-250 µm fraction. A multivariate statistical analysis indicated significant correlations between our proxy records and historical data, especially with the mean annual temperature data from Stykkishólmur (Iceland) and the storis index (historical observations of sea-ice export via the East Greenland Current). In particular, the biological proxies (calcite weight percent, IP25, and total organic carbon %) showed significant linkage with the storis index. Our records show two distinct intervals in the recent history of the SE Greenland coast. The first of these (ad 1850-1910) shows predominantly perennial sea-ice conditions in the area, while the second (ad 1910-1990) shows more seasonally open water conditions.

Pore water isotopic ratios of CO2 and SO4, and sediment CH4 content of ODP Leg 164 sites

A unique set of geochemical pore-water data, characterizing the sulfate reduction and uppermost methanogenic zones, has been collected at the Blake Ridge (offshore southeastern North America) from Ocean Drilling Program (ODP) Leg 164 cores and piston cores. The d13C values of dissolved CO2 (sum CO2) are as 13C-depleted as -37.7 per mil PDB (Site 995) at the sulfate-methane interface, reflecting a substantial contribution of isotopically light carbon from methane. Although the geochemical system is complex and difficult to fully quantify, we use two methods to constrain and illustrate the intensity of anaerobic methane oxidation in Blake Ridge sediments. An estimate using a two-component mixing model suggests that ~24% of the carbon residing in the sum CO2 pool is derived from biogenic methane. Independent diagenetic modeling of a methane concentration profile (Site 995) indicates that peak methane oxidation rates approach 0.005 µmol/cm**3/yr, and that anaerobic methane oxidation is responsible for consuming ~35% of the total sulfate flux into the sediments. Thus, anaerobic methane oxidation is a significant biogeochemical sink for sulfate, and must affect interstitial sulfate concentrations and sulfate gradients. Such high proportions of sulfate depletion because of anaerobic methane oxidation are largely undocumented in continental rise sediments with overlying oxic bottom waters. We infer that the additional amount of sulfate depleted through anaerobic methane oxidation, fueled by methane flux from below, causes steeper sulfate gradients above methane-rich sediments. Similar pore water chemistries should occur at other methane-rich, continental-rise settings associated with gas hydrates.

(Table 1) Effects of experimental warming on plant cover and diversity indices in an evergreen shrub at Alexandra Fiord

1. Identifying plant communities that are resistant to climate change will be critical for developing accurate, wide-scale vegetation change predictions. Most northern plant communities, especially tundra, have shown strong responses to experimental and observed warming. 2. Experimental warming is a key tool for understanding vegetation responses to climate change. We used open-top chambers to passively warm an evergreen-shrub heath by 1.0-1.3 °C for 15 years at Alexandra Fiord, Nunavut, Canada (79 °N). In 1996, 2000 and 2007, we measured height, plant composition and abundance with a point-intercept method. 3. Experimental warming did not strongly affect vascular plant cover, canopy height or species diversity, but it did increase bryophyte cover by 6.3% and decrease lichen cover by 3.5%. Temporal changes in plant cover were more frequent and of greater magnitude than changes due to experimental warming. 4. Synthesis. This evergreen-shrub heath continues to exhibit community-level resistance to long-term experimental warming, in contrast to most Arctic plant communities. Our findings support the view that only substantial climatic changes will alter unproductive ecosystems.