Impact of freshwater runoff from the southwest Greenland Ice Sheet on fjord productivity since the late 19th century

Climate warming and the resulting acceleration of freshwater discharge from the Greenland Ice Sheet are impacting Arctic marine coastal ecosystems, with implications for their biological productivity. To accurately project the future of coastal ecosystems, and place recent trends into perspective, paleo-records are essential. Here, we present late 19th century to present runoff estimates for a large sub-Arctic fjord system (Nuup Kangerlua, southwest Greenland) influenced by both marine- and land-terminating glaciers. We followed a multiproxy approach to reconstruct spatial and temporal trends in primary production from four sediment cores, including diatom fluxes and assemblage composition changes, biogeochemical and sedimentological proxies (total organic carbon, nitrogen, C / N-ratio, biogenic silica, δ13C, δ15N, grain size distribution). We show that an abrupt increase in freshwater runoff in the mid-1990’s is reflected by a 3-fold increase in biogenic silica fluxes in the glacier-proximal area of the fjord. In addition to increased productivity, freshwater runoff modulates the diatom assemblages and drives the dynamics and magnitude of the diatom spring bloom. Our records indicate that marine productivity is higher today than it has been at any point since the late 19th century and suggest that increased mass loss of the Greenland Ice Sheet is likely to continue promoting high productivity levels at sites proximal to marine-terminating glaciers. We highlight the importance of paleo-records in offering a unique temporal perspective on ice-ocean-ecosystem responses to climate forcing beyond existing remote sensing or monitoring time-series.

Metagenomics of Antarctic Marine Sediment Reveals Potential for Diverse Chemolithoautotrophy

The impacts of climate change in polar regions, like Antarctica, have the potential to alter numerous ecosystems and biogeochemical cycles. Increasing temperature and freshwater runoff from melting ice can have profound impacts on the cycling of organic and inorganic nutrients between the pelagic and benthic ecosystems.

Seasonal components of freshwater runoff in Glacier Bay, Alaska: diverse spatial patterns and temporal change

A high spatial resolution (250 m), distributed snow evolution and ablation model, SnowModel, is used to estimate current and future scenario freshwater runoff into Glacier Bay, Alaska, a fjord estuary that makes up part of Glacier Bay National Park and Preserve. The watersheds of Glacier Bay contain significant glacier cover (tidewater and land-terminating) and strong spatial gradients in topography, land cover, and precipitation. The physical complexity and variability of the region produce a variety of hydrological regimes, including rainfall-, snowmelt-, and ice-melt-dominated responses. The purpose of this study is to characterize the recent historical components of freshwater runoff to Glacier Bay and quantify the potential hydrological changes that accompany the worst-case climate scenario during the final decades of the 21st century. The historical (1979–2015) mean annual runoff into Glacier Bay is found to be 24.5 km3 yr−1, or equivalent to a specific runoff of 3.1 m yr−1, with a peak in July, due to the overall dominance of snowmelt processes that are largely supplemented by ice melt. Future scenarios (2070–2099) of climate and glacier cover are used to estimate changes in the hydrologic response of Glacier Bay. Under the representative concentration pathway (RCP) 8.5, the mean of five climate models produces a mean annual runoff of 27.5 km3 yr−1, 3.5 m yr−1, representing a 13 % increase from historical conditions. When spatially aggregated over the entire bay region, the projection scenario seasonal hydrograph is flatter, with weaker summer flows and higher winter flows. The peak flows shift to late summer and early fall, and rain runoff becomes the dominant overall process. The timing and magnitudes of modeled historical runoff are supported by a freshwater content analysis from a 24-year oceanographic conductivity–temperature–depth (CTD) dataset from the U.S. National Park Service's Southeast Alaska Inventory and Monitoring Network (SEAN). The hydrographs of individual watersheds display a diversity of changes between the historical period and projection scenario simulations, depending upon total glacier coverage, elevation distribution, landscape characteristics, and seasonal changes to the freezing line altitude.

Residual Transport of Suspended Material by Tidal Straining near Sloping Topography

Tidal straining is known to have an important impact on the generation of residual currents and the transport of suspended material in estuaries and the coastal ocean. Essential for this process is an externally imposed horizontal density gradient, typically resulting from either freshwater runoff or differential heating. Here, it is shown that near sloping topography, tidal straining may effectively transport suspended material across isobaths even if freshwater runoff and differential heating do not play a significant role. A combined theoretical and idealized modeling approach is used to illustrate the basic mechanisms and implications of this new process. The main finding of this study is that, for a wide range of conditions, suspended material is transported upslope by a pumping mechanism that is in many respects similar to classical tidal pumping. Downslope transport may also occur, however, only for the special cases of slowly sinking material in the vicinity of slopes with a slope angle larger than a critical threshold. The effective residual velocity at which suspended material is transported across isobaths is a significant fraction of the tidal velocity amplitude (up to 40% in some cases), suggesting that suspended material may be transported over large distances during a single tidal cycle.