Disruption of the sedimentary environment in Greenland fjords due to enhanced cryosphere melting caused by > 2oC climate warming

Widespread and fast melting of glaciers and ice sheets as a result of marked climate warming leads to a variety of possible hazards, both in proximal and (sea level) far-field regions. Past melting behavior of the Greenland Ice Sheet (GrIS)  has been strongly controlled by northern hemisphere insolation changes. In the early- and mid-Holocene relatively high insolation led to marked ice sheet retreat. The GrIS extent reached its minimum in NW Greenland  between  5 and 3 ka (ka = 1000 yrs before present) and in southern Greenland between 7 and 4 ka(1). During this regional ‘Holocene Thermal  Maximum’ (HTM) a more humid climate prevailed with summer  temperatures  3^o to 5^o C higher than in the mid-20^th century(2,3). Here we report sediment records from Greenland fjords indicative of drastically enhanced bottom current activity as well as local occurrence of massive silt deposition during above HTM periods. In Ameralik fjord near Nuuk a major sediment hiatus exists for the interval 6.8 to 4.4 ka(4), whereas nearby in this fjord massive deposition of silty melt water sediments subsequently occurred(5). In the outer part of Igaliku Fjord, South Greenland, sedimentation had stopped between approx. 7 and 3.7 ka(6). Nearby lake deposits display record-high accumulation rates of organic-rich sediment associated with a mild, stormy climate between 4.5 and 3.7 ka(7). On the shelf near Nuuk sediment geochemistry confirms significant melt water sediment transport from the adjacent mainland, markedly ceasing after 4 ka(8). Lacking of a hardground underlying the hiatus in the sediment core from Ameralik fjord points to erosion instead of long-term non-depositional conditions. Coastal deposits lack evidence of mid-holocene earthquake-induced tsunami activity, a potential trigger mechanism we thus may exclude. Instead we conclude that widespread glacier melting under a much warmer (> 2^o) climate must have led to repeated (sub)glacial  meltwater outburst surges producing high-energy, hyperpycnal  flow processes in the fjords. Sea level high-stand data from far-field regions around the Indian Ocean suggest most prominent melting episodes around 6 ka and near 4.3 ka(9).  Higher temperatures and increased precipitation rates in western Greenland presumably also favored widespread onshore permafrost thawing and consequently local destabilization of fjord slopes. Associated mud- and debris flow processes likewise have had a severe impact on the fjord sedimentary regime and benthic ecosystem. Based on above sedimentary records, we thus conclude that further ongoing global warming will have hazardous effects on the benthic environment of glacial fjords.

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