E8 Transport of Mass and Matter

Topic

The cryosphere exports mass in several distinct ways simultaneously: meltwater flowing as runoff into rivers, icebergs calving and drifting across the ocean, sediments scoured and transported beneath the ice, and organic carbon and nutrients released from material previously trapped within the ice.

A key concept for understanding how a glacier exports water is "peak water": the discharge from a retreating glacier initially rises as melting intensifies—reaching a maximum known as peak water—after which the discharge irreversibly declines as the glacier's volume continues to shrink.

Calving is the process by which a block of ice breaks off from the front of a glacier or ice shelf and enters the ocean directly; for marine-terminating glaciers, calving can account for between fifty and ninety percent of the glacier's total ice discharge. The rate of calving depends on three factors: the ice thickness at the glacier front, the speed at which the ice flows toward that front, and the conditions of the receiving ocean, including wave action and water temperature.

Each calved iceberg transports fresh water over long distances across the ocean; as the iceberg melts far from its point of origin, it creates local anomalies in salinity, temperature, and nutrient levels in the surrounding water—anomalies that ultimately affect the ocean's biological productivity in that area. In addition to transporting water and icebergs, ice also carries sediment: it plucks material from the bedrock through abrasion and direct block detachment, transports it both within the ice itself and along its base, and finally deposits it as a layer of subglacial sediment or as moraines at the glacier's margins and terminus. The rate at which a glacier erodes the underlying bedrock often far exceeds the erosion caused by rivers or wind, making glaciers extraordinarily efficient agents of landscape shaping.

The sediments a glacier transports and deposits—along with the varves that accumulate layer upon layer in the proglacial lakes fed by that same glacier—serve as valuable paleoclimatic proxies; thus, the very process of mass transport traced here, from meltwater to sediment, ultimately leaves behind a record of past climates as well.

ID:2639