Glacial-marine sediments (GMS) are deposits of rock fragments and fine particles that originate from terrestrial ice sheets and glaciers but accumulate in the ocean. These sediments are transported to the marine environment by various means, primarily meltwater and floating ice. A defining feature of GMS is the presence of dropstones, which are isolated, oversized rock fragments embedded within the otherwise fine-grained layers of the deep seafloor.
How Glaciers Gather Sediment
The source material for glacial-marine sediments begins as bedrock and loose debris on land, which is scoured and collected by moving ice sheets. Glaciers primarily erode the underlying landscape through two distinct mechanical processes: abrasion and plucking. Abrasion occurs when rock fragments frozen into the base of the glacier act like sandpaper, grinding against the bedrock beneath as the ice flows forward. This continuous scraping action produces a vast quantity of very fine silt and clay-sized particles known as rock flour.
The second process, plucking, is responsible for incorporating larger, blocky material into the ice mass. Meltwater penetrates existing cracks and joints in the bedrock, and as this water refreezes, its expansion wedges the rock apart. The glacier’s movement then pulls or “plucks” these loosened chunks of rock, ranging from pebbles to large boulders, into the basal ice.
Delivery of Fine Sediments to the Ocean
Meltwater forms channels both on the glacier surface and deep beneath the ice, where it flows under high pressure along the bedrock. This subglacial meltwater becomes heavily laden with suspended sediment before it reaches the grounding line, the point where the glacier lifts off the seabed and begins to float.
Upon exiting at the grounding line, the sediment-rich meltwater is less dense than the surrounding seawater, causing it to rise as a plume. This subglacial meltwater plume carries the fine silt and clay particles high into the water column. Ocean currents then distribute this turbid water widely across the continental shelf and into the deeper ocean basin. As the plume loses energy and the particles slowly settle out of suspension, they form widespread layers of fine glacial-marine mud on the seafloor.
The Origin and Release of Dropstones
Dropstones are large, isolated clasts that originate from the coarse debris collected by the glacier through the process of plucking. These rocks are frozen within the glacial ice, often traveling great distances embedded in the moving mass. When the glacier reaches the sea, sections break off through a process called calving, forming icebergs that float away from the glacier terminus.
These icebergs act as floating rafts carrying a load of entrained sediment, a process known as ice rafting. As the icebergs drift across the ocean, they gradually melt, releasing the rock fragments they hold. These large clasts, which can range from a few centimeters to several meters in size, sink vertically through the water column. They land randomly on the seafloor, often far from the coast and surrounded by the fine mud deposited by meltwater plumes.
The deposition of a dropstone is a stochastic event, meaning it occurs at random locations and times as the iceberg melts. When the stone impacts the soft, underlying sediment, it often creates an impact depression and deforms the fine layers below it. This isolated positioning and the resulting disturbance of the surrounding sediment layer make dropstones a distinct and telltale marker of past glacial activity in marine deposits.
Characteristics of Glacial-Marine Sediment Layers
The resulting layer is poorly sorted because it contains a wide range of grain sizes, from the clay and silt deposited by meltwater plumes to the occasional large dropstone. This contrasts sharply with sediments deposited by standard ocean currents, which tend to be well-sorted.
The fine-grained portion of the sediment often exhibits distinct layering or lamination. This layering, or rhythmites, results from seasonal variations in meltwater discharge, where more sediment is delivered during the summer melt season. Intermixed with the terrigenous, or land-derived, material are biogenic sediments, such as the shells of marine microorganisms, which accumulate from the water column. The definitive characteristic remains the presence of coarse, angular dropstones scattered randomly within the fine, often laminated mud matrix.