skip to main content


Title: In a PICL: The sedimentary deposits and facies of perennially ice‐covered lakes
Abstract

Perennially ice‐covered lakes can have significantly different facies than open‐water lakes because sediment is transported onto the ice, where it accumulates, and sand grains preferentially melt through to be deposited on the lake floor. To characterize the facies in these lakes, sedimentary deposits from five Antarctic perennially ice‐covered lakes were described using lake‐bottom observations, underwater video and images, and sediment cores. One lake was dominated by laminated microbial mats and mud (derived from an abutting glacier), with disseminated sand and rare gravel. The other four lakes were dominated by laminated microbial mats and moderately well to moderately sorted medium to very coarse sand with sparse granules and pebbles; they contained minor interstitial or laminated mud (derived from streams and abutting glaciers). The sand was disseminated or localized in mounds and 1 m to more than 10 m long elongate ridges. Mounds were centimetres to metres in diameter; conical, elongate or round in shape; and isolated or deposited near or on top of one another. Sand layers in the mounds had normal, inverse, or no grading. Nine mixed mud and sand facies were defined for perennially ice‐covered lakes based on the relative proportion of mud to sand and the style of sand deposition. While perennially ice‐covered lake facies overlap with other ice‐influenced lakes and glaciomarine facies, they are characterized by a paucity of grains coarser than granules, a narrow range in sand grain sizes, and inverse grading in the sand mounds. These facies can be used to infer changes in ice cover through time and to identify perennially ice‐covered lakes in the rock record. Ancient perennially ice‐covered lakes are expected on Earth and Mars, and their characterization will provide new insights into past climatic conditions and habitability.

 
more » « less
Award ID(s):
1637708
NSF-PAR ID:
10078050
Author(s) / Creator(s):
 ;  ;  ;  ;  ;
Publisher / Repository:
Wiley-Blackwell
Date Published:
Journal Name:
Sedimentology
Volume:
66
Issue:
3
ISSN:
0037-0746
Page Range / eLocation ID:
p. 917-939
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Summary

    Laminated microbial mats are important ecosystem components of perennially ice‐covered Antarctic dry valley lakes. In order to understand better their response to changing environment, we made observations and carried out a manipulation experiment to determine their response to variations in irradiance in Lake Hoare (77°38′ S, 162°53′ E).

    Ice transparency was the most variable parameter that affected benthic light dose, both spatially and between years. Patterns of lamina accrual corresponded to irradiance history, with laminae that were initiated in high transmission years thicker than those from low transmission years.

    A shading experiment confirmed that accrual of lamina thickness, calcite precipitation and ash‐free dry mass were determined by irradiance, but photosynthetic biomass and phototrophic species composition were less affected.

    Buried laminae decomposed only slowly over time, with potentially viable phototrophs many laminae down into the microbial mat. Decay rate increased only slightly with shading.

    We conclude that the microbial mats in Lake Hoare are characterised by remarkable stability, with slow accumulation rates and turnover of biomass over time. Photosynthetic biomass and species composition appeared to be stable across long time periods, with interannual variation in lamination pattern due to differential accumulation of extracellular polysaccharide and representing the visible expression of annual growth conditions.

     
    more » « less
  2. Pliocene sediments were recovered during IODP Expedition 379 within the Resolution Drift offshore the Amundsen Sea. Site U1533 was drilled on the margin of a submarine channel extending landward to the continental margin, and Site U1532 was drilled in a more distal position on the thicker portion of the drift. We present new data collected on both sites. Facies assemblages consist of greenish gray clast-bearing mud with a biosiliceous component, interbedded with dark brownish gray laminated silty clay. Due to the close proximity of Sites U1532 and U1533 and the continuous sedimentation in the early Pliocene, individual beds of each facies can be correlated between sites. The red-green channel (a*) in shipboard reflectance spectroscopy and colorimetry data for Site U1533 covaries with the facies descriptions, Ba/Rb and Br in XRF data, ICP-MS bulk elemental ratios such as Sm/Zr, and clay mineralogy. This suggests that a more greenish color of the facies is partially attributed to a larger biogenic component in the sediment relative to the terrigenous supply, and a different provenance from the gray facies. Terrigenous particle size distributions (0-2000 mu) of Site U1533 show that the gray facies are relatively uniform silty clay, whereas greenish gray units show more variability, and a sand component. Sand-rich beds are present in both facies between the top of the greenish units and the bottom of the overlying gray units, and these have a uniform fine-sand mode. Greenish gray units are tentatively interpreted as deposition during ice retreat, with reduced terrigenous supply and higher primary productivity. Although these greenish grey facies can be interpreted as interglacial units, beds with this character do not occur evenly spaced throughout the stratigraphy. Greenish grey facies coincide with low Al/Ti ratios in XRF data for Site U1533. However, Al/Ti ratios change over evenly spaced intervals with orbital frequency and likely record a more complete record of glacial-interglacial cyclicity in sediment delivery than the irregular occurrence of greenish grey facies. This would suggest that some early Pliocene interglacials did not yield suitable conditions for the deposition of the greenish gray facies, and highlights the complex interactions between the ice sheet and the ocean embedded within these paleoarchives. PLAIN LANGUAGE SUMMARY Layers of sediment extracted via deep-sea drilling from beneath the seafloor off the Amundsen Sea, Antarctica, were stacked up over millions of years. The layers were built by pulses of sediment supplied from land ice and biogenic blooms, with distribution of material by ocean currents. The changing color and composition of the layers is an indication of the dominant imprint of ice-related processes versus ocean processes on the sediments that were raining down on the seafloor at any given time. Sedimentation related to the ice and the ocean follows different rhythms related to distribution of heat over time at different latitudes on Earth. The climate archive studied here records how the interference of these rhythms produces ice ages in Antarctica in a previous warm period about 3 to 5 million years ago with atmospheric greenhouse conditions that were like those of today. Investigations of these polar geological climate archives help provide context for the current ice mass loss observed in this same area of Antarctica and its potential sea-level effects. 
    more » « less
  3. Persistent cold temperatures, a paucity of nutrients, freeze-thaw cycles, and the strongly seasonal light regime make Antarctica one of Earth’s least hospitable surface environments for complex life. Cyanobacteria, however, are well-adapted to such conditions and are often the dominant primary producers in Antarctic inland water environments. In particular, the network of meltwater ponds on the ‘dirty ice’ of the McMurdo Ice Shelf is an ecosystem with extensive cyanobacteria-dominated microbial mat accumulations. This study investigated intact polar lipids (IPLs), heterocyte glycolipids (HGs), and bacteriohopanepolyols (BHPs) in combination with 16S and 18S rRNA gene diversity in microbial mats of twelve ponds in this unique polar ecosystem. To constrain the effects of nutrient availability, temperature and freeze-thaw cycles on the lipid membrane composition, lipids were compared to stromatolite-forming cyanobacterial mats from ice-covered lakes in the McMurdo Dry Valleys as well as from (sub)tropical regions and hot springs. The 16S rRNA gene compositions of the McMurdo Ice Shelf mats confirm the dominance of Cyanobacteria and Proteobacteria while the 18S rRNA gene composition indicates the presence of Ochrophyta, Chlorophyta, Ciliophora, and other microfauna. IPL analyses revealed a predominantly bacterial community in the meltwater ponds, with archaeal lipids being barely detectable. IPLs are dominated by glycolipids and phospholipids, followed by aminolipids. The high abundance of sugar-bound lipids accords with a predominance of cyanobacterial primary producers. The phosphate-limited samples from the (sub)tropical, hot spring, and Lake Vanda sites revealed a higher abundance of aminolipids compared to those of the nitrogen-limited meltwater ponds, affirming the direct affects that N and P availability have on IPL compositions. The high abundance of polyunsaturated IPLs in the Antarctic microbial mats suggests that these lipids provide an important mechanism to maintain membrane fluidity in cold environments. High abundances of HG keto-ols and HG keto-diols, produced by heterocytous cyanobacteria, further support these findings and reveal a unique distribution compared to those from warmer climates. 
    more » « less
  4. Nares Strait, a major connection between the Arctic Ocean and Baffin Bay, was blocked by coalescent Innuitian and Greenland ice sheets during the last glaciation. This paper focuses on the events and processes leading to the opening of the strait and the environmental response to establishment of the Arctic‐Atlantic throughflow. The study is based on sedimentological, mineralogical and foraminiferal analyses of radiocarbon‐dated cores 2001LSSL‐0014PCandTCfrom northern Baffin Bay. Radiocarbon dates on benthic foraminifera were calibrated with ΔR = 220±20 years. Basal compact pebbly mud is interpreted as a subglacial deposit formed by glacial overriding of unconsolidated marine sediments. It is overlain by ice‐proximal (red/grey laminated, ice‐proximal glaciomarine unit barren of foraminifera and containing >2 mm clasts interpreted as ice‐rafted debris) to ice‐distal (calcareous, grey pebbly mud with foraminifera indicative of a stratified water column with chilled Atlantic Water fauna and species associated with perennial and then seasonal sea ice cover) glacial marine sediment units. The age model indicates ice retreat into Smith Sound as early asc. 11.7 and as late asc. 11.2 cal. kaBPfollowed by progressively more distal glaciomarine conditions as the ice margin retreated toward the Kennedy Channel. We hypothesize that a distinctIRDlayer deposited between 9.3 and 9 (9.4–8.9 1σ) cal. kaBPmarks the break‐up of ice in Kennedy Channel resulting in the opening of Nares Strait as an Arctic‐Atlantic throughflow. Overlying foraminiferal assemblages indicate enhanced marine productivity consistent with entry of nutrient‐rich Arctic Surface Water. A pronounced rise in agglutinated foraminifers and sand‐sized diatoms, and loss of detrital calcite characterize the uppermost bioturbated mud, which was deposited after 4.8 (3.67–5.55 1σ) cal. kaBP. The timing of the transition is poorly resolved as it coincides with the slow sedimentation rates that ensued after the ice margins retreated onto land.

     
    more » « less
  5. Abstract

    The McMurdo Dry Valleys, Antarctica, are a polar desert populated with numerous closed‐watershed, perennially ice‐covered lakes primarily fed by glacial melt. Lake levels have varied by as much as 8 m since 1972 and are currently rising after a decade of decreasing. Precipitation falls as snow, so lake hydrology is dominated by energy available to melt glacier ice and to sublimate lake ice. To understand the energy and hydrologic controls on lake level changes and to explain the variability between neighboring lakes, only a few kilometers apart, we model the hydrology for the three largest lakes in Taylor Valley. We apply a physically based hydrological model that includes a surface energy balance model to estimate glacial melt and lake sublimation to constrain mass fluxes to and from the lakes. Results show that lake levels are very sensitive to small changes in glacier albedo, air temperature, and wind speed. We were able to balance the hydrologic budget in two watersheds using meltwater inflow and sublimation loss from the ice‐covered lake alone. A third watershed, closest to the coast, required additional inflow beyond model uncertainties. We hypothesize a shallow groundwater system within the active layer, fed by dispersed snow patches, contributes 23% of the inflow to this watershed. The lakes are out of equilibrium with the current climate. If the climate of our study period (1996–2013) persists into the future, the lakes will reach equilibrium starting in 2300, with levels 2–17 m higher, depending on the lake, relative to the 2020 level.

     
    more » « less