Subglacially precipitated carbonates record geochemical interactions and pollen preservation at the base of the Laurentide Ice Sheet on central Baffin Island, eastern Canadian Arctic

Abstract

The mineralogy and isotopic compositions of subglacially precipitated carbonate crusts (SPCCs) provide information on conditions and processes beneath former glaciers and ice sheets. Here we describe SPCCs formed on gneissic bedrock at the bed of the Laurentide Ice Sheet (LIS) during the last glacial maximum on central Baffin Island. Geochemical data indicate that the Ca in the crusts was likely derived from the subglacial chemical weathering Ca-bearing minerals in the local bedrock. C and Sr isotopic analyses reveal that the C in the calcite was derived predominantly from older plant debris. The δ18O values of the SPCCs suggest that these crusts formed in isotopic equilibrium with basal ice LIS preserved in the Barnes Ice Cap (BIC). Columnar crystal fabric and the predominance of sparite over micrite in the SPCCs are indicative of carbonate precipitation under open-system conditions. However, the mean δ18O value of the calcite crusts is ~ 10‰ higher than those of primary LIS ice preserved in the BIC, demonstrating that SPCCs record the isotopic composition of only basal ice. Palynomorph assemblages preserved within the calcite and basal BIC ice include species last endemic to the Arctic in the early Tertiary. The source of these palynomorphs remains enigmatic.

Geochemical Processes Leading to the Precipitation of Subglacial Carbonate Crusts at Bossons Glacier, Mont Blanc Massif (French Alps)

“…These corresponding types of cryogenic carbonates have been described in the Vestfold Hills of Antarctica (Aharon, 1988), the Chinese Tien Shan Mountains (Liu et al, 2005), and the Barnes Ice Cap on central Baffin Island, in the eastern Canadian Arctic (Refsnider et al, 2014). For the first and third locations listed here, the respective authors reported carbon and oxygen isotopic signal of SPCCs.…”

“…This process has been suggested by Refsnider et al (2014) to explain the low δ 13 C of around −9‰ (VPDB) in the SPCCs from the eastern Canadian Arctic. The carbon isotopic composition measured in organic material of sample STB1, around −24.1 ± 0.2‰ (1σ), is fully consistent with the range of the modern upland Alpine (Urseren Valley, southern central Switzerland) soil organic matter from −27 to −21‰ (Schaub and Alewell, 2009).…”

“…Barnes Ice Cap SPCCs show a mean δ 18 O value of 6.6 ± 0.8‰ VSMOW and δ 13 C values between −11 and −7‰ VPDB. These isotopic signals have been related to subglacial carbonate precipitation reflecting kinetic processes (Aharon, 1988) or quasi-isotopic equilibrium with parent water under open system conditions (Refsnider et al, 2014). Refsnider et al (2014) also showed that the Barnes Ice Cap subglacial calcite crusts have a typical fabric (i.e., columnar, with sparite predominating over micrite) that supports near-equilibrium fractionation, and they suggested that the carbon incorporated in the SPCCs was predominantly derived from old soil carbon sources.…”

“…These isotopic signals have been related to subglacial carbonate precipitation reflecting kinetic processes (Aharon, 1988) or quasi-isotopic equilibrium with parent water under open system conditions (Refsnider et al, 2014). Refsnider et al (2014) also showed that the Barnes Ice Cap subglacial calcite crusts have a typical fabric (i.e., columnar, with sparite predominating over micrite) that supports near-equilibrium fractionation, and they suggested that the carbon incorporated in the SPCCs was predominantly derived from old soil carbon sources. Nearequilibrium fractionation is also deduced from SPCCs located in limestone bedrock areas in Holocene and Pleistocene examples (Lacelle, 2007) suggesting that the C-O isotopic signals of the SPCCs might be useful proxies for paleoclimatic (e.g., air temperature) and paleoenvironmental (e.g., carbon cycle) conditions.…”

Oxygen isotope geochemistry of Laurentide ice-sheet meltwater across Termination I

“…However, mixing models of meltwater contributions to the ocean typically assume a constant d 18 O w value for the LIS (À25 or À35‰; Aharon, 2006;Carlson, 2009;Carlson et al, 2007;Hill et al, 2006;Obbink et al, 2010), and do not attempt to incorporate the complex spatial heterogeneity in d 18 O w observed in modern continental ice sheets (Masson-Delmotte et al, 2008;Vinther et al, 2009). Attempts to estimate the oxygen isotope composition of parts of the LIS have used measurements of remnant ice such as the Barnes Ice Cap (Hooke, 1976;Hooke and Clausen, 1982), Pleistocene-age groundwater (Hooke and Clausen, 1982;Remenda et al, 1994) and preserved ice wedges (Kotler and Burn, 2000); as well as indirect proxies from subglacial carbonates (Refsnider et al, 2012(Refsnider et al, , 2014 and proglacial lakes (Birks et al, 2007;Buhay and Betcher, 1998;Last et al, 1994;Moore et al, 2000).…”