Uranium isotopes and dissolved organic carbon in loess permafrost: Modeling the age of ancient ice

Abstract

The residence time of ice in permafrost is an indicator of past climate history, and of the resilience and vulnerability of high-latitude ecosystems to global change. Development of geochemical indicators of ground-ice residence times in permafrost will advance understanding of the circumstances and evidence of permafrost formation, preservation, and thaw in response to climate warming and other disturbance. We used uranium isotopes to evaluate the residence time of segregated ground ice from ice-rich loess permafrost cores in central Alaska. Activity ratios of 234 U vs. 238 U ( 234 U/ 238 U) in water from thawed core sections ranged between 1.163 and 1.904 due to contact of ice and associated liquid water with mineral surfaces over time. Measured ( 234 U/ 238 U) values in ground ice showed an overall increase with depth in a series of five neighboring cores up to 21 m deep. This is consistent with increasing residence time of ice with depth as a result of accumulation of loess over time, as well as characteristic ice morphologies, high segregated ice content, and wedge ice, all of which support an interpretation of syngenetic permafrost formation associated with loess deposition. At the same time, stratigraphic evidence indicates some past sediment redistribution and possibly shallow thaw among cores, with local mixing of aged thaw waters. Using measures of surface area and a leaching experiment to determine U distribution, a geometric model of ( 234 U/ 238 U) evolution suggests mean ages of up to $200 ky BP in the deepest core, with estimated uncertainties of up to an order of magnitude. Evidence of secondary coatings on loess grains with elevated ( 234 U/ 238 U) values and U concentrations suggests that refinement of the geometric model to account for weathering processes is needed to reduce uncertainty. We suggest that in this area of deep ice-rich loess permafrost, ice bodies have been preserved from the last glacial period (10-100 ky BP), despite subsequent fluctuations in climate, fire disturbance and vegetation. Radiocarbon ( 14 C) analysis of dissolved organic carbon (DOC) in thaw waters supports ages greater than $40 ky BP below 10 m. DOC concentrations in thaw waters increased with depth to maxima of >1000 ppm, despite little change in ice content or cryostructures. These relations suggest time-dependent produchttp://dx. ScienceDirectGeochimica et Cosmochimica Acta 152 (2015) 143-165 tion of old DOC that will be released upon permafrost thaw at a rate that is mediated by sediment transport, among other factors.

Ground-ice stable isotopes and cryostratigraphy reflect late Quaternary palaeoclimate in the Northeast Siberian Arctic (Oyogos Yar coast, Dmitry Laptev Strait)

“…After cleaning the exposures from thawed material and debris, horizons were cryolithologically described (Murton and French, 1994;French and Shur, 2010;Murton, 2013) and samples were taken by axe and hammer in sub-profiles. The weight of the frozen sample compared to the weight of the sample after oven-drying was used to calculate the gravimetric ice content of the sediments, expressed as weight percentage (wt %) (Van Everdingen, 1998).…”

“…The stratigraphic position of Unit I below deposits of the Buchchagy Ice Complex of Unit II supports the age information obtained from IRSL dating, which is why we discard the radiocarbon age.To exclude a possible attribution of Unit I in the eastern part of the study region (sequence A) to the Zyryanian stadial known from Bol'shoy Lyakhovsky Island (Table 1), additional age control is greatly needed, if possible also using other approaches, e.g. uranium decay series in ground ice (Ewing et al, 2015). The Zyryanian at Bol'shoy Lyakhovsky Island shows similar sedimentary characteristics, pointing to floodplain deposits (Andreev et al, 2004), but contains toes of Yedoma Ice Complex ice wedges instead of the small truncated composite wedges found for Unit I at Oyogos Yar.…”

“…relative humidity, sea surface temperature) in the moisture source region (Merlivat and Jouzel, 1979). In the last years, stable-isotope data from ice wedges have been progressively used to reconstruct past climate changes in Arctic permafrost regions in northern Siberia (Meyer et al, 2002a(Meyer et al, , b, 2015Opel et al, 2011Opel et al, , 2017aWetterich et al, , 2014Vasil'chuk and Vasil'chuk, 2014;Streletskaya et al, 2015) as well as in Alaska (Meyer et al, 2010b;Lachniet et al, 2012;Schirrmeister et al, 2016) and in Canada (Fritz et al, 2012;Porter et al, 2016) on different timescales and with different temporal resolutions.…”

“…Moreover, the ages of late Pleistocene ice wedges are often close to or beyond the age limit of radiocarbon dating. However, new dating tools are in development and comprise Uranium isotopes (Ewing et al, 2015) as well as 36 Cl / Cl − dating for Middle to Late Pleistocene ground ice (Blinov et al, 2009). In many cases, syngenetic ice wedges are only indirectly dated by age determination of the surrounding host sediments.…”

Ice Complex permafrost of MIS5 age in the Dmitry Laptev Strait coastal region (East Siberian Arctic)

“…Some uncertainties are probably related to unknown influences of permafrost processes on chemical and physical parameters important to the age determination techniques. Ewing et al (2015) evaluated in a novel approach the use of radioactive disequilibrium among uranium isotopes in thaw water to estimate the age of intrasedimentary syngenetic ground ice in Alaskan loess. Developing dating techniques and increasing understanding of controlling periglacial processes allow for improved chronostratigraphies of permafrost sequences.…”