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Silt-rich meltwater plume deposits (MPDs) analyzed from marine sediment cores have elucidated relationships that are clearly connected, yet difficult to constrain, between subglacial hydrology, ice-marginal landforms, and grounding-zone retreat patterns for several glacial catchments. Few attempts have been made to infer details of subglacial hydrology, such as flow regime, geometry of drainage pathways, and mode(s) of sediment transport through time, from grain-scale characteristics of MPDs. Using sediment samples from MPD, till, and grounding-zone proximal diamicton collected offshore of six modern and relict glacial catchments in both hemispheres, we examine grain shape distributions and microtextures (collectively, grain micromorphology) of the silt fraction to explore whether grains are measurably altered from their subglacial sources via meltwater action. We find that 75 % of all imaged grains (n = 9400) can be described by 25 % of the full range of measured shape morphometrics, indicating grain shape homogenization through widespread and efficient abrasive processes in subglacial environments. Although silt grains from MPDs exhibit edge rounding more often than silt grains from tills, grain surface textures indicative of fluvial transport (e.g., v-shaped percussions) occur in only a modest number of grains. Furthermore, MPD grain surfaces retain several textures consistent with transport beneath glacial ice (e.g., straight or arcuate steps, (sub)linear fractures) in comparable abundances to till grains. Significant grain shape alteration in MPDs compared to their till sources is observed in sediments from glacial regions where (1) high-magnitude, potentially catastrophic meltwater drainage events are inferred from marine sediment records and (2) submarine landforms suggest supraglacial melt contributed to the subglacial hydrological budget. This implies that quantifiable grain shape alteration in MPDs could reflect a combination of high-energy flow of subglacial meltwater, persistent sediment entrainment, and/or long sediment transport distances through subglacial drainage pathways. Integrating grain micromorphology into analysis of MPDs in site-specific studies could therefore aid in distinguishing periods of persistent, well-connected subglacial discharge from periods of sluggish or disorganized drainage. In the wider context of deglacial marine sedimentary and bathymetric records, a grain micromorphological approach may bolster our ability to characterize ice response to subglacial meltwater transmission through time. This work additionally demonstrates that glacial and fluvial surface textures are retained on silt-sized quartz grains in adequate amounts for microtexture analysis, which has heretofore been conducted exclusively on the sand fraction. Therefore, grain microtextures can be examined on silt-rich glaciogenic deposits that contain little to no sand as a means to evaluate sediment transport processes. [ABSTRACT FROM AUTHOR]

We present the first study which directly compares carbonate radiocarbon (14C) dates with the Ramped PyrOx (RP) radiocarbon dating technique within a single sediment core, and we confirm the utility for the latter constructing chronologies of high latitude, Holocene marine margin sediment successions. Historically, the heavily detrital nature of Antarctic margin sedimentary organic material and lack of carbonate preservation have made these sediments difficult to date accurately. Here, we use archived cores with existing foraminiferal ages to compare with RP dates at equal or similar depth intervals. The lowest temperature RP splits were integrated over narrower intervals than in previous studies to reduce the amount of mixing with older, more thermochemically stable end-members during pyrolytic decomposition. Ages of the low-temperature RP splits coincide with their corresponding carbonate counterparts, suggesting that the RP 14C dating method is a reliable alternative to carbonate dates in sediments where carbonates are absent or not sufficiently preserved for 14C dating. The rarity of calcareous material in most Antarctic sediments often obligates the use of the bulk acid insoluble organic (AIO) fraction of the sediment, which can be problematic because of contamination by older carbon. The bulk AIO 14C ages, which are calculated using the weighted arithmetic mean of all RP splits of individual samples, show that age reversals and biases can occur using bulk AIO dates for age models because of variable proportions of pre-aged organic matter down-core. The application of the RP dating method can reduce these effects to produce a more reliable chronology that is statistically identical to the foraminiferal dating chronology. [ABSTRACT FROM AUTHOR]

Abstract: Three Holocene core sections from Maxwell Bay, South Shetland Islands, West Antarctica are investigated, using MS, grain size, TOC, foraminifera, δ18O and δ13C, 14C, and 210Pb, in order to examine Holocene oceanographic, glacial and climatic changes in the region. They come from 190, 221, and 333m water-depths and the bay margin-proximal locations, complementing earlier studies from the deep central part. The three records vary between locations and reflect combined marine and terrigenous influence from the open ocean and surrounding islands, respectively. The longest and best dated composite section comes from the inner part of the bay (core JTC/JPC-17) and extends back to 6.3calkyr BP, close to the termination of the Holocene Climatic Optimum. A combination of dating problems and mixed foraminiferal assemblages prevented reliable interpretation of core JPC-15 from the outer part of the bay. Core JPC-16, collected from the inner bay near the inlet between King George and Nelson Islands, revealed repeated highs in planktonic Neogloboquadrina pachyderma occurrence during last 3000yr. These highs are interpreted as indicating periods of decreased meltwater flow into the bay, and thus greater influence of open-marine surface-waters. Core JTC/JPC-17, located closest to King George Island and the Collins Ice Cap, records increased glacial conditions at ~2000calyr BP. This event is recorded by turbidite deposition at core site JPC-16. Major components of the benthic foraminiferal assemblage indicate decreasing tolerance to near-glacier or near-shore conditions, however, local climate variability appears to have overall little impact on benthic foraminiferal biota. Thus, it is unlikely that, in similar settings, recent warming would be strongly manifest as changes in benthic foraminiferal communities. Stable isotopic analysis is based on Cassidulinoides parkerianus and Globocassidulina biora. Subtle decrease in δ18O at ~3500calyr BP, followed by episodic increases at ~1000calyr BP and in the youngest part of the record, appear to correlate with the period prior to the beginning of the Neoglacial, with Medieval Warm Period, and with post Little Ice Age warming, respectively. The latest event is also marked by a significant increase in sediment accumulation rate in the second half of the 20th century. [Copyright &y& Elsevier]

Postglacial fluctuations of Southern Patagonian Icefield (SPI) glaciers are well constrained on the leeward side of the Andes, but they remain mostly unknown on the windward side of the icefield, where most glaciers are marine-terminating. Here, we reconstruct the postglacial fluctuations of the HPS19, Penguin, and Europa glaciers along the hyperhumid western side of the SPI using a multi-proxy sedimentological and geochemical analysis of a 12.2 m long sediment core from Wide Channel (50°S). Results show that the glaciers retreated into Penguin and Europa fjords by 11.2 cal kyr BP and that they were relatively stable and marine-terminating between 11.2 and 5.8 cal kyr BP. Thereafter, they fluctuated rapidly, with four marked episodes of glacier shrinkage at 5.8–4.8, 3.9–2.4, 1.0–0.2 cal kyr BP, and during the 20th century. Although the HPS19, Penguin, and Europa glaciers were calving into Penguin and Europa fjords during most of the Holocene, our data suggest that they retreated to land-based positions between 5.8 and 4.8 cal kyr BP. The comparison of our sediment record with geological archives from both sides of the Patagonian icefields (46°–56°S) suggests synchronous glacier variability on multi-centennial timescales during the Neoglacial period, which is particularly clear after 2.5 cal kyr BP. We conclude that western SPI outlet glaciers remained relatively stable during the first half of the Holocene but fluctuated considerably during the Neoglacial period, and that they retreated to locations further inland than today during the first retreat of the Neoglacial period between 5.8 and 4.8 cal kyr BP. • SPI outlet glaciers retreated into Penguin and Europa fjords by 11.2 cal kyr BP. • Glaciers were relatively stable between 11.2 and 5.8 cal kyr BP. • High glacier variability during the last 5.8 kyr. • Glacier shrinkage occurred at 5.8–4.8, 3.9–2.4, 1.0–0.2 cal kyr BP, and during the 20th century. • Glaciers on both sides of the Patagonian Andes fluctuated synchronously on multi-centennial timescales during the last 5.8 kyr. [ABSTRACT FROM AUTHOR]