Influence Of Glacial Meltwater Research Articles

Functional Feeding Groups of Macrofauna and Detritus Decomposition along a Gradient of Glacial Meltwater Influence in Tropical High-Andean Streams

Tropical Andean glaciers are retreating rapidly, with possible consequences for trophic structure and ecosystem processes in high Andean meltwater streams. Here, we measured the environmental characteristics, quantified pools of particulate organic matter (POM) and periphyton (Chl. a), sampled benthic macroinvertebrates, determined functional feeding groups (FFG), and performed mesh bag decomposition experiments with Calamagrostis grass detritus at 17 stream sites along a gradient of glacial influence (GI) with 0–23% glacier cover in the catchment at 4050–4200 m a.s.l. in the Andes of Ecuador. POM was unrelated to GI while Chl. a. showed a weak (non-significant) negative relationship to GI. The macrofauna abundance decreased while taxon richness and the number of FFGs per site showed a hump-shaped relationship with increasing GI. Taxa with an opportunistic and generalist feeding mode generally dominated benthic assemblages and were related to high GI levels and low Chl. a. Only shredders were negatively related to GI, but unrelated to POM. Decomposition rates were comparable to those found in temperate alpine streams, and for both fine (0.0010–0.0065; median 0.0028 d−1) and coarse (0.0019–0.0088; median 0.0048 d−1) mesh bags, peaked at intermediate GI values, while the difference between bag types was small and almost constant along the GI gradient. This indicates an overall minor effect of macroinvertebrate shredders compared to that of microbes, in particular at high GI. It also suggests that the relatively high average temperature of these high-altitude equatorial streams (7–10 °C) does not produce higher decomposition rates than those in comparable but colder streams at temperate latitudes. The results suggest that, at the lower end of glacier cover, tropical glacier loss will not change the dominant microbial role in detritus decomposition, but that part of the physical abrasion could be partially replaced by biological shredding.

Short-term dynamics of nutrients, planktonic abundances, and microbial respiratory activity in the Arctic Kongsfjorden (Svalbard, Norway)

Atlantification of Arctic ocean is causing a sharp increase in temperature and salinity around Svalbard Islands and in Kongsfjorden. Such phenomenon and the input of sediment-rich glacial meltwater influence salinity, water column turbidity, and light penetration with ecological implications on the microbial features. With the aim to address the temporal variability of the microbial assemblage in relation to environmental variables, a 7-day study was carried out in Kongsfjorden, in late summer 2013. Abiotic (temperature, salinity, nutrients, total suspended matter, particulate inorganic, and organic carbon) and biotic (phyto -, picophyto-, bacterio-, and virioplankton abundance and microbial respiration) parameters were investigated at a station in the inner fjord area, ca. 8.5 km away from the glacier front. Phyto-, picophyto-, and virioplankton showed low abundance. Dinoflagellates and coccolithophorids dominated the phytoplankton community while Synechococcus sp. the picophytoplankton alone, in relation with Atlantic water. Low virus to bacteria ratios were detected, presumably linked to the high sedimentation rates. Interesting variability for picophytoplankton with depth, virioplankton with both time and depth, and respiratory rates with time were found. Moreover, the organic matter turnover was slower on the first sampling day compared to that of the following days. Planktonic abundance depended on the variability of both hydrology (seawater mass inflow) and freshwater runoff from the glacier (relative turbidity degree). Differently, the metabolic rates of respiration appeared to be linked with the particulate carbon pool. Over 1-week study, the diverse microbial dynamics appeared to be conditioned on complex forcing, emphasizing the importance of high-resolution experiment.

Sinuous ridges in Chukhung crater, Tempe Terra, Mars: Implications for fluvial, glacial, and glaciofluvial activity

We present a geomorphic map of Chukhung crater (38.47°N, 72.42°W) in central Tempe Terra, Mars. Chukhung crater formed ~3.6–2.1 Ga, between the early Hesperian and early Amazonian periods of Mars' geologic history. It hosts dendritic networks of crater wall valleys, broad crater floor valleys, mid- to late-Amazonian-aged debris-covered glaciers, moraine-like deposits, and a radial assemblage of sinuous ridge landforms. We explore the origins of landforms in Chukhung crater, focusing in particular upon the sinuous ridges. In northern Chukhung crater, sinuous ridges extend downslope from fluvial valley systems on the northern crater wall. We interpret the northern sinuous ridges as inverted paleochannels: ridges formed by exhumation of resistant and/or indurated fluvial channel fill deposits. The origins of sinuous ridges on the southern floor of Chukhung crater are more ambiguous. They emerge from beneath moraine-like ridges which bound extant debris-covered glaciers extending from the southern wall of the crater. The southern sinuous ridges have numerous morphological and contextual similarities to eskers: ridges of glaciofluvial sediment deposited in meltwater conduits within or beneath wet-based glaciers. The close proximity of the northern and southern sinuous ridges, however, calls into question an interpretation which ascribes a different origin to each set. The similarity in the overarching process between esker and inverted channel formation (i.e., exposure by the removal of a bounding medium, be that ice or sediments/rock) results in convergence of form between eskers and inverted paleochannels. We discuss the esker-like characteristics of the southern sinuous ridges in detail, and argue that one of two ridge populations in southern Chukhung crater is best explained by the esker hypothesis while the other could be explained under either the esker or the inverted paleochannel hypothesis. Regardless of the specific formation mechanism for the southern sinuous ridges, we find that Chukhung crater has undergone significant modification by liquid water since its formation. The northern sinuous ridges and associated crater-wall valleys provide evidence for subaerial drainage of precipitation and/or snowmelt. This suggests that Chukhung crater, and possibly the surrounding region, experienced unusually warm and wet episodes between the early Hesperian and mid Amazonian. If some or all of the southern sinuous ridges are eskers, they could provide evidence for an additional influence of glacial meltwater in Chukhung crater during the mid-to-late Amazonian. If wet-based glaciation did occur in Chukhung crater, the location of the crater between major branches of the Tempe Fossae tectonic rift system would add to the growing body of evidence that elevated geothermal heat flux was an important driver of localized occurrences of recent wet-based glaciation on Mars.

Groundwater–glacier meltwater interaction in proglacial aquifers

Abstract. Groundwater plays a significant role in glacial hydrology and can buffer changes to the timing and magnitude of flows in meltwater rivers. However, proglacial aquifer characteristics or groundwater dynamics in glacial catchments are rarely studied directly. We provide direct evidence of proglacial groundwater storage, and quantify multi-year groundwater–meltwater dynamics, through detailed aquifer characterisation and intensive high-resolution monitoring of the proglacial system of a rapidly retreating glacier, Virkisjökull, in south-eastern Iceland. Proglacial unconsolidated glaciofluvial sediments comprise a highly permeable aquifer (25–40 m d−1) in which groundwater flow in the shallowest 20–40 m of the aquifer is equivalent to 4.5 % (2.6 %–5.8 %) of mean river flow, and 9.7 % (5.8 %–12.3 %) of winter flow. Estimated annual groundwater flow through the entire aquifer thickness is 10 % (4 %–22 %) the magnitude of annual river flow. Groundwater in the aquifer is actively recharged by glacier meltwater and local precipitation, both rainfall and snowmelt, and strongly influenced by individual precipitation events. Local precipitation represents the highest proportion of recharge across the aquifer. However, significant glacial meltwater influence on groundwater within the aquifer occurs in a 50–500 m river zone within which there are complex groundwater–river exchanges. Stable isotopes, groundwater dynamics and temperature data demonstrate active recharge from river losses, especially in the summer melt season, with more than 25 % and often >50 % of groundwater in the near-river aquifer zone sourced from glacier meltwater. Proglacial aquifers such as these are common globally, and future changes in glacier coverage and precipitation are likely to increase the significance of groundwater storage within them. The scale of proglacial groundwater flow and storage has important implications for measuring meltwater flux, for predicting future river flows, and for providing strategic water supplies in de-glaciating catchments.

Insolation and Glacial Meltwater Influence on Sea‐Ice and Circulation Variability in the Northeastern Labrador Sea During the Last Glacial Period

AbstractThe variable amounts of ice rafted debris (IRD) and foraminifers in North Atlantic sediments are related to the abrupt, millennial‐scale alteration from Greenland stadials to interstadials during the last glacial period and indicate past ice sheet instabilities, changes in sea‐ice cover and productivity. In the Norwegian Sea, Greenland stadials were likely characterized by an extensive, near‐perennial sea‐ice cover whereas Greenland interstadials were seasonally ice‐free. The variability in other areas, such as the Labrador Sea, remains, however, obscure. We therefore investigated deep‐sea sediment core GS16‐204‐22CC retrieved south of Greenland. Using a multiproxy approach, we distinguish two sediment regimes and hence different environmental conditions between ca. 65 and 25 ka b2k. Regime 1 (~65‐49 ka b2k) is characterized by the dominance of planktic foraminifers in the sediments. During late MIS4 and early MIS3, the site was covered by near‐perennial sea‐ice with occasional periods of iceberg discharge. During the younger part of regime 1 the northeastern Labrador Sea was seasonally ice‐free with hardly any icebergs melting near the site and long‐term environmental conditions were less variable. Regime 2 (~49‐25 ka b2k) is characterized by pronounced stadial‐interstadial variability of foraminifer and IRD fluxes, suggesting an extensive sea‐ice cover during most Greenland stadials and seasonally ice‐free conditions during most Greenland interstadials. During MIS2 environmental conditions were very similar to those of the younger part of regime 1. While all Heinrich (H) related Greenland stadials are marked by depleted oxygen isotope values at our core site, only H4 and H3 are associated with pronounced IRD peaks.

Rare earth element geochemistry of Australian Neoproterozoic carbonate: Constraints on the Neoproterozoic oxygenation events

Carbonate successions are principal geochemical archives of the extraordinary environmental changes that occurred during the Neoproterozoic Era. In this study, we present a stratigraphic record of carbonate rare earth element (REE) compositions from Neoproterozoic and Cambrian strata in central and southern Australia. The new REE results provide insight into the redox conditions of the Neoproterozoic-Cambrian ocean, the influence of glacial meltwater on carbonate precipitation, and Neoproterozoic-Cambrian paleoenvironmental changes. High Ce/Ce* and low Y/Ho suggest that the major Neoproterozoic and Cambrian carbonate accumulations of central and southern Australia formed in restricted marine environments that were largely dysoxic. Elevated ∑REE of mid-Cryogenian and early Ediacaran carbonate from the Amadeus Basin (central Australia) and King Island (Tasmania), respectively, are interpreted to reflect increased detrital mineral flux following periods of Cryogenian glaciation. REE data from the basal Ediacaran Cumberland Creek Dolostone of King Island suggest a transient episode of ocean oxidation in the wake of the terminal Cryogenian glaciation, but the balance of evidence suggests dominantly low oxygen ocean conditions during carbonate deposition in the remainder of the Neoproterozoic Era. An apparent mismatch in timing between carbonate deposition in central and southern Australia versus previously proposed mid-to-late-Ediacaran oxygenation events based on geochemical results from strata of South China highlights what may be important factors in reconciling varied geochemical records of Neoproterozoic-Paleozoic ocean oxygenation.

Influence of Glacial Meltwater on Summer Biogeochemical Cycles in Scoresby Sund, East Greenland

Greenland fjords receive considerable amounts of glacial meltwater discharge from the Greenland Ice Sheet due to present climate warming. This impacts the hydrography via freshening of the fjord waters and biological processes due to altered nutrient input and the addition of silts. We present the first comprehensive analysis of the summer carbon cycle in the world's largest fjord system situated in southeastern Greenland. During a cruise onboard RV Maria S. Merian in summer 2016, we visited Scoresby Sund and its northernmost branch, Nordvestfjord. In addition to direct measurements of hydrography, biogeochemical parameters and sediment trap fluxes, we derived net community production (NCP) and full water column particulate organic carbon (POC) fluxes, and estimated carbon remineralization from vertical flux attenuation. While the narrow Nordvestfjord is influenced by subglacial and surface meltwater discharge, these meltwater effects on the outer fjord part of Scoresby Sund are weakened due to its enormous width. We found that subglacial and surface meltwater discharge to Nordvestfjord significantly limited NCP to 32 – 36 mmol C m-2 d-1 compared to the outer fjord part of Scoresby Sund (58 – 82 mmol C m-2 d-1) by inhibiting the resupply of nutrients to the surface and by shadowing of silts contained in the meltwater. The POC flux close to the glacier fronts was elevated due to silt-ballasting of settling particles that increases the sinking velocity and thereby reduces the time for remineralization processes within the water column. By contrast, the outer fjord part of Scoresby Sund showed stronger attenuation of particles due to horizontal advection and, hence, more intense remineralization within the water column. Our results imply that glacially influenced parts of Greenland's fjords can be considered as hotspots of carbon export to depth. In a warming climate, this export is likely to be enhanced during glacial melting. Additionally, entrainment of increasingly warmer Atlantic Water might support a higher productivity fjord systems. It therefore seems that future ice-free fjord systems with high input of glacial meltwater may become increasingly important for Arctic carbon sequestration.

An assessment of landform composition and functioning with the first proglacial systems dataset of the central European Alps

Proglacial systems are enlarging as glacier masses decline. They are in a transitory state from glacier-dominated to hillslope and fluvially-dominated geomorphological processes. They are a very important meltwater, sediment and solute source. This study makes the first quantitative, systematic and regional assessment of landform composition and functioning within proglacial systems that have developed in the short term since the Little Ice Age (LIA). Proglacial system extent was thus defined as the area between the LIA moraine ridges and the contemporary glacier. We achieved this assessment via a series of topographic analyses of 10 m resolution digital elevation models (DEMs) covering the central European Alps, specifically of Austria and Switzerland. Across the 2812 proglacial systems that have a combined area of 933 km2, the mean proportional area of each proglacial system that is directly affected by glacial meltwater is 37%. However, there are examples where there is no glacial meltwater influence whatsoever due to complete disappearance of glaciers since the LIA, and there are examples where >90% of the proglacial area is probably affected by glacial meltwater. In all of the major drainage basins; the Inn, Drava, Venetian Coast, Po, Rhine, Rhone and Danube, the proportions of the combined land area belonging to each landform class is remarkably similar, with >10% fluvial, ~35% alluvial and debris fans, ~50% moraine ridges and talus/scree, and ~10% bedrock, which will be very helpful for considering estimates of regional sediment yield and denudation rates. We find groupings of the relationship between proglacial system hypsometric index and lithology, and of a slope threshold discriminating between hillslope and fluvial-dominated terrain, both of which we interpret to be due to grain size. We estimate of contemporary total volume loss from all of these proglacial systems of 44 M m3a−1, which equates to a mean of 0.3 mm·a−1 contemporary surface lowering. Overall, these first quantifications of proglacial landform and landscape evolution will be an important basis for inter- and intra-catchment considerations of climate change effects on proglacial systems such as land stability, and changing water, sediment and solute source fluxes. Our datasets are made freely available.

Parameterization of Spectral Particulate and Phytoplankton Absorption Coefficients in Sognefjord and Trondheimsfjord, Two Contrasting Norwegian Fjord Ecosystems

We present here parameterizations of particulate and phytoplankton absorption coefficients as functions of pigment concentrations (Tchla) in Sognefjord and Trondheimsfjord along the northwestern coast of Norway. The total particulate and non-algal optical densities were measured via quantitative filter technique (QFT) in a spectrophotometer with integrating sphere. The spectral parameter coefficients A(λ) and E(λ) of the power law describing variations of particulate and phytoplankton absorption coefficients as a function of Tchla, were not only different from those provided for open ocean case 1 waters, but also exhibited differences in the two fjords under investigation. Considering the influence of glacial meltwater leading to increased inorganic sediment load in Sognefjord we investigate differences in two different parameterizations, developed by excluding and including inner Sognefjord stations. Tchla are modelled to test the parameterizations and validated against data from the same cruise and that from a repeated campaign. Being less influenced by non-algal particles parameterizations performed well in Trondheimsfjord and yielded high coefficients of determination (R2) of modelled vs. measured Tchla. In Sognefjord, the modelled vs. measured Tchla resulted in better R2 with parameter coefficients developed excluding the inner-fjord stations influenced by glacial meltwater influx.

Influence of glacial melt and Atlantic water on bacterioplankton community of Kongsfjorden, an Arctic fjord

The Kongsfjorden, an Arctic fjord is experiencing warming due to increased input of Atlantic water masses. High-throughput sequencing was performed to examine bacterial diversity from the outer and inner zone of the fjord in summer and fall of 2012. A total of 11,999 operational taxonomic units (OTUs) were assigned into 19 known phyla and 5 genera incertae sedis. Significant variation (p=0.001, n=4) was observed between the bacterial community structure of outer and inner fjord while variation between summer and fall was minimum. Proteobacteria was the most abundant phylum (55.9-61.0%) in summer and fall. The most dominant alphaproteobacterial member of this phylum (OTU 263 Pelagibacteriaceae) contributed maximum to the observed dissimilarity between the outer and inner fjord community. Characterised by relatively fresher and warmer water, glacial meltwater input could be a major source of predominance of OTU 6968 Flavobacteriaceae, OTU 5552 Psychrobacter, OTU 7148 Sphingomonadales and OTU 5011 Loktanella in the inner fjord in summer. Thus, the significant variation in the bacterioplankton community composition of outer and inner fjord indicates strong and localized influence of glacial melt water in shaping the community structure.

Bio-optical characterization and light availability parameterization in Uummannaq Fjord and Vaigat–Disko Bay (West Greenland)

Abstract. This study investigated the bio-optical conditions of Uummannaq Fjord and Vaigat–Disko Bay, two neighboring, semi-enclosed coastal systems in West Greenland. Though close to each other, the systems differ in their hydrographic structure influencing the bio-optical conditions and, subsequently, the biological activities. Both systems showed high inorganic suspended particulate matter (SPMi) concentrations near river runoff or meltwater influxes (max. of 15.28 mg L−1 at the surface) and low colored dissolved organic matter (aCDOM@350nm, < 1.50 m−1) abundance throughout the systems. High chlorophyll levels (as an indicator of phytoplankton biomass) were measured in the Vaigat (max. of 11.44 µg L−1), which represents the outflow arm of Disko Bay. Light penetration depth as indicated by the 1 % depth of photosynthetically available radiation (PAR) was dominated by chlorophyll and SPMi alike, ranging from 12.2 to 41.2 m. Based on these characteristics, an effective two-component parameterization for the diffuse attenuation coefficient kPAR was developed in order to model light penetration depth as a relevant factor for bio-optical studies in Arctic environments under glacial meltwater influence.

The influence of glacial melt water on bio-optical properties in two contrasting Greenlandic fjords

Scattering alters the path of photons, but ultimately their removal from the water column occurs by absorption by one of four components: water itself, coloured dissolved organic matter (CDOM), phytoplankton pigments or non-algal particulate matter (NAP). We calculated absorption budgets for two fjordal systems, Godthåbsfjord (SW Greenland) and Young Sound (NE Greenland), based on these components and evaluated the fate of solar radiation in each system. Absorption by phytoplankton pigment accounted for 15–32% of photons in Godthåbsfjord whilst in Young Sound the corresponding fraction was only 5–8%. NAP accounted for 13–25% of absorption in Young Sound and only 7–8% in Godthåbsfjord whilst fractions of absorption by CDOM were more similar: 6–13% in Godthåbsfjord and 6–18% in Young Sound.In typical temperate estuarine systems, nutrients, CDOM and particulate matter are supplied by riverine sources. Increased nutrient supply will tend to increase productivity whilst increased concentrations of CDOM and particles will increase light attenuation, thereby reducing productivity. The two Greenlandic fjords differ from typical estuarine systems in that their supply of nutrients and particles is decoupled. Freshwater feeding them comes from glacial melt and contains particles but low concentrations of nutrients. New nutrients are supplied by entrainment of oceanic water at the mouths of the fjords and sediment–water exchange of remineralized nutrients. Attenuation and thereby light availability in the two fjords is strongly correlated with turbidity (Godthåbsfjord: r2 = 0.90 p < 0.01, Young Sound: r2 = 0.82 p < 0.001) and we conclude that loading of particulate matter controls light attenuation, and through this may influence primary production. Previous studies argue that warming due to climate change will increase productivity in the fjords. We suggest that increased runoff and particle load may have an opposite effect.

Spatio-temporal shifts of macroinvertebrate drift and benthos in headwaters of a retreating glacier

Recent alpine stream studies have aimed to predict biotic responses to changing environmental conditions, hydrological regime and climate, but there is little information on macroinvertebrate drift shifts in this context and most research was largely restricted to snow-free period. We compared seasonal drift–benthos patterns in proglacial streams of Val Roseg (Switzerland) between 2008/2009 and 1997/1998. Samples were collected monthly (including winter) in a glacial lake outlet stream, a proglacial stream, and their post-confluence stream (main channel) in both periods. Measured physical and chemical parameters indicated the largest glacial meltwater influence in the proglacial stream and its lower inflow in 2008/2009 compared to 1997/1998. Drift largely coincided with benthic structure at each site, although drift–benthos patterns were at times asynchronous, depending on site and flow conditions. Our results indicate that the initial stages of hydrological “freeze-melt” events and the period of snow cover are most favorable for macroinvertebrates in glacial streams. Drift density was significantly lower in 2008/2009 than in 1997/1998. In 2008/2009, we observed lower chironomid, but higher ephemeropteran and plecopteran drift densities. The drift–benthos changes between periods likely reflect behavioral and life-history responses of proglacial stream macroinvertebrates to seasonal shifts in environmental conditions in response to glacial retreat.

Surface Drifters Track the Fate of Greenland Ice Sheet Meltwater

Understanding the fate and influence of glacial meltwater in heavily ice‐covered fjord systems has proven difficult because previous measurement platforms were con­strained to deeper water to keep instrumentation safe from drifting icebergs. Now, using novel, satellite‐tracked devices that can with­ stand multiple collisions with ice blocks (see Figure 1) without incurring much damage, scientists have obtained new and detailed data about the role of Greenland Ice Sheet meltwater and its trajectories through God­thåbsfjord in western Greenland.

The influence of glacial meltwater on alpine aquatic ecosystems: a review

The recent and rapid recession of alpine glaciers over the last 150 years has major implications for associated aquatic communities. Glacial meltwater shapes many of the physical features of high altitude lakes and streams, producing turbid environments with distinctive hydrology patterns relative to nival systems. Over the past decade, numerous studies have investigated the chemical and biological effects of glacial meltwater on freshwater ecosystems. Here, we review these studies across both lake and stream ecosystems. Focusing on alpine regions mainly in the Northern Hemisphere, we present examples of how glacial meltwater can affect habitat by altering physical and chemical features of aquatic ecosystems, and review the subsequent effects on the biological structure and function of lakes and streams. Collectively or separately, these factors can drive the overall distribution, diversity and behavior of primary producers, triggering cascading effects throughout the food web. We conclude by proposing areas for future research, particularly in regions where glaciers are soon projected to disappear.

Detrital input, productivity fluctuations, and water mass circulation in the westernmost Mediterranean Sea since the Last Glacial Maximum

By presenting sea surface temperatures, planktonic oxygen isotope profiles, and bulk geochemical composition of core sediments, we offer a multiparameter reconstruction of Western Mediterranean oceanography from the Last Glacial Maximum until the Middle Holocene (20,000–5000 cal years B. P.). Sediments from Ocean Drilling Program Site 975 in the Algero‐Balearic basin have been compared with three Alboran basin cores (TTR‐300G, TTR‐302G, and TTR‐304G), all of them investigated at high resolution. This multiproxy approach has allowed two different modes of circulation to be recognized: (1) during the LGM and from ∼8.0 cal. ka B. P. onward, no surface gradient in δ18OG. bulloides is found associated with low productivity, in close analogy to modern conditions; (2) during the Bølling‐Allerød and early Holocene, significant surface isotopic gradients are found with periods probably indicating an unstable water column, associated with enhanced productivity and low bottom oxygen conditions. The close synchrony between the occurrence of the surface isotopic offset and organic rich layer formation implicates that the origin of these features is linked, probably via shoaling of the regional thermohaline circulation. Paleo‐SSTs, derived from planktonic foraminifer assemblages, indicate abrupt changes in surface conditions during the analyzed time interval. Fluctuations in marine productivity based on Ba and total organic carbon are related to water column stability and atmospheric conditions. A sharp warming and δ18OG. bulloides excursion at the end of the Younger Dryas is probably linked to glacial meltwater influence. The riverine input has been reconstructed using the Mg/Al ratio, and Mg/Al peaks during arid periods (Greenland Stadial‐2a and Younger Dryas) related to “bypass” margin processes.

Late Holocene palaeoclimatic records from lake sediments on James Ross Island, Antarctica

Sediment profiles from several lakes/ponds in the northern, ice-free area of James Ross Island, Antarctica, were the subject of a multi-disciplinary palaeoenvironmental investigation. The results, obtained from lithological, geomagnetic, geochemical and diatom analyses, and the frequency of Branchinecta eggs, were evaluated with multivariate statistics and provide a fairly detailed picture of climate change during the last 5000 years. New radiocarbon dates are combined with previously published 14C dates, so as to date the palaeoenvironmental and palaeohydrological changes identified by the stratigraphic studies. In combination with the findings of glacial geological studies our data suggest that the Brandy Bay glacier began to recede rapidly just before 5000 yr B.P., due to calving and moisture starvation caused by cold and arid conditions. In one of the ponds studied gypsum began to precipitate as a consequence of these conditions when the influence of glacial meltwater ceased. This hypothesis was tested by hydrochemical modeling. At approximately 4200 yr B.P. the climate became more humid (and warmer) and the pond supported a fairly diverse biota, which included a variety of diatom species and, for example, Branchinecta gaini, a crustacean that feeds on benthic cyanobacterial mats. The Brandy Bay glacier probably advanced during this humid period, but the advance was interrupted by subsequent arid conditions which started at 3000 yr B.P. During this arid phase, glaciers became completely absent from the study area. As a result formerly large lakes, fed by glacial meltwater, transformed into small, enclosed, brackish water bodies that contained B. gaini. The arid and fairly cold conditions lasted for 1500–2000 years, until approximately 1200 yr B.P. when the aquatic systems expanded and their salinity was lowered as a result of increased snow accumulation and glacial expansion. The increased deposition of minerogenic matter caused by glacial erosion and meltwater inflow probably caused the disappearance of Branchinecta on James Ross Island. We regard this most recent phase as a period of increased humidity and warmth compared to the previous period, although the conditions appear to be less warm and humid than those which prevailed during the climatic optimum between 4200 and 3000 yr B.P. In spite of certain correlation problems we tentatively correlate our results to other studies of Holocene environments in Antarctica, and from that it can be suggested that the climatic optimum was of circumpolar significance. Therefore the climatic oscillations recorded in sediments on James Ross Island may reflect oscillations in the anticyclonic stormtracks and the strength of the high pressure cell over the inland ice sheet.

Sensitivity and response time of natural systems to climatic change in the late quaternary

Although the leading theory for the cause of climatic change — the Milankovitch perturbations in insolation — indicates variable rates of change at different latitudes, any discussion of the sensitivity and response time of different natural systems is most practical when the climatic change is considered to be relatively abrupt and global rather than gradual and regional. The oxygen-isotope record in glacial ice should have the swiftest response time, but the stratigraphic record in ice sheets at particular localities may be obscured or confused if the glacier flow is irregular. The oxygen-isotope stratigraphy of ocean sediment cores is a sensitive reflection primarily of contemporaneous ice volume, and to a lesser extent ocean temperature. Ocean temperature, however, is recorded more specifically by microfossils in ocean sediments and may reflect global insolation directly, except that in the North Atlantic the influence of glacial meltwater and various feedbacks may override the insolation factor. The differential lags in the response of isotopes and faunas to insolational change are apparent in the ocean sediment stratigraphy. Ice sheets themselves at their terminus may respond to climatic change only slowly if a change in snow accumulation is the factor, because of the time involved in building a thickness sufficient for flow to great distances, but if wastage near the margin is the factor then the response may be more rapid. Some glaciers, however, may advance rapidly without regard to climatic change, as in the case of surging, or they may retreat just as rapidly if they terminate in deep water, through iceberg formation. World sea level is depressed with glacial growth and thus reflects the volume of ice sheets, but isostatic changes in the crust complicate the sea level response not only locally beneath the ice load but elsewhere as well. The pluvial lakes of the American Southwest, correlated in their high levels with intervals of glaciation, show a direct response to climatic patterns resulting from the expansion of the North American ice sheets and thus may lag with respect to the primary climatic change that causes ice-sheet growth. North African lakes, on the other hand, respond to the intensity of monsoonal circulation, which is controlled directly by insolational shifts. The response of plants and insects to deglaciation can be immediate in the case of pioneers on newly exposed terrain, although the deglaciation itself may lag behind climatic change. Succession can follow rapidly, but then more slowly as other plants immigrate from great distances. It is heare where the more realistic assumption of continuing climatic change may be necessary to explain the dynamics of long-range vegetational change following the last deglaciation. The distribution and abundance of vertebrate populations reflect primarily their food resources, and the ‘disharmonious’ faunas of the last glacial period in North America — with components that no longer have overlapping ranges — perhaps may be explained by vegetational associations that likewise may have no modern analogues. The cause may be a different climatic seasonality. Wastage of the ice-sheet — itself lagging behind the climatic change — may have changed the seasonal climatic patterns sufficiently to bring about the precipitous extinction of many vertebrate species.