Interglacial Time Periods Research Articles

How have recent temperature changes affected the efficiency of ocean biological carbon export?

AbstractThe ocean's large, microbially mediated reservoirs of carbon are intimately connected with atmospheric CO2 and climate, yet quantifying the feedbacks between them remains an unresolved challenge. Through an idealized mechanistic model, we consider the impact of documented climate change during the past few decades on the efficiency of biological carbon export out of the surface ocean. This model is grounded in universal metabolic phenomena, describing export efficiency's temperature dependence in terms of the differential temperature sensitivity of phototrophic and heterotrophic metabolism. Temperature changes are suggested to have caused a statistically significant decrease in export efficiency of 1.5% ± 0.4% over the past 33 yr. Larger changes are suggested in the midlatitudes and Arctic. This interpretation is robust across multiple sea surface temperature and net primary production data products. The same metabolic mechanism may have resulted in much larger changes e.g., in response to the large temperature shifts between glacial and interglacial time periods.

Glacial ocean circulation and stratification explained by reduced atmospheric temperature

Earth's climate has undergone dramatic shifts between glacial and interglacial time periods, with high-latitude temperature changes on the order of 5-10 °C. These climatic shifts have been associated with major rearrangements in the deep ocean circulation and stratification, which have likely played an important role in the observed atmospheric carbon dioxide swings by affecting the partitioning of carbon between the atmosphere and the ocean. The mechanisms by which the deep ocean circulation changed, however, are still unclear and represent a major challenge to our understanding of glacial climates. This study shows that various inferred changes in the deep ocean circulation and stratification between glacial and interglacial climates can be interpreted as a direct consequence of atmospheric temperature differences. Colder atmospheric temperatures lead to increased sea ice cover and formation rate around Antarctica. The associated enhanced brine rejection leads to a strongly increased deep ocean stratification, consistent with high abyssal salinities inferred for the last glacial maximum. The increased stratification goes together with a weakening and shoaling of the interhemispheric overturning circulation, again consistent with proxy evidence for the last glacial. The shallower interhemispheric overturning circulation makes room for slowly moving water of Antarctic origin, which explains the observed middepth radiocarbon age maximum and may play an important role in ocean carbon storage.

How to survive the glacial apocalypse: Hominin mobility strategies in late Pleistocene Central Asia

Previous research concerning the biogeography of hominin populations in Central Asia indicates persistence across interglacial/glacial sequences. Hominin groups are present on the landscape during the coldest episodes of the Last Glacial Period. Moreover, the Inner Asian Mountain Corridor (IAMC) likely served as a geographic conduit for human groups that found refuge in the foothill regions of the Altai Mountains as well as those of the southwestern horn of the Tien Shan; this conduit can be construed as the stage upon which hominin admixture occurred. The present study broadens the geographic focus of previous work to include the steppe and steppe/desert zones immediately adjacent to the biologically productive foothills of the IAMC. Using an ecological threshold model, four abiotic variables that best predict hominin site locations are analyzed to examine differences in fundamental niche structure when the IAMC foothills are compared to the adjacent steppic zones. Our null hypothesis is that the foothills and adjacent steppe present similar abiotic profiles. Our results, however, indicate significant differences between these regions, suggesting the foothills would have presented hominins with a more attractive landscape in both glacial and interglacial time periods than the steppe. Counterintuitively, these differences are actually more extreme during interglacial time periods. This preliminary model of hominin-environment interactions serves as a useful example for the ways by which mid-scale hominin dispersal trajectories are mapped and interpreted.

Empirical evidence of climate's role in Rocky Mountain landscape evolution

Climate may be the dominant factor affecting landscape evolution during the late Cenozoic, but models that connect climate and landscape evolution cannot be tested without precise ages of landforms. Zircon (U‐Th)/He ages of clinker, metamorphosed rock formed by burning of underlying coal seams, provide constraints on the spatial and temporal patterns of Quaternary erosion in the Powder River basin of Wyoming and Montana. The age distribution of 86 sites shows two temporal patterns: (1) a bias toward younger ages because of erosion of older clinker and (2) periodic occurrence of coal fires likely corresponding with particular climatic regimes. Statistical t tests of the ages and spectral analyses of the age probability density function indicate that these episodes of frequent coal fires most likely correspond with times of high eccentricity in Earth's orbit, possibly driven by increased seasonality in the region causing increased erosion rates and coal exhumation. Correlation of ages with interglacial time periods is weaker. The correlations between climate and coal fires improve when only samples greater than 50 km from the front of the Bighorn Range, the site of the nearest alpine glaciation, are compared. Together, these results indicate that the interaction between upstream glaciation and downstream erosion is likely not the dominant control on Quaternary landscape evolution in the Powder River basin, particularly since 0.5 Ma. Instead, incision rates are likely controlled by the response of streams to climate shifts within the basin itself, possibly changes in local precipitation rates or frequency‐magnitude distributions, with no discernable lag time between climate changes and landscape responses. Clinker ages are consistent with numerical models in which stream erosion is driven by fluctuations in stream power on thousand year timescales within the basins, possibly as a result of changing precipitation patterns, and is driven by regional rock uplift on million year timescales.

SHELL TAPHONOMY AND FIDELITY OF LIVING, DEAD, HOLOCENE, AND PLEISTOCENE LAND SNAIL ASSEMBLAGES

Variations in the taxonomic composition of ancient land snail assemblages can potentially reflect changes in past ecosystems. The use of fossil associations as a paleoenvironmental-paleoecological proxy assumes that the original biological signature is retained, but postmortem processes can distort it. In this study, the fidelity of land snail assemblages was tested by comparing taphonomic and ecological variables recorded by live and dead, middle Holocene and Upper Pleistocene land snail shelly assemblages from San Salvador Island (Bahamas). Shells of living organisms were practically unaltered whereas dead and fossil shells were primarily affected by fragmentation, ornament loss, color loss, and carbonate coating. Taphonomic features fluctuated across space and time likely due to variable environmental conditions and/or time of exposure prior to shell burial. Live assemblages showed good taxonomic agreement with dead assemblages, although the later exhibited a higher number of taxa and individuals than the former. Assemblages that were moderately (dead and Holocene) and strongly (Pleistocene) taphonomically altered did not differ in species abundances, suggesting that the original biological signal was preserved. In contrast, unaltered (live and some dead) assemblages differed taxonomically from moderately and strongly damaged assemblages, likely as a consequence of different scales of time-averaging rather than variable shell-specific destruction rates. Taxonomic richness and simple dominance of time-averaged land snail assemblages were similar at various interglacial time periods (∼125 ka, ∼5–6 ka, and today). Such apparently equivalent snail richness may suggest that the climatic-environmental and/or ecological conditions at those times were comparable to the present.

Late Quaternary environmental and landscape dynamics revealed by a pingo sequence on the northern Seward Peninsula, Alaska

A terrestrial sediment sequence exposed in an eroding pingo provides insights into the late-Quaternary environmental history of the northern Seward Peninsula, Alaska. We have obtained the first radiocarbon-dated evidence for a mid-Wisconsin thermokarst lake, demonstrating that complex landscape dynamics involving cyclic permafrost aggradation and thermokarst lake formation occurred over stadial–interstadial as well as glacial–interglacial time periods. High values of Picea pollen and the presence of Larix pollen in sediments dated to 50–40 ka BP strongly suggest the presence of forest or woodland early in MIS 3; the trees grew within a vegetation matrix dominated by grass and sedge, and there is indirect evidence of grazing animals. Thus the interstadial ecosystem was different in structure and composition from the Holocene or from the preceding Last Interglacial period. An early Holocene warm period is indicated by renewed thermokarst lake formation and a range of fossil taxa. Multiple extralimital plant taxa suggest mean July temperatures above modern values. The local presence of spruce during the early Holocene warm interval is evident from a radiocarbon-dated spruce macrofossil remain and indicates significant range extension far beyond the modern tree line. The first direct evidence of spruce in Northwest Alaska during the early Holocene has implications for the presence of forest refugia in Central Beringia and previously assumed routes and timing of post-glacial forest expansion in Alaska.

Model insight into glacial–interglacial paleodust records

We present a new method of analyzing model results to help identify the sensitivity of the location of different paleodust records to estimate local to regional scale variability of dust and climate variables. We use model simulations of global dust distribution from the last glacial maximum, preindustrial, current, and predicted future. The dust model has been previously shown to match available observational data for the current and last glacial maximum climate. Here, the model is compared to available source provenance data and is shown to agree with these limited observations. Using correlations and slopes across different time periods, the modeled relationships between deposition at specific observational sites and regional deposition and dustiness are shown. In addition, we evaluate the modeled relative slope of these cores to determine the location of paleodust sites that are especially easy to interpret as regional indicators of dustiness. Model predictions suggest that deposition in Antarctic ice cores is usually better than dust concentration to capture regional deposition and dustiness variability over glacial–interglacial time periods, in agreement with ice core interpretations. For Greenland, the model predicts a possible shift from dominantly wet deposition under modern conditions to dominantly dry deposition during glacial climate conditions indicating that deposition may be better suited to capture dustiness variability under LGM conditions in Greenland. The model also identifies specific regions that are not well covered by observations for glacial/interglacial or anthropocene dust variability. In addition, we evaluate the modeled relative slope of the location of these cores to determine regions that would provide ideal localities for pursuing records that would provide easily interpretable paleo-proxy records of regional dustiness.

Glacial cycling, large mammal community composition, and trophic adaptations in the Western Cape, South Africa

Some of the earliest evidence for modern human behavior has been recovered from the Western Cape Province, South Africa. Archaeological and paleontological sites in the Western Cape are typically described as “glacial” or “interglacial” in aspect based on the numbers of grazers found in the faunal assemblage, as glacial periods are often thought to have been characterized by spreading C 4 grasslands that replaced endemic C 3 shrubland vegetation found in the Western Cape today. Here, we test the hypothesis that glacial and interglacial time periods were associated with a predictable change in large mammal trophic adaptations by analyzing the proportions of grazing larger mammals from 118 levels of 15 Western Cape fossil assemblages sampling marine isotope stage (MIS) 6 to the present time to determine whether there is a change in composition in these communities that might reflect a shift in ecology and habitat. Our results indicate that trophic proportions did not significantly change over time in the Western Cape as a whole, and thus the hypothesis for habitat changes affecting the subsistence ecology of modern humans during the development of modern behavior is not supported. However, our results show that the southwestern subregion of the Western Cape was characterized by the presence of more grazing species through time than the western subregion. Thus, if ecological and population isolation during glacial periods were integral to catalyzing the development of modern behaviors in the Western Cape region of South Africa, then a complex model including the development of possible mosaic habitats is needed.

Modern stalagmite δ18O: Instrumental calibration and forward modelling

The δ 18O isotopic composition of stalagmite calcite frequently contains a climate signal, but one that is difficult to interpret in terms of either rainfall or temperature. Over glacial–interglacial time periods and decadal sampling resolution, stalagmite δ 18O clearly contains a ‘first order’ climate signal where the magnitude of changes in temperature and ocean/atmospheric circulation dominate the δ 18O signature. However, at annual–biennial resolution sampling, and within interglacial periods, the magnitude of climate changes is smaller, and variability in δ 18O introduced by soil, karst groundwater and cave processes can introduce considerable uncertainty into the climatic interpretation of stalagmite δ 18O records. Here, two approaches that can quantify the climate signal contained with stalagmite δ 18O are discussed. Firstly, linear regression based approaches are reviewed, which correlate stalagmite δ 18O with instrumental climate parameters such as temperature and rainfall. The advantages and disadvantages of complex linear regression approaches that attempt to account for groundwater mixing within the karst aquifer are discussed. Secondly, a forward modelling approach is introduced, where stalagmite δ 18O is modelled from rainfall δ 18O, surface climate parameters and a simple karst hydrology model. Using a case study from Gibraltar, this latter approach demonstrates that between stalagmite variability in δ 18O of the order of 1‰ can be explained solely by differences in karst hydrology. Forward modelling suggests that a similar variability in δ 18O might be observed between stalagmites within a cave, or between caves within a homogenous climate region, and highlights the difficulty in attempting to use δ 18O as a paleo-thermometer.

Isolation of the syndepositional magnetic susceptibility signals from loessic paleosols of China

Bulk magnetic susceptibility (MS) has been widely used to differentiate the loess and soil units, and acts as a useful proxy of the intensity of summer monsoon to reconstruct the paleoclimate on the Chinese Loess Plateau. Since soil development occurs not only at the actual ground surface but also to a greater depth, bulk MS is overprinted by the post-depositional pedogenesis and can only be regarded as representative of a time period, rather than a specific point in time. Therefore, the bulk MS record, as a naturally smoothed curve, can illustrate the low-frequency Milankovitch cycles, but may be unable to adequately resolve high-frequency sub-orbital climatic changes. Based on the pedosedimentary processes of soil formation, the Magnetic Susceptibility Variation (MSV) model is proposed to isolate the syndepositional enhancements of MS that may be directly linked to the climatic conditions of dust deposition. After an indirect test of the concept underlying the MSV model, using an atmospheric methane data set from Taylor Dome, Antarctica, we further verify the model directly from a loessic paleosol by the tentative correlation between the MSV record of Chinese loess at Luochuan profile and δ 18O of the NGRIP ice core within the last interglacial period. Our results show that MSV model may improve our understanding of widely used MS proxy itself, and climate variation within the interglacial time periods.

Glacial–interglacial variability of particle accumulation in the Canary Basin: a time-slice approach

Abstract Particle supply and sediment accumulation in the Canary Basin are strongly modified by seasonal coastal upwelling and dust input from the Sahara. Along an E–W depth profile and fertility gradient north of the Canary Islands, we demonstrate an increase in total mass accumulation rates from a low of 1–2 g cm−2 ka−1 in the outer oceanic domain to 6–12 g cm−2 ka−1 closer to the Moroccan shelf in Holocene sediments. During the Last Glacial Maximum (LGM, oxygen isotopic event 2.2), a steeper gradient was found, with two to four times higher mass accumulation rates at the near-shore sites. Total mass accumulation rates during the Last Interglacial Climax (LIC, isotopic event 5.5) and end of the Penultimate Glaciation (ePG, isotopic event 6.2) resemble Holocene values, with lowered near-shore maxima of 3 and 4 g cm−2 ka−1, respectively. Analyses of separate sediment components show that total organic carbon, total carbonate, lithogenic and diatom accumulation rates were generally enhanced during glacial times, especially during the LGM. In contrast, glacial accumulation rates of biogenic carbonate particles (coccoliths and planktic foraminifera shells) remained constant or were lowered with respect to interglacial time periods at the site closest to the Moroccan shelf, despite a doubling of bulk carbonate accumulation. This could indicate lowered supply from the productive euphotic zone or a latitudinal, offshore shift of the main sedimentary depocentre. However, enhanced carbonate dissolution could by itself account for this decrease in biogenic carbonate accumulation. Increased glacial input of detrital carbonate, derived from the exposed Moroccan shelf, appears to be responsible for the overall enhanced carbonate accumulation at the same sites, east of Lanzarote.

Age and isotopic constraints on Pleistocene pluvial episodes in the Western Desert, Egypt

Abstract North Africa has undergone drastic climatic changes over the past several hundred thousand years. The timing of humid intervals called pluvials was investigated by uranium-series disequilibrium dating of travertines from the Kurkur Oasis, Western Desert, Egypt. The youngest and best dated travertines (70–160 ka) are found in Wadi Kurkur and include spring and lacustrine units exposed as 2 to 3 m high terraces. Travertines having an age of approximately 191–220 ka are exposed by differential erosion as linear mounds produced by spring systems over fracture zones in ancient wadis. The oldest travertines, having ages >260 ka, are extensive, cap limestone units above the oasis, and were deposited in paludal and lacustrine environments. Oxygen isotope ratios were measured for the wadi travertines (δ18O values ranging from 16.7 to 19.1‰ SMOW) and for spring mound travertines (18.5–20.5‰). Equilibrium oxygen isotope fractionation calculations indicate that the Kurkur travertines were deposited from waters having δ18O values similar to ancient Western Desert groundwaters (∼ −11‰, Sonntag et al., 1978a,b; Thorweithe, 1990; Sultan et al., 1996). The ages of the travertines correspond to times of monsoonal maxima, eustatic sea level high stands and interglacial maxima. Rainfall producing these groundwaters (and travertines) was significantly fractionated during atmospheric transport, in contrast to modern meteoric waters (−2.09‰, Joseph et al., 1992), implying a distant source for the pluvial waters. Increased precipitation, recharge of Western Desert groundwaters, and resultant travertine deposition are interpreted to be consequences of Milankovitch insolation cycle forcing, through enhanced Atlantic and Indian Ocean monsoons during interglacial time periods.