•Aragonite cements in all sediment cores beneath New Harbor are analyzed.•Cement patterns indicate diverse brine-forming conditions in outlet glacial valleys.•Brine formation further illustrates how ...climate and ice sheets varied over time.•Guide to identify similar cryogenic processes elsewhere is provided.
Cryogenic-brine precipitated carbonates (cryogenic-brine carbonates) have proven to be a valuable proxy for tracing the origin, nature, and distribution of Cenozoic brines that reside beneath McMurdo Sound (MMS) and adjacent McMurdo Dry Valleys (MDV), Antarctica. Of particular significance is the predominance of cryogenic-brine aragonite cements found in several stratigraphic sections in this region, reflecting the initial chemistry and conditions under which the brines formed. To fully explore this aspect, this paper presents a comprehensive examination of cryogenic-brine aragonite cements in all sediment cores recovered in New Harbor, an embayment that connects the MDV and MMS. Petrographic and isotopic analyses of the cements reveal a diverse range of brine-forming environments in the glacial outlet system during the middle Miocene Climatic Transition and the Early–Middle Pleistocene Transition. These environments span subglacial and ice-contact proglacial lakes to sub-ice-shelf fjords, occurring as cold-based, continent-sized Antarctic ice sheets expanded over the continental margin. Subsurface infiltration of the brines triggered almost instantaneous precipitation of aragonite cements below the sediment-water interface at low temperatures near 0 °C. Driven by the hydraulic gradient, the brines facilitated slow yet pervasive precipitation of an unusual form of aragonite characterized by coarse, blocky and poikilotopic morphologies—distinct from the typical acicular and fibrous aragonite dominating near-surface strata. The oxygen isotopic compositions of brines, as derived from aragonite precipitates, vary among different environments, displaying an increasing trend from ∼ –30 to –5 ‰ VSMOW toward MMS. The 18O depletion is primarily controlled by the availability of periodic saline water replenishment and the relative contribution of seawater versus meltwater during the cryoconcentration process. The ubiquity of brine-forming environments on the Antarctic continental margin suggests their potential existence during past ice ages, which may be identified by the presence of distinct cryogenic-aragonite cement patterns as documented in this study.
The Early Middle Pleistocene Transition (EMPT) is the term used to describe the prolongation and intensification of glacial–interglacial climate cycles that initiated after 900,000 years ago. During ...the transition glacial–interglacial cycles shift from lasting 41,000 years to an average of 100,000 years. The structure of these glacial–interglacial cycles shifts from smooth to more abrupt ‘saw-toothed’ like transitions. Despite eccentricity having by far the weakest influence on insolation received at the Earth's surface of any of the orbital parameters; it is often assumed to be the primary driver of the post-EMPT 100,000 years climate cycles because of the similarity in duration. The traditional solution to this is to call for a highly nonlinear response by the global climate system to eccentricity. This ‘eccentricity myth’ is due to an artefact of spectral analysis which means that the last 8 glacial–interglacial average out at about 100,000 years in length despite ranging from 80,000 to 120,000 years. With the realisation that eccentricity is not the major driving force a debate has emerged as to whether precession or obliquity controlled the timing of the most recent glacial–interglacial cycles. Some argue that post-EMPT deglaciations occurred every four or five precessional cycle while others argue it is every second or third obliquity cycle. We review these current theories and suggest that though phase-locking between orbital forcing and global ice volume may occur the chaotic nature of the climate system response means the relationship is not consistent through the last 900,000 years.
This review serves as an explanatory note to the sketch map of loessoids, i.e. loess and various loess-like deposits. The map is a byproduct of the ongoing project of Quaternary Map of the Russian ...Federation, scale 1:2,500,000 in GIS-format. The primary information is derived from the standard National Geological Map scale 1:1,000,000 of the last two generations supplemented by literary data. The map polygons present all mappable loessoid units, including thick southern carbonate loess with paleosols, loess-like silts of northern terrains, intercalations of loessic sediments with waterlain facies and icy silts of the Arctic and Subarctic type. The loessoid formations are classified according to their stratigraphic intervals: 1) the radiocarbon dated Upper Pleistocene of northern terrains, 2) the Middle plus Upper Pleistocene of central Russia, 3) the late Lower Pleistocene to Upper Pleistocene series of southern regions and 4) the Lower Pleistocene red clays of the south. The distribution of diverse loessoids is an imprint of: i) the diverse sedimentary environments of the basically arid periglacial zones of the Pleistocene and ii) the modern geographical zonality changing from subtropics to the icy High Arctic and responsible for post-depositional alteration of subaerial dust. Whatever the origin of the primary dust and geographical varieties of the diagenesis, all loessoids share the same fundamental feature – paleosols, whether numerous and expressive or scarce and weak. They underline the climatically driven intermittent process of loessoid formation – basically subaerial and independent of any aqueous agencies.
Early researchers of radiocarbon levels in Southern Hemisphere tree rings identified a variable North-South hemispheric offset, necessitating construction of a separate radiocarbon calibration curve ...for the South. We present here SHCal20, a revised calibration curve from 0–55,000 cal BP, based upon SHCal13 and fortified by the addition of 14 new tree-ring data sets in the 2140–0, 3520–3453, 3608–3590 and 13,140–11,375 cal BP time intervals. We detail the statistical approaches used for curve construction and present recommendations for the use of the Northern Hemisphere curve (IntCal20), the Southern Hemisphere curve (SHCal20) and suggest where application of an equal mixture of the curves might be more appropriate. Using our Bayesian spline with errors-in-variables methodology, and based upon a comparison of Southern Hemisphere tree-ring data compared with contemporaneous Northern Hemisphere data, we estimate the mean Southern Hemisphere offset to be 36 ± 27 14C yrs older.
This work showed that cave sediments are useful for geomorphologic studies and for reconstructing depositional environments. While the cave entrance facies have been extensively studied for their ...relationship with the fossil and archaeological record, the cave interior facies have received much less attention, although they can provide much information on the geomorphological evolution of the karst. This work presents the stratigraphic and sedimentological study of a section >6 m thick and 10 m long of cave interior sediments of Galería Complex (Sierra de Atapuerca, Spain). Galería Complex is a cavity infill of the Sierra de Atapuerca (Spain), composed of three sections filled by at least 30 m of Pleistocene sediments. This sequence is divided into 5 lithostratigraphic units named from bottom to top: GI – GV. GI unit is 19 m thick of interior facies in the base of the Galería Complex, divided into two sub-unit, GIa and GIb, by the Matuyama-Bruhnes paleomagnetic boundary. GI unit shows an issue with the chronology since has uncoherent between TT-OSL and ESR/U-series and paleomagnetism dates. This work has been done by combining field observation with laboratory sedimentary analysis to characterize the texture and structure of the sediments. Based on these studies, 12 layers and 9 sedimentary facies have been identified. The facies associations indicate a clear separation between GIa and GIb sub-units. GIa sub-unit is dominated by epiphreatic conditions and represents continuous relativity sedimentation during the Early Pleistocene; meanwhile, GIb shows important erosion events and facies with reworked materials that indicate vadose conditions during the Middle Pleistocene. This environmental change is related to the geomorphological evolution of the Arlanzón River. In addition, soft-sediment deformation structures have been described, including faults and low-angle folds. An important leaching process has been identified by the presence of phosphates that could explain the underestimated ages obtained in other works.
The Hadley Circulation and associated westerlies strengthened and moved equatorward across the mid‐Pleistocene transition (MPT). However, the evolution of the intertropical convergence zone (ITCZ) is ...still elusive due to the scarcity of long‐term hydrological records from regions sensitive to the ITCZ change. Here, high‐resolution sea surface salinity estimates derived from surface‐dwelling planktic foraminiferal δ18O and Mg/Ca in Ocean Drilling Program Site 871 reveal a long‐term freshening trend in the central equatorial Pacific across the MPT. We attribute this secular reorganization of the precipitation‐evaporation balance to the gradual southward migration and intensification of ITCZ in the North Pacific. It is inferred that the long‐term evolution of the ITCZ was modulated by the increased meridional sea surface temperature gradients and the enhancements of trade winds across the MPT.
Plain Language Summary
The Hadley circulation and westerlies have been revealed to strengthen and shift equatorward across the mid‐Pleistocene transition (MPT), yet how the ITCZ, the uplifting branches of the Hadley Circulation, changed remains elusive. As precipitation is a key indicator of the ITCZ, here we use the residual oxygen isotope (δ18O) of calcite, calculated from planktic foraminiferal δ18O and Mg/Ca, to reconstruct the sea surface salinity (SSS) change and track precipitation change in the central equatorial Pacific during the past ∼1,780 kyr. In contrast to the stable SSS condition in the far western part of the Western Pacific, our new SSS record shows a striking secular decreasing trend across the MPT and represents a hydrological transition from evaporation‐control to precipitation‐control state. We suggest that the long‐term hydrological reorganization was the result of the gradual southward migration and intensification of the North Pacific ITCZ across the MPT, which was closely related to the increased meridional sea surface temperature gradients and the enhancements of trade winds.
Key Points
Sea surface salinity (SSS) in the central equatorial Pacific since ∼1,780 ka was reconstructed by surface‐dwelling foraminiferal δ18O and Mg/Ca
The reconstructed SSS shows a long‐term freshening trend across the mid‐Pleistocene transition
The freshening trend primarily reflects the southward shift and intensification of the North Pacific intertropical convergence zone
A reliable chronology for the oldest settlements of Europe from the Early to the beginning of the Middle Pleistocene is an important requirement for understanding human evolution and especially the ...routes used to reach Western Europe from Africa. The paucity of sites, plus the limited number of available samples for dating, and the many contexts which do not favor the use of particular dating methods, further complicate the picture. This contribution addresses several questions: Which dating methods are relevant for establishing the chronology of the main sites? Are the first settlements older than the Jaramillo event? Which routes were used to enter Europe? Were the earliest human occupations intermittent or continuous?
•Contribution of dating methods to the oldest settlements in Europe during Pleistocene period.
The Thar Desert, located in western South Asia, marks a major global biogeographic boundary and a major adaptive threshold for the eastward expansions of modern humans from Africa across Asia. ...Examining the framework of palaeoenvironmental change in this region, both in terms of the regional manifestation of global climate change and the physical geography of the landscape, is therefore important to understand how modern humans first engaged with this significant shift in ecology. Here, we present evidence for the oldest chronometrically dated evidence for fluvial activity within this region, located at the Nal Quarry site in the central Thar Desert. We use luminescence dating of alluvial facies to demonstrate phases of fluvial activity at the site at ∼172–174, 140–150, 79–95 and 26 thousand years ago. This result substantially extends existing evidence for fluvial activity within the Thar Desert, as well as overlapping with evidence from the southern and eastern Thar desert indicating increased fluvial activity during the Last Interglacial (Marine Isotope Stage 5), whereas the cessation of fluvial deposition at Nal Quarry is contemporaneous with the onset of activity within the Ghaggar-Hakkra channel in the northern and western Thar Desert. Critically, the phases of fluvial activity identified at Nal overlaps with substantial behavioural change across South Asia, as well as the wider expansion of modern humans across the continent. This research illuminates a dynamic fluvial landscape that existed in the late Middle Pleistocene and early Late Pleistocene at a key threshold for modern human dispersals.
•We identify the deep Quaternary fluvial deposits in the central Thar Desert.•OSL dating constrains major episodes of fluvial activity to MIS 6 and 5.•Fluvial activity at Nal predates other fluvial deposits across the Thar Desert.•First ground truthing and dating of results of remote sensing studies.•Results highlight fluvial dynamism across Thar Desert at key threshold for human dispersal.
Choosing the most conservative technique to extract unequivocally identifiable foraminiferal tests is crucial to avoid biases in sedimentary sequence dating and paleoenvironmental interpretations. ...However, for problematic samples containing heavily encrusted specimens, the concentration and isolation of microfossils might be challenging. In this work, we analyzed Early-Middle Pleistocene samples from the International Ocean Discovery Program (IODP) Site U1460, located on the southwestern Australian shelf platform, characterized by extensive early marine diagenesis. At this site, foraminiferal preservation varies between the glacial and interglacial phases. In particular, tests are highly encrusted in samples representing sea-level lowstands of glacials while exhibiting better preservation in samples corresponding to interglacials. As the application of previously established, very conservative preparation techniques (e.g., sieving technique, soaking in H2O2 solution and gentle sonication) did not produce satisfactory results, it was necessary to set up a new procedure for foraminifera isolation specifically for the cautious cleaning of cemented benthic and planktic foraminiferal tests. This new methodology combines the use of a freeze-dryer with repeated soakings in highly-diluted H2O2 solution to disaggregate the material. To evaluate the efficiency of our technique objectively, we considered the improvements obtained with our procedure on the worst-preserved samples of our record (corresponding to glacials) and the best-preserved (associated with interglacials). Despite being more time-consuming than other preparation techniques, this newly developed procedure produces excellent results in samples exhibiting a high level of encrustation for reliable quantitative studies and isotope analysis on foraminiferal assemblages. Our new methodology is highly conservative and thus preserving even delicate taxa.
•Different laboratory methods are considered to isolate Early-Middle Pleistocene benthic and planktonic foraminifera.•A new method is set up to extract tests from highly-encrusted material for reliable quantitative and isotopic analyses.•The new technique allows the extraction of tests even from samples exhibiting high level of shallow marine diagenesis.
Interglacials of the last 800,000 years Berger, A.; Crucifix, M.; Hodell, D. A. ...
Reviews of geophysics (1985),
March 2016, Letnik:
54, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Interglacials, including the present (Holocene) period, are warm, low land ice extent (high sea level), end‐members of glacial cycles. Based on a sea level definition, we identify eleven ...interglacials in the last 800,000 years, a result that is robust to alternative definitions. Data compilations suggest that despite spatial heterogeneity, Marine Isotope Stages (MIS) 5e (last interglacial) and 11c (~400 ka ago) were globally strong (warm), while MIS 13a (~500 ka ago) was cool at many locations. A step change in strength of interglacials at 450 ka is apparent only in atmospheric CO2 and in Antarctic and deep ocean temperature. The onset of an interglacial (glacial termination) seems to require a reducing precession parameter (increasing Northern Hemisphere summer insolation), but this condition alone is insufficient. Terminations involve rapid, nonlinear, reactions of ice volume, CO2, and temperature to external astronomical forcing. The precise timing of events may be modulated by millennial‐scale climate change that can lead to a contrasting timing of maximum interglacial intensity in each hemisphere. A variety of temporal trends is observed, such that maxima in the main records are observed either early or late in different interglacials. The end of an interglacial (glacial inception) is a slower process involving a global sequence of changes. Interglacials have been typically 10–30 ka long. The combination of minimal reduction in northern summer insolation over the next few orbital cycles, owing to low eccentricity, and high atmospheric greenhouse gas concentrations implies that the next glacial inception is many tens of millennia in the future.
Key Points
We have reviewed the occurrence, strength, shape, and timing of interglacials
Despite spatial variability, MIS 5 and 11 stand out as strong/warm
The current interglacial is expected to be longer than any of those reviewed
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