Glacial Sea Research Articles

Shallow carbonate geochemistry in the Bahamas since the last interglacial period

The carbon isotope composition (δ13C) of ancient shallow-water carbonates frequently is used to reconstruct changes to Earth's global carbon cycle and to perform chemostratigraphic correlation. However, previous work demonstrates that local banktop processes also exert an important control on shallow carbonate δ13C as well as other isotope systems like δ18O. To effectively interpret ancient δ13C records, we must understand how both global carbon cycle perturbations and changes to local conditions are translated to the stratigraphic record. Modern environments, while imperfect analogues, can serve as a guide for interpreting physical and geochemical records of more ancient environments. Shallow carbonate strata from the most recent Pleistocene glacial cycles, which drove high-amplitude perturbations to sea level, temperature, and pCO2 without significantly altering the δ13C of global-mean seawater DIC, present an opportunity to begin untangling signals of global and local processes. However, the geochemistry of Pleistocene platform carbonates largely was overprinted by dissolution and meteoric diagenesis during glacial sea level lowstands. To understand how shallow carbonate geochemistry has changed during the Pleistocene, we instead look to the periplatformal slope and proximal basins. These deep environments serve as a refuge for carbonate produced on the shelf and exported to the slope, and contain a record of shallow carbonate geochemistry that persists across glacial cycles. We study 21 short piston cores from around Bahamian platforms to quantify differences in banktop production and geochemistry between the Holocene interglacial, the last glacial period, and the last interglacial (LIG) period. We show that mud production persists on the periplatformal slopes during the last glacial period, but differences in geochemistry between glacial and interglacial carbonates are a complex function of surface conditions and diagenesis. In contrast, Holocene and LIG carbonates show no evidence of post-depositional alteration, and offer the chance to study differences in δ13C and δ18O between interglacials. We find that while the δ13C of aragonite mud is the same during the Holocene and LIG, the LIG carbonate factory may have delivered more aragonite mud to the periplatform. In addition, the mean δ18O of this mud is elevated compared to the Holocene. We posit that these differences are caused by changes to regional climate and LIG surface conditions.

Glacial sea level low-stands regulated the upper ocean hydrology in the southern South China Sea over the past ∼ 280 kyr

The southern South China Sea (SCS) is an ideal region for investigating the low-latitude climatic and oceanic dynamics in the context of local and global effects. However, records of upper-water hydrography beyond the last two glacial cycles are still limited from the southern SCS. In this study, we present shell δ18O, Mg/Ca and Nd/Ca records of three planktonic foraminiferal species (Globigerinoides ruber, Globigerinoides sacculifer and Pulleniatina obliquiloculata) from Core TX05 to explore changes in upper-water hydrography in the southern SCS over the past ∼280 kyr. The results show that shell δ18O of the three species exhibit more positive values in glacials than interglacials. The δ18O residual (i.e. δ18Osw-ice) calculated by shell δ18O subtracting the signals of ice volume and temperature suggests fresher upper seawaters (lower salinity) during glacials than interglacials. This may be caused by terrestrial freshwater discharge from the paleo-Sunda continent that exposed during glacial sea level low-stands. The freshwater discharge may further be ascribed to relatively abundant precipitation over the paleo-Sunda continent during glacials, possibly pointing to the collective effects of the southward shift of the Intertropical Convergence Zone and the enhanced Pacific Walker circulation. The vertical thermal gradients (ΔTrub-obl and ΔTsac-obl) at Site TX05 show lower values during glacials than interglacials. The SST differences of the southern SCS from the northern SCS (ΔSSTS-N) and from the western tropical Pacific (ΔSSTSCS-WTP) correlate well with indicators from the Chinese Loess section, indicating relatively strong East Asian winter monsoon during glacials. The good correlation of ΔT, ΔSSTS-N and ΔSSTSCS-WTP to the relative sea-level suggests a remarkable effect of sea level low-stands and the strengthened East Asian winter monsoon during glacials on the upper ocean hydrography in the southern SCS.

Glacial changes in sea level modulated millennial-scale variability of Southeast Asian autumn monsoon rainfall

Most paleoclimate studies of Mainland Southeast Asia hydroclimate focus on the summer monsoon, with few studies investigating rainfall in other seasons. Here, we present a multiproxy stalagmite record (45,000 to 4,000years) from central Vietnam, a region that receives most of its annual rainfall in autumn (September-November). We find evidence of a prolonged dry period spanning the last glacial maximum that is punctuated by an abrupt shift to wetter conditions during the deglaciation at ~14 ka. Paired with climate model simulations, we show that sea-level change drives autumn monsoon rainfall variability on glacial-orbital timescales. Consistent with the dry signal in the stalagmite record, climate model simulations reveal that lower glacial sea level exposes land in the Gulf of Tonkin and along the South China Shelf, reducing convection and moisture delivery to central Vietnam. When sea level rises and these landmasses flood at ~14 ka, moisture delivery to central Vietnam increases, causing an abrupt shift from dry to wet conditions. On millennial timescales, we find signatures of well-known Heinrich Stadials (HS) (dry conditions) and Dansgaard-Oeschger Events (wet conditions). Model simulations show that during the dry HS, changes in sea surface temperature related to meltwater forcing cause the formation of an anomalous anticyclone in the Western Pacific, which advects dry air across central Vietnam, decreasing autumn rainfall. Notably, sea level modulates the magnitude of millennial-scale dry and wet phases by muting dry events and enhancing wet events during periods of low sea level, highlighting the importance of this mechanism to autumn monsoon variability.

A Modeling Perspective on the Lingering Glacial Sea Surface Temperature Conundrum

AbstractThe strong cooling during the Last Glacial Maximum (LGM, 21 ka BP) provides a rigorous test of climate models' ability to simulate past and future climate changes. We force an atmospheric general circulation model with two recent global LGM sea surface temperature (SST) reconstructions, one suggesting a weak and the other a more pronounced cooling, and compare the simulated land surface temperatures (LSTs) to reconstructed data. Our results do not confirm either SST reconstruction. The cold SST data set leads to good agreement between simulated and observed LSTs at low latitudes, but is systematically too cold at mid‐latitudes. The opposite is true for the warm SST data set. Differences between the simulated LSTs are caused by varying land surface albedos, which is lower for the warmer SST reconstruction. The inconsistency between reconstructed and simulated climate points to a potentially significant bias in the proxy reconstructions and/or the climate sensitivity of current climate models.

Sensitivity of coastal southern African climate to changes in coastline position and associated land extent over the last glacial

Paleoclimatic changes in South Africa, especially around the southern Cape region, are of intense interdisciplinary interest; as this is an important area in the context of human evolution, hosting several prominent archaeological sites such as Blombos Cave (located near today's shoreline). In this study, we investigate the sensitivity of climate of coastal southern Africa to changes in glacial variations in sea level and associated land extent, by performing a model sensitivity experiment. Employing a high-resolution (12 km grid spacing) regional climate model (WRF), output from a global model simulation at 70 thousand years ago (ka) is downscaled twice: First using the present-day coastline, then with an estimated coastline position at 70 ka; keeping all other forcing factors identical. Thus we focus on the response of the local climate to an expanded land surface area equivalent to a glacial sea level low stand scenario. Our results reveal that the climate of previously coastal localities shows strong continental characteristics when sea level drops, as the coastline moves away, and a coastal plain is exposed. This is most evident from the year-round warmer days and cooler nights, with up to 6 °C increases (decreases) in daily maximum (minimum) temperatures. This result also extends to extremes. For instance, at the archaeological site of Blombos Cave, temperature extremes (1st and 99th percentiles) of the modelled marine climate become 25- to 75-fold more probable as the coastline shifts. We also find year-round reductions (5–40%) in the amount of precipitation within the region, owing to local modifications in near-surface atmospheric circulation caused by the exposed land. The reductions in precipitation are accompanied by a significant drop in the annual number of rainy days (31 days locally at Blombos Cave). Simulated changes also vary seasonally, with more pronounced and widespread changes in temperature and precipitation occurring during summer. Through isolating and quantifying the effects of land extent variation, our approach demonstrates, for the first time, the role of coastline position in shaping local climate around near-coastal environs. Our results have significant implications for future studies exploring the influence of local coastline changes on early human settlement and mobility patterns.

Phylogeography of Mesophotic Coral Ecosystems: Squirrelfish and Soldierfish (Holocentriformes: Holocentridae)

Mesophotic coral ecosystems (MCEs: ~30 to 100+ m depth) may be older and more stable than shallow coral ecosystems that are more prone to disturbances in both the long term (glacial sea level cycles) and short term (heavy weather and anthropogenic activities). Here, we assess the phylogeography of two MCE fishes, the soldierfish Myripristis chryseres (N = 85) and the squirrelfish Neoniphon aurolineatus (N = 74), with mtDNA cytochrome oxidase C subunit I. Our goal is to resolve population genetic diversity across the Central and West Pacific and compare these patterns to three shallow-reef species in the same taxonomic family (Holocentridae). Significant population structure (ΦST = 0.148, p = 0.01) was observed in N. aurolineatus, while no structure was detected in M. chryseres (ΦST = −0.031, p = 0.83), a finding that matches the shallow-water congener M. berndti (ΦST = −0.007, p = 0.63) across the same range. Nucleotide diversity in the MCE fishes was low (π = 0.0024–0.0028) compared to shallow counterparts (π = 0.003–0.006). Coalescence times calculated for M. chryseres (~272,000 years) and N. aurolineatus (~284,000 years) are more recent or comparable to the shallow-water holocentrids (~220,000–916,000 years). We conclude that the shallow genetic coalescence characteristic of shallow-water marine fishes cannot be attributed to frequent disturbance. We see no evidence from holocentrid species that MCEs are older or more stable habitats.

Coupled Oceanic and Atmospheric Controls of Deglacial Southeastern South America Precipitation and Western South Atlantic Productivity

Various mechanisms were proposed as substantial drivers of (sub)tropical South American hydroclimate changes during the last deglaciation. However, the interpretation of past precipitation records from the regions affected by the South American Summer Monsoon, the dominant hydroclimatic system in (sub)tropical South America, still insufficiently consider feedbacks between oceanic and atmospheric processes evident in modern observational data. Here, we evaluate ocean-atmosphere feedbacks active in the region from 19 to 4 ka based on a multi-proxy record comprising lipid biomarker, bulk sediment elemental composition and foraminiferal geochemistry from a sediment core retrieved from the tropical western South Atlantic offshore eastern Brazil at ~22°S. Our proxy data together with existing paleoclimate records show that the consideration of large scale synoptic climatic features across South America is crucial for understanding the past spatio-temporal rainfall variability, especially during the last deglaciation. While the paleohydrological data from our study site show relatively stable precipitation across the deglaciation in the core region of the South Atlantic Convergence Zone, distinct hydroclimatic gradients developed across the continent during Heinrich Stadial 1, which climaxed at ~16 ka. By then, the prevalent atmospheric and oceanic configuration caused more frequent extreme climatic events associated with positive rainfall in the northern portion of eastern South America and in the southeastern portion of the continent. These climatic extremes resulted from substantial warming of the sub(tropical) western South Atlantic sea surface that fostered oceanic moisture transport towards the continent and the reconfiguration of quasi-stationary atmospheric patterns. We further find that enhanced continental precipitation in combination with low glacial sea level strongly impacted marine ecosystems via enhanced terrigenous organic matter input in line with augmented nutrient release to the ocean. Extreme rainfall events similar to those that occurred during Heinrich Stadial 1 are likely to recur in South America as a consequence of global warming, because the projected reduction of the intra-hemispheric temperature gradient may lead to the development of atmospheric patterns similar to those in force during Heinrich Stadial 1.

Development of the Nile Littoral Cell during the past 8.2 kyr

The depositional history of the Nile Littoral Cell (NLC) in the past 8.2 kyr was reconstructed from two radiocarbon-dated cores retrieved down-flow the Nile Delta at the southeastern Mediterranean-Levantine continental shelf. The two cores are located ∼80 km apart at the inner shelf off-shore Israel: core V115 at a southern proximal site and core V101 at a northern distal site. Grain-size distribution, chemical composition, and εNd and 87Sr/86Sr isotope ratios and marine derived variables such as carbonate content, total organic carbon and its δ13C composition record the temporal and spatial variations in transport processes and sediment provenance.From ∼8.2 to ∼5.5 ka BP the record is dominated by coarse quartz sands from the Nile Delta front that were recycled along the NLC due to shelf transgression along with the post glacial sea level rise. Oligotrophic conditions prevailed in the inner shelf, and the relatively high carbonate content reflects the decrease in Nilotic fine particles due to increased vegetation and decrease in surface erosion related to the African Humid Period (AHP) wetter conditions. The chemical and isotope signature of the fine fractions indicates mostly remobilization of dust-driven surface cover soils from the surrounding lands that originated from the granitoid terrains of the Sahara-Arabia deserts.Sea level stabilization, termination of the AHP at ∼5.5 ka BP and the partial drying of the Ethiopian Highlands resulted in the increase of fine sediments with a basaltic signature. After ∼5.5 ka BP, fine sediments were supplied seasonally by the Nile, creating a distinct mud belt along the Israeli coast and Nile Delta at water depth exceeding ∼35 m. Occasional sediment coarsening in the distal NLC likely reflects periodical decrease in Nile floods discharge or contribution from shallower water depth due to increased storminess. With the continuation of the regional aridification and decreasing intensity of the Nile River flow during the last 2 kyr, higher contribution of surface cover particles from the nearby lands was recorded in the distal northern site, while the southern site continued to be affected mostly by the Nilotic contribution.A distinct increase in grain size and carbonate content, and decrease in TOC content and δ13Corg values at the topmost part of the sedimentary record represent the Nile River damming during the modern period, which resulted in halting of the Nile fine suspension and lead to ultra-oligotrophic conditions in the Levantine basin.

Les faunes terrestres quaternaires des Îles du Japon

This paper explains a brief history of research on Quaternary terrestrial mammals in Japan and fossil occurrence of taxa at major localities of each time period at first. Based on these data, then, the changing history of Quaternary terrestrial mammals in Japan is reviewed, especially on such points as (1) importance of strait in western Japan as coming over route, (2) changing history of Plio-Pleistocene terrestrial mammals in Japan, (3) two faunas during the Late Pleistocene, (4) dates of extinctions of large mammals near the end of the Late Pleistocene. Formation process of Quaternary terrestrial mammal fauna in Japan must have been affected by condition of land connection with Asian continent at seaway west to Japan and changing history of climate and vegetation in East Asia, and it should be considered by different time period, e.g. before and after ca. 1.7 Ma. The time period after 1.7 Ma is one affected by glacial sea level fluctuations. Although a new fauna came over from the west to Japan when sea level was low, this time period is basically the age of insularization, and it is presumed that the fauna was becoming endemic during this time period. During the late Late Pleistocene, there were two faunas existed in Japan, e.g. one with Palaeoloxodon naumanni having come over from the west and another with Mammuthus primigenius having come over from the north. The former dwelled mainly in deciduous broad-leaved forest and mixed forest with conifers in temperate climate, while the latter dwelled in steppe and coniferous forest in subboreal climate. Both faunas changed their ranges to north and south repeatedly as climate changed, and the extinction of large mammals of the fauna with Palaeoloxodon naumanni occurred at the same time of the beginning of the LGM (ca. 25–16 ka). On the other hand, large mammals of the fauna with Mammuthus primigenius became extinct or moved to north as climate became warm quickly after the LGM. Thus, it is suggested that the extinction of large mammals at the late Late Pleistocene occurred by “two pulses.” The extinction process of large mammals in Japan seems likely that they went extinct finally near the end of the Pleistocene going through the reduction of habitat and fragmentation of populations caused by the repeated temperature change during the late Late Pleistocene, rather than a single drastic event.

Ice-sheet melt drove methane emissions in the Arctic during the last two interglacials

AbstractCircum-Arctic glacial ice is melting in an unprecedented mode, and release of currently trapped geological methane may act as a positive feedback on ice-sheet retreat during global warming. Evidence for methane release during the penultimate (Eemian, ca. 125 ka) interglacial, a period with less glacial sea ice and higher temperatures than today, is currently absent. Here, we argue that based on foraminiferal isotope studies on drill holes from offshore Svalbard, Norway, methane leakage occurred upon the abrupt Eurasian ice-sheet wastage during terminations of the last (Weichselian) and penultimate (Saalian) glaciations. Progressive increase of methane emissions seems to be first recorded by depleted benthic foraminiferal δ13C. This is quickly followed by the precipitation of methane-derived authigenic carbonate as overgrowth inside and outside foraminiferal shells, characterized by heavy δ18O and depleted δ13C of both benthic and planktonic foraminifera. The similarities between the events observed over both terminations advocate for a common driver for the episodic release of geological methane stocks. Our favored model is recurrent leakage of shallow gas reservoirs below the gas hydrate stability zone along the margin of western Svalbard that can be reactivated upon initial instability of the grounded, marine-based ice sheets. Analogous to this model, with the current acceleration of the Greenland ice melt, instabilities of existing methane reservoirs below and nearby the ice sheet are likely.

Holocene paleoenvironmental changes in the marginal marine basin of Great Rann of Kachchh, western India: Insights from sedimentological and mineral magnetic studies on a ∼60 m long core

We carried out sedimentological and mineral magnetic studies on a ∼60 m long core recovered from the central part of the Great Rann of Kachchh (GRK), a marginal marine basin, located on the western continental margin of India to understand the Holocene paleoenvironmental changes. We critically analysed the sedimentation pattern, sediment characteristics along with paleoclimatic signatures of global and regional relevance. In GRK basin, the sedimentation started in early Holocene (∼10.6–9.3 kyr BP) under rapidly rising post glacial sea level with very high sedimentation rate (8.71 cm/yr to 2.37 cm/yr) which is also seen in several marginal marine basins across the globe. The abundant laminations, coarse grained sediment flux and environmental magnetic parameters (χlf, S-ratio) suggests that the sedimentation occurred in developing monsoonal conditions under wetter yet fluctuating climate during this period. After ∼9 kyr BP, the environmental magnetic proxies show transition toward relatively wetter condition with peak at ∼6.5 kyr BP. This wetter climatic phase on the other hand corroborates with reduction of sedimentation rate to 0.46 cm/yr. This reduction in sedimentation rate under strengthening monsoonal conditions probably occurred due to rapid filling up of the basin on account of earlier high sedimentation in the basin. The sedimentation rate is reduced significantly after the onset of aridity at ∼4 kyr BP except a minor wet spell recorded during ∼1.5 to 1 kyr BP before drying of the GRK basin.

Simulated stability of the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

Abstract. The response of the Atlantic Meridional Overturning Circulation (AMOC) to freshwater perturbations critically depends on its mean state. Large swaths of icebergs melting in the North Atlantic during the last deglaciation constituted such perturbations and can, thus, provide important constraints on the stability of the AMOC. However, the mean AMOC state during the Last Glacial Maximum (LGM), preceding the rapid disintegration of the ice sheets during the deglaciation, as well as its response to these perturbations remain debated. Here, we investigate the evolution of the AMOC as it responds to freshwater perturbations under improved LGM boundary conditions in the Bern3D intermediate complexity model. Particularly, we consider the effect of an open versus a closed Bering Strait and the effect of increased tidal dissipation as a result of the altered bathymetry due to the lower glacial sea level stand. The vigorous and deep AMOC under these glacial boundary conditions, consistent with previous simulations with different models, reacts more strongly to North Atlantic freshwater forcings than under preindustrial conditions. This increased sensitivity is mostly related to the closed Bering Strait that cuts off the freshwater escape route through the Arctic into the Pacific, thereby facilitating faster accumulation of freshwater in the North Atlantic and halting deep-water formation. Proxy reconstructions of the LGM AMOC instead indicate a weaker and possibly shallower AMOC than today, which is in conflict with the particularly strong and deep circulation states coherently simulated with ocean circulation models for the LGM. Simulations with reduced North Atlantic deep-water formation, as a consequence of potentially increased continental runoff from ice sheet melt and imposed changes in the hydrological cycle, more closely resemble the overturning circulation inferred from proxies. These circulation states also show bistable behavior, where the AMOC does not recover after North Atlantic freshwater hosing. However, no AMOC states are found here that either comprise an extreme shoaling or vigorous and concurrent shallow overturning as previously proposed based on paleoceanographic data.

Environmental and coastline changes controlling Holocene carbon accumulation rates in fjords of the western Strait of Magellan region

Organic-rich sediments of the southernmost Chilean Pacific coast and its fjord system constitute an important component of the global marine carbon budget. Sediment records from Trampa and Caribe bays and Churruca fjord in the western Magellan fjord system have been analyzed with the goal of understanding the factors controlling carbon accumulation and its regional fluctuation throughout the Holocene. The individual response in paleoproductivity at the different sites and related variations in accumulation rates document a very complex interplay among local and regional-scale environmental changes, and coastline elevation across the Holocene. Shallow sill basins close to the Pacific coast, as the ones studied here, are particularly sensitive to these processes, having responded with strong productivity changes throughout the Holocene.A Bayesian mixed model approach, using sediment archived provenance proxies, indicates that components of terrestrial plants and soils washed-out into these basins contribute with a variable proportion (20–80 wt%) of the total accumulated organic carbon. Accumulation rates of terrestrial carbon increase with the amount of precipitation in the hyper-humid mountain area, but also reflect distinct Holocene plant successions as well as long-term development of soil and vegetation cover that strongly overprint the direct precipitation impact. Over the Holocene accumulation rates of biogenic carbonate and aquatic-marine organic carbon range between 5 and 118 kg m−2 kyr−1 and 0.3–20 kg m−2 kyr−1, respectively. This variability depends on water column structure and conditions, which are regulated by the degree of marine transgression as a function of post glacial sea level rise and isostatic uplift as well as precipitation-related surface water freshening. In the Bahia Trampa record, a significant change in accumulation rates indicates a marine transgression at ca. 12.2 kyr BP, when the global sea level was 60–70 m lower than today and eustatic rise overcame isostatic rebound rates. In Caribe and Trampa records, CaCO3 accumulation rates were higher at ca. 7 kyr BP. The Churruca record shows organic carbon accumulation rates up to 36.2 kg m−2 kyr−1 during the early Holocene.

Evidence of Sundaland’s subsidence requires revisiting its biogeography

AbstractIt is widely accepted that sea level changes intermittently inundated the Sunda Shelf throughout the Pleistocene, separating Java, Sumatra and Borneo from the Malay Peninsula and from each other. On this basis, the dynamics of the biodiversity hotspot of Sundaland is consistently regarded as solely contingent on glacial sea level oscillations, with interglacial highstands creating intermittent dispersal barriers between disjunct landmasses. However, recent findings on the geomorphology of the currently submerged Sunda shelf suggest that it subsided during the Pleistocene and that, over the Late Pliocene and Quaternary, is was never submerged prior to Marine Isotope Stage 11 (MIS 11, 400 ka). This would have enabled the dispersal of terrestrial organisms regardless of sea level variations until 400 ka and hampered movements thereafter, at least during interglacial periods. Existing phylogeographic data for terrestrial organisms conform to this scenario: available divergence time estimates reveal an 8‐ to 9‐fold increase in the rate of vicariance between landmasses of Sundaland after 400 ka, corresponding to the onset of episodic flooding of the Sunda shelf. These results highlight how reconsidering the paleogeographic setting of Sundaland challenges understanding the mechanisms generating Southeast Asian biodiversity.

High-resolution seismic imaging reveals infill history of a submerged Quaternary fjord system in the subantarctic Auckland Islands, New Zealand

AbstractQuaternary processes and environmental changes are often difficult to assess in remote subantarctic islands due to high surface erosion rates and overprinting of sedimentary products in locations that can be a challenge to access. We present a set of high-resolution, multichannel seismic lines and complementary multibeam bathymetry collected off the eastern (leeward) side of the subantarctic Auckland Islands, about 465 km south of New Zealand's South Island. These data constrain the erosive and depositional history of the island group, and they reveal an extensive system of sediment-filled valleys that extend offshore to depths that exceed glacial low-stand sea level. Although shallow, marine, U-shaped valleys and moraines are imaged, the rugged offshore geomorphology of the paleovalley floors and the stratigraphy of infill sediments suggests that the valley floors were shaped by submarine fluvial erosion, and subsequently filled by lacustrine, fjord, and fluvial sedimentary processes.

Diagenesis and reservoir quality of carbonates rocks and mixed siliciclastic as response of the Late Carboniferous glacio-eustatic fluctuation: A case study of Xiaohaizi Formation in western Tarim Basin

Abstract Late Carboniferous-Permian was a longest-lived and most widespread ice age. The depositional environment, diagenesis and reservoir quality of carbonate rocks as response to the glacio-eustatic changes was analyzed in this study. The Upper Carboniferous Xiaohaizi Formation in the western Tarim Basin is an important oil reservoir. The dolostones from the Xiaohaizi Formation have very good physical properties, with maximum porosity and permeability of 16.6% and 214mD, respectively. The sedimentary facies of the Xiaohaizi Formation consist of carbonate shoals and lagoon on a mixed siliciclastic-carbonate platform. There was a strong heterogeneity of carbonate rocks deposited on shoals. Porous dolostones always alternate with tight sparry allochemical limestones. Based on the petrographic observation and stable isotope measurement, the controlling factors for the heterogeneity and reservoir quality were revealed. The palaeosol, vadose silt, karst breccia, detrital kaolinite, darkening of dolostones and enrichment of 18O indicate the short-lasting subaerial emergence during the glacial period. During the glacial sea level fall, the mixed siliciclastic and carbonate allochems deposited on bioclastic and intraclastic shoals, while dolomitization of calcic grains occurred under a relatively low-temperature. Through the correlation analysis of physical property, lithology, rock components, it's suggested that the reservoir quality of the Xiaohaizi Formation was controlled by mixed sedimentation and dolomitization associated with glacio-eustatic fluctuation. At the burial diagenesis, the effect of hydrothermal fluid on physical properties of dolostones with relative high primary porosity was positive, whereas resulted in the poikilotopic calcite cementation. The assembly of fluorite, dickite and authigenic pyrite were indicators for hydrothermal fluid accumulation. The control of glacio-eustatic fluctuations on sedimentation and diagenesis in carbonates could be used to forecast the reservoir quality.

Water Mass Versus Sea Level Effects on Benthic Foraminiferal Oxygen Isotope Ratios in the Atlantic Ocean During the LGM

AbstractDepth transects of benthic foraminiferal oxygen isotopes from the Atlantic Ocean show that glacial‐interglacial changes are larger at deep (> ~2,000 m) than at intermediate water levels. Our model results suggest that the smaller changes in the upper 1,000 m of the water column are a result of the glacial sea level lowering of about 120 m, leading to warmer temperatures of around 1 °C and hence a smaller glacial‐interglacial stable oxygen isotope difference. In contrast, a shoaling of the water mass boundary to ~2,000‐m water depth between the northern and southern source waters is caused by the expansion of a cold (close to the freezing point) southern source water in the abyssal ocean, increasing the oxygen isotope values of benthic foraminifera from the Last Glacial Maximum in the deep Atlantic. These two effects explain the different amplitudes of glacial‐interglacial stable oxygen isotope differences in the upper and deeper water column of the Atlantic Ocean.

First evidence for onshore marine isotope stage 3 aeolianite formation on the southern Cape coastline of South Africa

The southern Cape coast of South Africa boasts an impressive suite of Plio-Pleistocene aeolian dune deposits (aeolianite). Previous research has shown that in this region onshore dune accumulation was generally focused around interglacial sea level highstands, with the locus of coastal dune accumulation shifting onto the adjacent continental shelf during glacial sea level lowstands. Here, using new luminescence dating results, we present the first evidence for preserved onshore glacial age dunes. Specifically, on the Robberg Peninsula, a rocky headland 28 km east of Knysna, two phases of aeolianite formation are identified, corresponding to early (45–60 ka) and late (35–30 ka) marine isotope stage (MIS) 3. Subsequently, during the Holocene, all substantive dune accumulation occurred between 10.2 and 7.0 ka, forming cliff-fronting dunes and filling the limited accommodation space on the headland, including an archaeological rock-shelter. Combining these ages with bathymetric data, we infer that this distinct onshore glacial age aeolianite record reflects: 1) restricted accommodation space during sea level highstands; 2) a regional narrowing of the continental shelf, and 3) liberation of sediments lying on a prominent −45 to −60 m offshore terrace, which would have been exposed during MIS 3. This demonstrates that despite broad regional-scale trends in the timing of coastal aeolian activity - driven by commonalities in relative sea level trends and climate - distinct local variations in late Quaternary coastal evolution can be identified. This is ascribed to local controls on preservation (accommodation space) and sediment supply (shoreline position and antecedent offshore sediment supplies). Such findings may have wider implications for interpretations of site context/resource availability at several notable coastal archaeological sites, and more broadly suggest that local offshore or onshore geologic contexts can at times assume greater influence on a preserved coastal aeolianite record than the regional-scale trends in sea level and climate.

Geomorphological evolution of the Rimac River’s alluvial fan, Lima, Peru

The alluvial fan of Lima is a complex landform, resulting from the sediment contributions of the Rimac River and the coalescence of the alluvial fans of the tributaries of the Rimac River. Depositional zones in the fan and changing main channel and distributary channels are influenced by the palaeo-relief inherited from a semi-arid climate and by the climatic changes. The upper sedimentary sequence of the fan, dominant on the Costa Verde, is of Upper Pleistocene—Holocene age. The sediments forming it are non-cohesive and are highly mobile during floods and earthquakes. The dominant features in this sequence, intertwined channel facies and laminar flows, were influenced by the Pleistocene—Holocene postglacial marine transgressions. A deeper understanding of the evolution of the Lima alluvial fan provides insight in to the fan’s future evolution in the framework of active tectonics and climate change. The Lima fan is an area with high human population density and is subjected to floods and debris flows resulting in subsequent loss of human life and properties. Therefore, the improved understanding of the fan’s evolution, resulting from this study, will contribute to a better definition of high risk areas of potential human disaster caused by these natural processes. Cyclic-fan-development, presently controlled by glacial sea level lows and palaeo-topography will continue regardless of human intervention in attempts to prevent natural disasters in Lima.

Greenland records of aerosol source and atmospheric lifetime changes from the Eemian to the Holocene

The Northern Hemisphere experienced dramatic changes during the last glacial, featuring vast ice sheets and abrupt climate events, while high northern latitudes during the last interglacial (Eemian) were warmer than today. Here we use high-resolution aerosol records from the Greenland NEEM ice core to reconstruct the environmental alterations in aerosol source regions accompanying these changes. Separating source and transport effects, we find strongly reduced terrestrial biogenic emissions during glacial times reflecting net loss of vegetated area in North America. Rapid climate changes during the glacial have little effect on terrestrial biogenic aerosol emissions. A strong increase in terrestrial dust emissions during the coldest intervals indicates higher aridity and dust storm activity in East Asian deserts. Glacial sea salt aerosol emissions in the North Atlantic region increase only moderately (50%), likely due to sea ice expansion. Lower aerosol concentrations in Eemian ice compared to the Holocene are mainly due to shortened atmospheric residence time, while emissions changed little.