Marine Isotope Stage Research Articles

Glacial expansion of carbon-rich deep waters into the Southwestern Indian Ocean over the last 630 kyr

Oceanic carbon storage is one of the main sinks for atmospheric CO2, and thought to be the major contributing factor for CO2 drawdown during past glacial periods. Both physical and biogeochemical processes control the capacity of carbon storage in the ocean. During glacial periods of the Pleistocene the larger volume of deep-water masses of Southern Hemisphere origin in the Atlantic has been shown to promote carbon storage in the Southern Ocean. However, the latitudinal extension of this water mass in the Indian Ocean has been scarcely studied. In this study, we combine foraminiferal εNd and benthic δ13C of two sediment cores in the southwest Indian Ocean (MD96–2077, 33°S, 3781 m water depth; MD96–2052, 19°S, 2627 m water depth), to reconstruct the spatial and temporal evolution of glacial carbon-rich deep waters in the SW Indian over the last 630 kyr. The combined use of foraminiferal εNd and benthic δ13C allows to distinguish δ13C changes related to water mass mixing and from respired carbon accumulation within the water masses. Nutrient-rich deep waters, which cannot be explained by the enhanced proportion of southern-sourced waters, were present at core sites deeper than 2700 m during glacial periods and extended at least until 33°S into the SW Indian Ocean. From Marine Isotope Stage (MIS) 14 to MIS 10, glacial carbon storage increased gradually until reaching its highest capacity during the extreme glacial periods MIS 12 and 10. Orbital forcing (100-kyr eccentricity, 41-kyr obliquity), restricted air-sea exchange and enhanced ocean stratification, fostered higher carbon storage during periods of relatively lower eccentricity and obliquity. Furthermore, after MIS 10, a progressive transition was observed from 100-kyr eccentricity to 41-kyr obliquity cycles in benthic δ13C and δ18O records of core MD96–2077 and sea-ice cover changes derived from ice-rafted debris of the Agulhas Plateau composite core site.

Moisture control of tropical cyclones in high-resolution simulations of paleoclimate and future climate

The intensity of tropical cyclones (TCs) is expected to increase in response to greenhouse warming. However, how future climate change will affect TC frequencies and tracks is still under debate. Here, to further elucidate the underlying sensitivities and mechanisms, we study TCs response to different past and future climate forcings. Using a high-resolution TC-resolving global Earth system model with 1/4° atmosphere and 1/10° ocean resolution, we conducted a series of paleo-time-slice and future greenhouse warming simulations targeting the last interglacial (Marine Isotope Stage (MIS) 5e, 125 ka), glacial sub-stage MIS5d (115 ka), present-day (PD), and CO2 doubling (2×CO2) conditions. Our analysis reveals that precessional forcing created an interhemispheric difference in simulated TC densities, whereas future CO2 forcing impacts both hemispheres in the same direction. In both cases, we find that TC genesis frequency, density, and intensity are primarily controlled by changes in tropospheric thermal and moisture structure, exhibiting a clear reduction in TC genesis density in warmer hemispheres.

The Pauzhetka tephra (South Kamchatka): A key middle Pleistocene isochron for the Northwest Pacific and Okhotsk Sea sediments

The distal Pauzhetka tephra, formed by a large caldera-forming volcanic eruption in South Kamchatka, has been identified in eleven recently recovered marine sediment cores based on major and trace element compositions of tephra glass. Ten SO264 cores form a transect along the Emperor Seamount Chain (ESC) in the Northwest (NW) Pacific between ∼50.3° and ∼45°N, 800–1200 km southeast of the Pauzhetka caldera. One additional core LV28-41-4 was retrieved in the Okhotsk Sea, ∼600 km west of the caldera. The Pauzhetka tephra glass shards have a characteristic medium-K rhyolite composition and trace element content compatible with the rear-arc position of the source volcano that ensures their identification. In the NW Pacific SO264 cores, the tephra is preserved as layers in cores 33, 47, 49, 53, 55, 56 and 62, as a lens in core 45, and as cryptotephra in cores 57 and 66. It forms a cryptotephra in the Okhotsk Sea core LV28-41-4. Distinctively high XRF-retrieved K/Ti and K/Fe ratios compared to those for the host sediments help identify the Pauzhetka tephra. According to our refined stable oxygen isotope (δ18O)- and magneto-stratigraphy of two studied and two reference cores, the Pauzhetka tephra occurs within a local δ18O maximum during a transition from marine isotope stage 12 to 11c (Termination V) and below a paleointensity minimum referred to as the Bermuda excursion, at ca. 418 ka. Using the tephra age as an isochron, we show that average linear sedimentation rates decrease southward along a transect of the SO264 cores, except in core 55. It partially reflects an intensification of mid-depth currents causing winnowing, erosion or non-deposition along the ESC over the past 418 kyr. An increased linear sedimentation rate in core 55, recovered from the southern leeward side of the Minnetonka Seamount, appears to record the pelagic accumulation protected from the mid-depth current influence. Our findings expand the former ash dispersal area farther southeast in the NW Pacific and southwest in the Okhotsk Sea. The new data on the tephra thickness supports the axis direction of the fallout zone southeast of the Pauzhetka caldera. Our results suggest the Pauzhetka tephra as a key middle Pleistocene isochron for the stratigraphy and correlation of the NW Pacific and Okhotsk Sea sediments.

Southern ocean response to glacial and interglacial forcing

The Southern Ocean is particularly vulnerable to climate change. Atmospheric CO2 fluctuations have been highly correlated with climate on million years time scales or caused or at least amplified, the climatic change. Here we used two simulations with the UVic ESCM 2.9 model for the Last Glacial Maximum (LGM) and Marine Isotope Stage 11c (MIS11c) climate using orbital forcing and CO2 prescribed for the ages. We have clearly identified the differences between the decrease and increase inf CO2 concentration in the Southern Ocean. The LGM simulation produces a colder, saltier, and more alkaline Southern Ocean than MIS11c. The MIS11c simulations show a reduction in O2 content with respect to LGM, which may be related to less primary productivity and more CaCO3 dissolution. According to the MIS11c simulation, the surface ocean carbonate factory is expected to be more sensitive to carbon oscillations due to an increase in CO2 dissolved in sea water. As expected, it will decrease the pH due to decreased alkalinity and ion carbonate concentrations. In this scenario, the aragonite concentration in the deep sediments would be more vulnerable to dissolution as a response of shoaling of the depth of aragonite saturation. The increase in CO2 dissolved in sea water in MIS11c, decreases the pH and saturation depth of CaCO3 becomes more than 1000 m shallower than found in the LGM simulation. This implies in a negative feedback of CO2 absorption with the saturation horizon becoming shallower, less Southern Ocean capacity has to uptake atmospheric CO2. Since the CaCO3 formation in the surface ocean and the dissolution in the deeper both affect TCO2 and TA as also the pCO2. Our results indicated that, under high CO2 conditions, the surface of the Southern Ocean appears to be more sensitive to a carbon perturbation, generally due to a decrease in ion carbonate concentration that leads to changes in the carbonate buffer system that will result in a deepening more than 1000 m the aragonite depth saturation in relation to MIS11c results.

Ontake-Katamachi tephra: Marine-terrestrial correlation of a time marker of marine isotopic stage 5b in NE Japan, the Japan Sea, and the NW Pacific

We characterized Ontake volcano-derived tephras on the basis of their shard major- and trace-element compositions to distinguish between tephras by different multiple large eruptions of Ontake volcano during marine isotopic stage (MIS) 5. Then we reconsidered the ages of the tephras and their correlations between proximal and distal areas in light of their stratigraphic relationships with other widespread tephras and the δ18O stratigraphy of deep-sea sediments of the Pacific Ocean. As a result, we were able to discriminate Ontake-Katamachi tephra (On-Kt), a previously identified Ontake volcano-derived tephra in deep-sea sediments of Holes C9001C and ODP1150A off NE Japan, from other tephras in the proximal area. In addition, we suggest that previous studies may have miscorrelated On-Kt in terrestrial sequences at some locations in NE Japan to proximal Ontake-Pumice 1 (On-Pm1). Therefore, On-Kt is recognized here for the first time as a widespread tephra covering NE Japan and parts of the Japan Sea and the Pacific Ocean. The On-Kt horizon in Pacific Ocean deep-sea sediments corresponds to MIS 5b in the reported δ18O stratigraphy, and it occurs below Aso-4 (ca. 87 ka), a widespread tephra derived from Aso caldera in the Kyushu arc. As a widespread tephra covering a wider area than previously thought, On-Kt is expected to be useful for constraining the ages of regional tephras in central Japan, NE Japan, the Japan Sea, and the NW Pacific.

Stage or Sub-Stage: The Contribution of Small Mammals to the Characterization of Middle Pleistocene Local Climate Variation

In western Europe, the Middle Pleistocene is marked by Acheulean settlement and their diversification after the MIS 12. The Arago cave recovery of numerous human settlements correlate to MIS 14, 13 and MIS 12 making it an important site for the understanding of the Lower Palaeolithic in Southwestern Europe. It is also an important site for the understanding of palaeoenvironments and palaeobiodiversity as it has yielded rich faunal associations. The faunal associations allow us to observe three climatic stages within this study: two cold ones and a mild one. Small vertebrates, with their abundance and their diversity, are particularly useful for observing these periods, which historically have been correlated to glacial or interglacial stages. If the first cold phase, dated 438 ± 31 ka, is correlated to the Marine Isotopic Stage 12 (MIS 12), the correlation of the following phases to isotopic stages can be discussed. They may correspond to climatic variations of the MIS 12. Indeed, the latest studies about palaeoclimatic reconstitution which allow us to define the evolution of the palaeo-temperature show that these differences are relatively small. Therefore, instead of a correlation to MIS 12, 13 and 14, the medium complex of the Arago cave could belong solely to MIS 12. The correlation of these environmental changes to other global data, notably the isotopic curve, is challenging because there are various local factors influencing faunal association. We propose here both hypotheses and discuss the various factors which influence the distribution and the representation of the small vertebrate species present on the site.

Unravelling the morphogenesis of coastal terraces at Cape Laundi (Sumba Island, Indonesia): Insights from numerical models

AbstractThe morphology of coastal sequences provides fundamental observations to unravel past sea level (SL) variations. For that purpose, converting morphometric observations into a SL datum requires understanding their morphogenesis. The long‐lasting sequence of coral reef terraces (CRTs) at Cape Laundi (Sumba Island, Indonesia) could serve as a benchmark. Yet, it epitomizes a pitfall that challenges the ultimate goal: the overall chronology of its development remains poorly constrained. The polycyclic nature of the terraces, involving marine erosion and reoccupation of old coral colonies by more recent ones hinders any clear assignment of Marine Isotope Stages (MIS) to specific terraces, in particular the reference datum corresponding to the last Interglacial maximum (i.e., MIS 5e). Thus, to overcome these obstacles, we numerically model the genesis of the sequence, testing a range of eustatic SL (ESL) reconstructions and uplift rates, as well as exploring the parameter space to address reef growth, erosion and sedimentation. A total of 625 model runs allowed us to improve the morpho‐chronological constraints of the coastal sequence and, more particularly, to explain the morphogenesis of the several CRTs associated with MIS 5e. Our results suggest that the lowermost main terrace was first constructed during the marine transgression of MIS 5e and was later reshaped during the marine regression of MIS 5e, as well as during the MIS 5c and MIS 5a highstands. Finally, we discuss the general morphology of the sequence and the implications it may have on SL reconstructions. At Cape Laundi, as elsewhere, we emphasize the necessity of addressing the development of CRT sequences with a dynamic approach, that is, considering that a CRT is a landform built continuously throughout the history of SL oscillations, and not simply during a singular SL maximum.

Coupled climate-glacier modelling of the last glaciation in the Alps

Abstract Our limited knowledge of the climate prevailing over Europe during former glaciations is the main obstacle to reconstruct the past evolution of the ice coverage over the Alps by numerical modelling. To address this challenge, we perform a two-step modelling approach: First, a regional climate model is used to downscale the time slice simulations of a global earth system model in high resolution, leading to climate snapshots during the Last Glacial Maximum (LGM) and the Marine Isotope Stage 4 (MIS4). Second, we combine these snapshots and a climate signal proxy to build a transient climate over the last glacial period and force the Parallel Ice Sheet Model to simulate the dynamical evolution of glaciers in the Alps. The results show that the extent of modelled glaciers during the LGM agrees with several independent key geological imprints, including moraine-based maximal reconstructed glacial extents, known ice transfluences and trajectories of erratic boulders of known origin and deposition. Our results highlight the benefit of multiphysical coupled climate and glacier transient modelling over simpler approaches to help reconstruct paleo glacier fluctuations in agreement with traces they have left on the landscape.

Glacial-interglacial environmental changes in the Drake Passage over the past 600 kyrs

Oceanographic conditions in the Southern Ocean are closely related to the oceanic frontal system. Glacial-interglacial changes in cryosphere influence variations in oceanic fronts in the Southern Ocean and vice versa, causing changes in surface water productivity, nutrient utilization, bottom water chemistry, and/or bottom current intensity. Here, we documented geochemical, nitrogen isotope, and grain size records over the last 600 kyrs from the Drake Passage with previously published data to compare between glacial and interglacial oceanic conditions from surface to bottom. Biogenic opal (diatom) and export productions were high during interglacial periods and low during glacial periods. Surface water production was dominated by open ocean species during both glacial and interglacial periods without a clear glacial-interglacial change in sea ice species. After Marine Isotope Stage (MIS) 9, interglacial biogenic opal and diatom abundance increased distinctively with decreased terrestrial influence. Nutrient utilization increased (decreased) during interglacial (glacial) periods with increased (decreased) surface water production, particularly after MIS 9. This indicates that surface water production was regulated by light availability in association with sea ice duration/extent. According to good correlation between sortable silt mean grain size (SS MGS) and %sortable silt, SS MGS can be used for bottom current intensity indicator in this study; increased (decreased) bottom current during glacial (interglacial) periods. Increased glacial bottom current is likely related to southwestward flowing bottom current. Interglacial bottom current intensity became also significantly weakened after MIS 9. However, outstanding interglacial warmth in the Drake Passage occurred from MIS 9, not following the global trend to occur after MIS 11. This implies that more studies from the Southern Ocean in response to the Mid-Brunhes Event are required in the future.

Marine Isotope Stage 4 (71–57 ka) on the Western European margin: Insights to the drainage and dynamics of the Western European Ice Sheet

Marine Isotope Stage 4 (71–57 ka) on the Western European margin: Insights to the drainage and dynamics of the Western European Ice Sheet

Humid phases on the southwestern Arabian Peninsula are consistent with the last two interglacials

Past environmental and climatic conditions within the Arabian Peninsula are key to understanding the setting for hominin dispersal across the Saharo-Arabian dryland belt. The tufa deposits within the volcanic harrats on the southwest coast of Saudi Arabia fill a significant spatial gap in the distribution of palaeoenvironmental records on the west coast of the Arabian Peninsula adjacent to the Red Sea. In the catchment of Wadi Dabsa in the Harrat Al Birk, there are widespread fossil palustrine to shallow-lacustrine tufa deposits with fluvial elements. Several phases of tufa accumulation, separated by fluvial downcutting, are observable within these powerful palaeoenvironmental proxies. U–Th dating of targeted dense, banded tufa facies, yield ages that are stratigraphically consistent at the landscape scale, and indicate that tufa accumulation occurred during distinct humid phases broadly coeval with the last two warm interglacial Marine Isotope Stages (MIS 7 and MIS 5). For the first time this shows humid intervals in southwest Arabia coincident with the southern coast. There is a simlar pattern emerging further north in the Arabian Peninsula, The Sinai and Levant and further on into continental Europe. Furthermore, tufa δ18O ranges from −14.6 to −1.9‰, covering a range similar to those reported for tufa from north African oasis sites and speleothems elsewhere on the Arabian Peninsula and The Levant. The lowest δ18O values are derived from MIS 5e samples, a pattern in agreement with speleothems in Yemen and Oman, and consistent with an isotopic-enabled climate model simulation for this time slice. The δ13C and Sr isotopic compositions of dated tufa samples indicate deposition from shallow-circulating meteoric water, with no geothermal influence. This, along with the δ18O values, suggest a freshwater supply that was a potable water source in this landscape. The δ13C signatures at Wadi Dabsa are more negative than for parts of north Africa, suggesting Wadi Dabsa may have experienced comparatively higher biomass, thicker soils and wetter conditions with lower evaporative losses. This new record of tufa deposition during the middle and late Pleistocene, suggests for the first time that the west coast of Arabia experienced a similar history of humid phases over the past 250 ka as southern Arabia and the Nefud in the northern interior. These regional changes in hydroclimatic regime occur at timescales coincident with hominin dispersals.

Late Quaternary river evolution and its response to climate changes in the upper Mekong River of the Qinghai–Tibetan Plateau

Multiple continental-scale large rivers originate from the Qinghai–Tibetan Plateau (QTP), known as the “Asia Water Tower”, because of glacial melting. However, the timing of fluvial processes in the headwaters of these large rivers and their correlation with global climate changes remain uncertain. Among these rivers, the Mekong River, the third-longest in Asia, flows through the southeastern QTP. In this study, we investigated fluvial terraces, buried paleochannels, hillslope-derived colluvium, and remnants of dammed lake in the Nangqian Basin, which is in the headwaters of the Mekong. We reconstructed the fluvial aggradation and incision history employing optically stimulated luminescence (OSL) dating. Our results suggest that orbital-scale climate changes have significantly influenced fluvial processes in the Nangqian Basin since the Late Pleistocene. During marine isotope stage (MIS) 3 (57–29 ka), a relatively warm period within the last glacial epoch, the Nangqian Basin experienced aggradation of fluvial sediment. Subsequently, during the later part of the Last Glacial Maximum (LGM) (21–17 ka), episodic landslides and debris flows caused valley infilling, resulting in river blockage and the formation of a dammed lake. At around 16 ka, the Mekong in the Nangqian Basin shifted from aggradation to fluvial incision. Since the late Holocene, the basin returned to aggradation, forming a new terrace at around 1.1 ka. This may be due to the combination of decreased stream power and increased sediment supply, potentially linked to intensified human activities and its associated vegetation degradation. Overall, our study highlights the close links between glaciation and fluvial processes in the region. Specifically, we propose that the observed aggradation pattern during the last glacial period on the plateau primarily resulted from substantial detrital input from glacier activities. The transition from aggradation to incision during the last deglacial period is likely influenced by increased discharge from glacier meltwater. The presence of colluvium interbedded within the late LGM fluvial sediment sequences highlights the significant impact of climate changes on fluvial processes in the headwater areas of the QTP, especially those events stemming from hillslope activity, such as landslides and debris flows.

Multi-proxy speleothem-based reconstruction of mid-MIS 3 climate in South Africa

Abstract. The southern coast of South Africa displays a highly dynamical climate as it is at the convergence of the Atlantic and Indian oceans, and it is located near the subtropical/temperate zone boundary with seasonal influence of easterlies and westerlies. The region hosts some key archeological sites with records of significant cognitive, technological and social developments. Reconstructions of the state and variability of past climate and environmental conditions around sites of archeological significance can provide crucial context for understanding the evolution of early humans. Here we present a short but high-resolution record of hydroclimate and temperature in South Africa. Our reconstructions are based on trace elements, calcite and fluid inclusion stable isotopes, as well as fluid inclusion microthermometry, from a speleothem collected in Bloukrantz cave, in the De Hoop Nature Reserve in the southern Cape region of South Africa. Our record covers the time period from 48.3 to 45.2 ka during marine isotope stage 3. Both δ18Oc and δ13Cc show strong variability and covary with Sr/Ca. This correlation suggests that the control on these proxies originates from internal cave processes such as prior carbonate precipitation, which we infer to be related to precipitation amount. The hydroclimate indicators furthermore suggest a shift towards overall drier conditions after 46 ka, coincident with cooling in Antarctica and drier conditions in the eastern part of South Africa corresponding to the summer rainfall zone (SRZ). Fluid inclusion-based temperature reconstructions show good agreement between the oxygen isotope and microthermometry methods, and results from the latter display little variation throughout the record, with reconstructed temperatures close to the present-day cave temperature of 17.5 ∘C. Overall, the BL3 speleothem record thus suggests relatively stable temperature from 48.3 to 45.2 ka, whereas precipitation was variable with marked drier episodes on sub-millennial timescales.

Improving tephrostratigraphy and cryptotephrostratigraphy since 1 Ma of Hole U1437B in the Izu-Bonin arc, NW Pacific: Differentiation of widespread tephras with similar shard chemistries

We investigated the deep-sea sedimentary sequence of Hole U1437B, corresponding to the last 1 Ma, in the Izu-Bonin arc, NW Pacific Ocean, and refined its tephrostratigraphy. We correlated many tephras and cryptotephras with terrestrial and marine tephras on the basis of the major- and trace-element compositions of their glass shards by referring to the reported tephrostratigraphy of marine sediments in core MD012422 (since 0.35 Ma) off Shikoku Island, in Holes ODP1151C (0.2–0.8 Ma) and C9001C (since ca. 0.8 Ma) off northern and northernmost Honshu, respectively, and on the Boso Peninsula (since 1 Ma). Many widespread tephras and cryptotephras were detected in the hole such as those originated from the Kyushu volcanic zone (from north to south, Yufu-Tsurumi, Shishimuta, Aso, Kakuto, Kobayashi, Aira, Ata, and Kikai volcanoes/calderas), Hakkoda-Kokumoto tephra originated from Hakkoda caldera, northernmost Honshu, and some tephras probably originated from source(s) in northern Honshu. Among the widespread tephras, those with similar shard chemistries occurred at different horizons. Kasamori 5 tephra (Ks5) was newly detected by this study in a position above the previously reported "Ks5" horizon in Hole U1437B. The newly detected Ks5 tephra occurs in the marine isotopic stage (MIS) 12 segment of the δ18O stratigraphy, consistent with the reported stratigraphic age of Ks5 (MIS 12b–c) in Hole C9001C. Although the reported "Ks5" horizon also occurs in the MIS 12 segment in Hole U1437B, it is actually a different unidentified tephra, as indicated by its slightly different shard chemistry. Further, we correlated Kasamori 10 tephra (Ks10: mid-MIS 13) in Hole U1437B to subspike G19.3 in Hole C9001C but not to subspike G4.4 in Hole ODP1151C. Although subspike G4.4 in Hole ODP1151C was previously estimated on the basis of its shard major-element composition to correspond to subspike G19.3 in Hole C9001C, their shard trace-element compositions differ. Instead, on the basis of their shard chemistries, we correlated subspike G4.4 in Hole ODP1151C to spike G90 in Hole U1437B (MIS 14), which occurs below the Ks10. This tephra has not been reported in the marine Kasamori Formation on the Boso Peninsula. Further, spike G7 in Hole ODP1151C, which is below subspike G4.4 and above spike G9 (a correlative of Ks22 and Kaisho-Kamitakara tephra, MIS 16), is here correlated to Ks18 in the MIS 15 horizon of Hole U1437B on the basis of their shard chemistries. The stratigraphic ages of Ks5, Ks10, the spike G90 tephra, and Ks18 at most (but not all) locations are roughly consistent with the age of a globally correlated biohorizon, the last appearance datum of Pseudoemiliania lacunosa (ca. 0.45 Ma). These findings emphasize that Hole U1437B sediments are as good as or better than the marine sediments on the Boso Peninsula as archives of the regional tephrostratigraphy. Thus, they are useful for improving the Middle Pleistocene tephrostratigraphy in the NW Pacific region.

Anomalous 234U/238U isotopic composition in Southern Ocean sediments

Uranium is a redox-sensitive trace element that under certain sedimentary conditions will reflect changes in past biological productivity and/or deep ocean ventilation. Applications for the isotopic composition of U (234U:238U activity ratio, δ234U or the per mil deviation from secular equilibrium) as a paleoceanographic proxy, however, remain largely unexplored in pelagic sediments. We present δ234U of bulk U for a 504-kyr record of South Atlantic core ODP 1094, a record that has exhibited sensitivity to bottom water formation with minimal 238U remobilization due to changes in redox condition. Sedimentary δ234U at time of burial was modeled given authigenic and detrital sources, where it was found that δ234U has a reoccurring pattern of anomalous behavior around deglacial transitions. In many cases, the anomalous δ234U compositions in sediment exceeded seawater, which is not consistent with its expected sources. A mass balance of 234U across these anomalous sections showed that vertical rearrangement of 234U can explain anomalous sedimentary δ234U during Marine Isotope Stage (MIS) 7/8 and 11/12 transitions, as well as partially during MIS 5/6 and MIS 9/10. The distribution of these anomalies is likely consistent with the idea that deglacial spikes in aU, resulting from periods of high organic carbon delivery and thereby strong reduction in the sediments, act as a persistent sink of 234U alpha-recoiled from vertically adjacent sediment layers after burial. While this mechanism explains the negative δ234U anomalies, there is a large subset of positive δ234U anomalies that cannot be explained by vertical diffusion of 234U in pore waters alone. This excess 234U points to an extremely high δ234U source to seawater during deglaciation or additional diagenetic effects such as diffusion from lateral heterogeneity in the redox state of sediment at depth. We find that δ234U of pelagic sediment may be useful to identify fine-scale pore water migration of alpha-recoil products.

Demographic processes, refugia and dispersal routes during the Pleistocene in a sigmodontine rodent assemblage from the South American Pampas

Abstract The Pampas of South America represents one of the most extended subtropical–temperate grasslands in the world. This ecoregion was influenced by Pleistocene climate oscillations. The sigmodontine rodent assemblage inhabiting this region is a good model system to analyse the impact of these climate changes on evolutionary histories. We performed a comparative phylogeographical study on seven species to evaluate the concordance of phylogeographical patterns among them, to assess if they experienced congruent and synchronous demographic changes, and posit putative centres of origin and dispersal routes. Four species (Calomys musculinus, Oligoryzomys flavescens, Oxymycterus nasutus and Oligoryzomys nigripes) showed evidence of demographic expansion. In the first three, population increases started during Marine Isotope Stage 5 (MIS 5) while in Ol. nigripes, the rise started during MIS 7; this rise would have continued to the present in all cases. Necromys lasiurus, Scapteromys tumidus and Scapteromys aquaticus did not show a pattern of expansion. Four centres of origin were identified; in general, populations sharing the same putative refugium followed common dispersal routes. Our results indicate that the Pampas offered relatively stable habitats over time, mainly in areas associated with watercourses or coastlines, suggesting that recent Pleistocene climate oscillations have had a moderate impact on this ecoregion compared to other regions of South America.

Vegetation cover dynamics on the northeastern Qinghai-Tibet Plateau since late Marine Isotope Stage 3

Knowledge of long-term changes in vegetation cover is essential for paleoenvironmental reconstruction and Earth system modeling. The vegetation on the northeastern Qinghai-Tibet Plateau (QTP) is highly sensitive to climatic changes, but studies of long-term changes in the vegetation cover of this region are lacking. To better understand the changes in the regional vegetation cover since late Marine Isotope Stage (MIS) 3, we obtained a pollen record from Luanhaizi Lake and used it to quantitatively reconstruct changes in the cover of trees and grasses using the random forest method. The pollen spectra show that the vegetation of the Luanhaizi Lake area was probably alpine tundra from late MIS3 to the Last Glacial Maximum, after which it changed to alpine steppe with alpine shrub and sparse forest at lower elevations during the last deglaciation. Alpine steppe and alpine meadow dominated the vegetation during the Holocene, with sparse forest in the surrounding low-elevation areas. The quantitative vegetation cover reconstruction suggests that the vegetation cover of the Luanhaizi Lake area was low (20–30%) during 47.0–20.0 ka. The tree cover then increased from ∼3% to ∼10%, and the grass cover increased from ∼20% to ∼45% during the last deglaciation (20.0–11.9 ka). The increasing Northern Hemisphere summer insolation caused increases in the regional temperature, meltwater supply, and monsoon precipitation, which promoted the development of steppe vegetation. The continued increases in insolation and monsoonal precipitation during the Holocene further increased the tree cover, which reached a maximum of 17.4% at ∼7.2 ka. Vegetation cover reconstructions from two other sites on the eastern QTP for the last deglaciation indicate contrasting patterns of changes, likely due to contrasts in elevation, climate, and environment.

One million years of Mediterranean outflow strength

An orbitally tuned age model is established, based principally on sortable silt mean grainsize (SS¯, proxy for flow speed), for sediments deposited under the influence of the Mediterranean Outflow (MO) at IODP Site U1389 in the Gulf of Cadiz. The possibility of tuning sortable silt grainsize to insolation was indicated by close agreement between these two parameters on an earlier isotopically-based age model for the upper 130 m of the core. This led to tuning of the whole 390 m-long record with addition of a magnetic susceptibility control at Marine Isotope Stage 16, and the last occurrence of R. asanoi datum at around 900 ka (now dated at 910 ka in this core).The strongest variability in SS¯ is precessional with speed maxima at insolation minima superimposed on an increase in flow speed from 620 ka to the present. Although the Gibraltar sill exercises a major control on the inflow and outflow to the Mediterranean, there is little change in flow depth over most 20 ka precessional cycles, and therefore minor hydraulic control. Accelerations in average flow speed are also seen in interglacial to glacial (IG-G) cycles, particularly after 550 ka, with strong speed maxima at maximum glacial lowering of sealevel. This has allowed calculation of the sensitivity of flow speed to sea-level (SL) change of ∼0.074 cm s−1/m. From 990 to 620 ka the average amplitude of precessional speed changes is 3.7 cm s−1, but this increases to 7.7 cm s−1 from 620 ka to present.Deep ocean flow speeds are mainly density driven. Previous work has demonstrated that the major drivers of this system's density on precessional timescales are the evaporation - precipitation - inflow balance in the eastern Mediterranean plus cooling in the Gulf of Lions. Published benthic δ18O for 125–235 ka (corrected for ice volume changes) which tracks the combination of seawater temperature and salinity that controls density, demonstrates that flow speed and density maxima are coincident for that period and probably for the whole record. Although precessional SL changes are too small to affect flow speed significantly, SL does control accelerations at glacial maxima because shallower flow depth reduces inflow and leads to longer evaporation time, resulting in increased outflow density and speed. The sensitivity of flow speed to sill depth changes has allowed estimation of the long-term shallowing of the Gibraltar sill of ∼68 m over the last 620 ka in agreement with dated terrace uplift.

Sediment Grain-Size Composition in the Permafrost Region of the Greater Khingan Range and Its Significance as a Material Source

Sediment grain-size distribution (GSD) provides rich information about sedimentary sources and can potentially do the same with regard to environmental and climatic changes. However, neither traditional descriptive statistics nor curving-fitting methods can fully address its complexity. We selected the Greater Khingan Range in northeastern China as the study area and used parameterized end-member analysis (EMA) of the GSD of four drilling cores to extract different end-member (EM) components. The results show that EM1 (mode particle size (Mo): 1.26–1.66 μm) originates from weathering and pedogenesis. The EMs with Mo values of 4.37–5.01 μm represent components transported by the upper westerly wind. EMs with Mo values of 7.58 μm and 11.48 μm represent wet dust deposition and dry dust deposition. The wind transport of particles in winter consists of low-level near-source transport and local-source transport (possibly from the flood plain of the Amur River). Due to the limitations of the EM model, the two sources have one or two EM components: AEM3 + AEM4, BEM3 + BEM4, CEM4, and DEM4. DEM5 is the only large particle-size component and may represent coarse-grained detritus generated via rock weathering. The components related to the winter monsoon and the 14C dating data suggest a weak-strong-weak-strong trend of the winter monsoon since the Marine Isotope Stage 3a (MIS 3a). Our results suggest that the strengthening of the winter monsoons in the previous few thousand years has caused the transportation of coarser grain sizes and further exacerbated permafrost degradation, providing a scientific reference for understanding climate change and the formation and evolution of permafrost in the Greater Khingan Mountains since the MIS 3a.

Iberian Margin surface ocean cooling led freshening during Marine Isotope Stage 6 abrupt cooling events

The high-resolution paleoclimate records on the Iberian Margin provide an excellent archive to study the mechanism of abrupt climate events. Previous studies on the Iberian Margin proposed that the surface cooling reconstructed by the alkenone-unsaturation index coincided with surface water freshening inferred from an elevated percentage of tetra-unsaturated alkenones, C37:4%. However, recent data indicate that marine alkenone producers, coccolithophores, do not produce more C37:4 in culture as salinity decreases. Hence, the causes for high C37:4 are still unclear. Here we provide detailed alkenone measurements to trace the producers of alkenones in combination with foraminiferal Mg/Ca and oxygen isotope ratios to trace salinity variations. The results indicate that all alkenones were produced by coccolithophores and the high C37:4% reflects decrease in SST instead of freshening. Furthermore, during the millennial climate changes, a surface freshening did not always trigger a cooling, but sometimes happened in the middle of multiple-stage cooling events and likely amplified the temperature decrease.