Paleoclimate Modeling Research Articles

Reconstructing environmental signals across the Permian-Triassic boundary in the SE Germanic Basin: A Quantitative Provenance Analysis (QPA) approach

Global tectonic and climatic models for the Permian-Triassic boundary (PTB) are highly debated. One of the most disputed topics is the temperature increase associated with CO 2 emissions generated by the Siberian Trap volcanism and its potential influence on chemical weathering and associated variations in sediment fluxes. By integrating crustal architecture, plate modelling, structural kinematics, and two climatic models, we reconstruct the drainage evolution of Variscan tectonostratigraphic units from the SE portion of the Germanic Basin, from which we also extract the parameters necessary to calculate sediment flux across a time-scale of 28 Ma (Guadalupian-Lower Triassic). We reconstruct the sedimentary response to climatic and tectonic perturbations using Quantitative Provenance Analysis (QPA) and integrate compositional data into a sedimentological framework, paleodrainage and paleoclimatic models. Raman heavy mineral analysis, as well as geochronology and geochemistry of detrital apatite, zircon, and rutile, document variation in drainage lithologies and sediment flux which are controlled by regional extensional tectonics and increasingly humid conditions at the PTB. The sedimentary successions of the SE Germanic Basin record climatic perturbations on a 10 4 years timescale, while the effects of tectonics are visible on a 10 6 years timescale. The interplay of climate, tectonics and lithology, and their effects on sediment production and drainage evolution resulted in changes in sediment flux from 2.3 Mt./yr during the Guadalupian (Capitanian), to 3.80 Mt./yr in the Lopingian (Changhsingian) to 7.44 Mt./yr at the end of the Lower Triassic (Olenekian). The multifaceted workflow provided in this study represents the first step towards more precise reconstructions of sediment routing systems in deep-time and provides the first ground-truthed quantification of sediment flux across the Permian-Triassic Boundary. • Drainage modifications are efficiently identified via Quantitative Provenance Analysis. • Constraining Environmental Signals in deep-time requires paleoclimatic/tectonic models. • Paleodrainage analysis provides the parameters to calculate sediment flux. • Sediment flux increases in the Lower Triassic in response to climate change.

Middle Miocene (∼14 Ma) and Late Miocene (∼6 Ma) Paleogeographic Boundary Conditions

AbstractDuring the Miocene, major global cooling occurred during two intervals: the middle Miocene (∼14–13 Ma) and the late Miocene (∼7‐6 Ma). The Antarctic Ice Sheet expanded substantially at ∼14–13 Ma, and glaciation in the Northern Hemisphere was initiated at ∼7–6 Ma. Although the causes of these two global cooling events remain unclear, paleoclimate modeling provides an important approach for investigating the mechanisms. However, paleoclimate modeling relies on paleogeographic boundary conditions, and many uncertainties remain regarding the land–sea distribution, topography, and bathymetry of the Miocene; specifically, previous published paleogeographic reconstructions for the middle and late Miocene show several discrepancies with each other. Here, we present two new sets of global topographic and bathymetric boundary conditions for the middle Miocene (∼14 Ma) and late Miocene (∼6 Ma). Our new reconstructions use a published plate kinematics model, oceanic lithospheric paleo‐ages, and oceanic sediment thicknesses and incorporate global fossil records, stratigraphy, lithofacies, paleoenvironment, and paleoelevation data. With these data, we further constrain the conditions of several important intercontinental and marginal seas surrounding Eurasia and the Arctic, the depth of major seaways in tropical latitudes and middle‐high latitudes of the Northern Hemisphere, and global paleotopography, especially that of the Tibetan Plateau and the Andes.

San Josecito Cave and Its Paleoecological Contributions for Quaternary Studies in Mexico

San Josecito Cave (2250 m elevation) is located nearby Aramberri, Nuevo León, northeastern Mexico, with excavations occurring in 1935–1941 and 1990. It is a paleontological cave and the significance of its faunal data rests in the understanding of the Quaternary ecosystems of the Mexican Plateau and the Southern Plains. This significance is underpinned by a consideration of associated stratigraphic and geochronological data. The fauna is composed of mollusks, amphibians, reptiles, birds, and mammals. More than 30 extinct vertebrate species have been identified, constituting one of the most important Quaternary localities in the Americas. Radiocarbon dates and faunal correlations indicate the excavated deposits represent an interval of time between 45,000 and 11,000 14C years BP. The current synthesis demonstrates that the previous view of the assemblage as a single local fauna is erroneous and that, instead, several successive local faunas are present within a stratigraphic framework. This finding underscores the need for detailed studies of single localities in building paleoenvironmental models. As a corollary, results point to the necessity of including all vertebrate classes represented from a locality in building those models. In addition, the field and analytical methodologies demonstrate the importance of very detailed paleontological excavations, with precise spatial and temporal controls, to assess the taphonomic history of a locality, construct a stratigraphic and geochronological framework, and infer the paleoecological conditions during the time span considered based on the number of local faunas represented. The recognition of San Josecito Cave as an important Late Pleistocene vertebrate paleontological locality is enhanced with the consideration of its faunal data for paleoenvironment reconstruction and possible contribution to Quaternary paleoclimatic modeling.

Modeling water crossings leading to the arrival of early Homo in Sulawesi, Indonesia, via paleoclimate drift experiments

Lithic tools found at Talepu, Sulawesi, in Wallacea suggest that an unknown hominin occupied Sulawesi between ∼ 118–194 ka. The Philippines, Borneo, and Flores are identified as possible sources of the colonizing hominin. Climatic reconstructions indicate that the period between ∼ 118–194 ka was marked by large variation in sea level in Wallacea, with portions of the Sunda shelf exposed at low sea level but with coastlines and land extent comparable to that of the present day at high sea level stages. A series of simulated 30-day oceanic drift experiments are conducted to determine which of the three proposed regions would be the most likely origin for hominin dispersal to Sulawesi. Drifts are simulated for low and high sea level cases. Present day sea level, winds and currents from atmosphere and ocean reanalysis products are used as proxy of environmental conditions during high sea level states. Output from Last Glacial Maximum (∼21 ka) simulations generated by three paleoclimate models is used to reconstruct the environment during low sea level conditions. Simulated drifters are released from multiple locations within three possible source regions: Borneo, Mindanao (Philippines) and the Banda Arc (including Flores). Probabilities of success and trip durations of direct arrivals as well as island-hops to Sulawesi are calculated. Results indicate that crossings are facilitated by low sea level, but the possibility of crossings at high sea level cannot be discarded. All of the three analyzed departure areas could be considered feasible sources for arrivals at Sulawesi but, Borneo is by far the most likely source area and Mindanao departures more likely to arrive in Sulawesi than those from the Banda Arc. The shortest simulated period voyagers would have to survive at sea are 3–8 days, 14–19 days and 12–20 days for Borneo, Mindanao and Band Arc departures respectively. These results have implications for the trajectory of hominin evolution in the region, including inputs to modern human diversity.

A First Intercomparison of the Simulated LGM Carbon Results Within PMIP‐Carbon: Role of the Ocean Boundary Conditions

AbstractModel intercomparison studies of coupled carbon‐climate simulations have the potential to improve our understanding of the processes explaining the drawdown at the Last Glacial Maximum (LGM) and to identify related model biases. Models participating in the Paleoclimate Modeling Intercomparison Project (PMIP) now frequently include the carbon cycle. The ongoing PMIP‐carbon project provides the first opportunity to conduct multimodel comparisons of simulated carbon content for the LGM time window. However, such a study remains challenging due to differing implementation of ocean boundary conditions (e.g., bathymetry and coastlines reflecting the low sea level) and to various associated adjustments of biogeochemical variables (i.e., alkalinity, nutrients, dissolved inorganic carbon). After assessing the ocean volume of PMIP models at the pre‐industrial and LGM, we investigate the impact of these modeling choices on the simulated carbon at the global scale, using both PMIP‐carbon model outputs and sensitivity tests with the iLOVECLIM model. We show that the carbon distribution in reservoirs is significantly affected by the choice of ocean boundary conditions in iLOVECLIM. In particular, our simulations demonstrate a GtC effect of an alkalinity adjustment on carbon sequestration in the ocean. Finally, we observe that PMIP‐carbon models with a freely evolving and no additional glacial mechanisms do not simulate the drawdown at the LGM (with concentrations as high as 313, 331, and 315 ppm), especially if they use a low ocean volume. Our findings suggest that great care should be taken on accounting for large bathymetry changes in models including the carbon cycle.

Climate and environmental reconstruction of the Epipaleolithic Mediterranean Levant (22.0–11.9 ka cal. BP)

This study presents, for the first time, an environmental reconstruction of a sequence spanning nearly the entire Mediterranean Epipaleolithic (∼22.0–11.9 ka cal. BP). The study is based on a well-dated, high-resolution pollen record recovered from the waterlogged archaeological site Jordan River Dureijat (JRD), located on the banks of Paleolake Hula. JRD's continuous sequence enabled us to build a pollen-based paleoclimate model providing a solid background for the dramatic cultural changes that occurred in the region during this period. Taxonomic identification of the waterlogged wood assemblage collected from JRD was used to fine-tune the paleoenvironmental reconstruction. The chronological framework is based on radiocarbon dating and the typology of archaeological findings. The LGM (∼22–19 ka cal. BP) was found to be the coldest period of the sequence, marked by a distinct decrease in the reconstructed January temperatures of up to 5°C lower than today, while mean annual precipitation was only slightly lower than the present-day average (∼450 vs. 515 mm, respectively). The wettest and warmest period of the record was identified between ∼14.9 and 13.0 ka cal. BP, with maximum values of 545 mm mean annual precipitation reached at ∼14.5 ka cal. BP. This time interval is synchronized with the global warm and moist Bølling-Allerød interstadial as well as with the onset of the Natufian culture and the emergence of sedentism in the Levant. The Younger Dryas began around 12.9 ka cal. BP and was identified as an exceptional period by the JRD sequence with low temperatures and minimal climatic seasonality contrast: an increase in rain contribution during spring, summer, and autumn was documented concurrently with a significant decrease in winter precipitation. The detailed vegetation and climatological reconstruction presented in this study serves as backdrop to seminal events in human history: the transition from small nomadic groups of hunter-gatherers to the sedentary villages of the Natufian, eventually transitioning to the agricultural, complex communities of the Neolithic.

Migration of the Northern Boundary of the East Asian Summer Monsoon Over the Last 21,000 years

AbstractInvestigating the past evolution of the northern boundary of the East Asian summer monsoon (EASM) is important in understanding the East Asian climate of the past, present, and future. In this study, using a set of transient simulations from TraCE‐21ka, we investigate the migration of the EASM northern boundary over eastern China via three metrics and its responses to individual external forcings during the last 21,000 years. The northern boundary was located farther southeastward from 21 ka to 18 ka and more northwestward since 15 ka compared with the present boundary. For two key time slices, the boundary exhibited a southeastward migration ranging from 120 to 140 km in the last glacial maximum (LGM) and a 110–140 km northwestward shift in the mid‐Holocene relative to the present boundary. Temporally, the evolution of the boundary was characterized first by a poleward migration and then an equatorward shift from the LGM to the present, which was mainly driven by orbital insolation. This long‐term trend was punctuated by several millennial fluctuations in response to meltwater fluxes. The boundary migration during the LGM and the mid‐Holocene generally lies within the range of the multimodel results from the Paleoclimate Modeling Intercomparison Project Phase 4 (PMIP4). In terms of dynamic mechanisms, an orbitally induced increase of summer insolation led to an enhanced land‐sea thermal contrast between the East Asian continent and adjacent ocean and, in turn, intensified low‐level southerly winds, which facilitated the northwestward migration of the EASM northern boundary and more precipitation over eastern China. The boundary responses to the changing meltwater fluxes were similar but weak. Additionally, the EASM northern limit was closely linked to the forest‐steppe boundary. The reconstructed forest‐steppe boundary estimated from pollen records and the simulated EASM northern edge display spatially consistent positions, orientations, and change trends.

Fluvial responses to external and internal forcing: Upper Holocene dynamics in a low latitude semi-arid region in South America

River streams are sensitive to environmental changes in drainage basins in response to external and internal forcing. These changes lead to drainage channel adjustments and may alter erosion-sedimentation cycles along valleys, as well as short and long term geomorphological evolution. Concerning the low latitude semi-arid region in South America, fluvial responses to environmental changes during the Upper Holocene have still not been adequately assessed, contrasting with evaluations in river drylands located in middle latitudes. To collaborate with assessments on Upper Holocene climatic fluctuations, alluvial deposits at the Itapicuru River, located in the Brazilian semi-arid, were analyzed to understand this river's dynamics in the last 5 ka and its relationship with both intrinsic and extrinsic factors. The following analyses were performed: [1] spatial terrace distribution throughout the river's longitudinal profile, [2] stratigraphic section assessments, [3] OSL dating, to estimate surface time elaboration, and [4] age correlations with regional paleoclimatic models. In the last 2.2 ka, downcutting and lateral migration occurred at the same time due to fluvial discharge changes over centuries and decades. Semi-arid conditions, such as those currently observed, prevailed throughout the drainage basin, with wet intervals identified upstream. Humidity variations were responsible for middle valley incision and deposition, forming terrace, bars, and natural levees. These findings indicate that, at least in our case study, intrinsic factors had big importance to control the fluvial dynamics since the establishment of the semi-arid low latitude climatic conditions.

Frequency-Dependent Estimation of Effective Spatial Degrees of Freedom

AbstractClimate variability occurs over wide ranges of spatial and temporal scales. It exhibits a complex spatial covariance structure, which depends on geographic location (e.g., tropics vs extratropics) and also consists of a superposition of (i) components with gradually decaying positive correlation functions and (ii) teleconnections that often involve anticorrelations. In addition, there are indications that the spatial covariance structure depends on frequency. Thus, a comprehensive assessment of the spatiotemporal covariance structure of climate variability would require an extensive set of statistical diagnostics. Therefore, it is often desirable to characterize the covariance structure by a simple summarizing metric that is easy to compute from datasets. Such summarizing metrics are useful, for example, in the context of comparisons between climate models or between models and observations. Here we introduce a frequency-dependent version of a simple measure of the effective spatial degrees of freedom. The measure is based on the temporal variance of the global average of some climate variable, and its novel aspect consists in its frequency dependence. We also provide a clear geometric interpretation of the measure. Its easy applicability is demonstrated using near-surface temperature and precipitation fields obtained from a paleoclimate model simulation. This application reveals a distinct scaling behavior of the spatial degrees of freedom as a function of frequency, ranging from monthly to millennial scales.

Milankovitch, the father of paleoclimate modeling

Abstract. The history of the long-term variations in the astronomical elements used in paleoclimate research shows that, contrary to what might be thought, Milutin Milankovitch is not the father of the astronomical theory but he is definitely the father of paleoclimate modeling. He did not calculate these long-term variations himself but used them extensively for calculating the “secular march” of incoming solar radiation. He advanced our understanding of Quaternary climate variations by two important and original contributions fully described in his Canon of insolation. These are the definition and use of caloric seasons and the concept of the “mathematical climate”. How his mathematical model allowed him to give the caloric summer and winter insolation a climatological meaning is illustrated.

Shallowing Glacial Antarctic Intermediate Water by Changes in Sea Ice and Hydrological Cycle

AbstractThe Antarctic Intermediate Water (AAIW) is an essential global ocean water mass at intermediate depths. Coupled climate models in isotope‐enabled (δ18O, δD), fully coupled Community Earth System Model and Paleoclimate Model Intercomparison Project Phase 3 consistently show shallower AAIW depth at the Last Glacial Maximum (LGM) due to the northward shift of AAIW. More importantly, modeling results suggest that the northward shift of AAIW can be caused by sea ice expansion and the weakened hydrological cycle under the glacial climate. On the contrary, the AAIW under global warming tends to shift poleward compared to the pre‐industrial period driven by the retreating sea ice and strengthened hydrological cycle. However, the AAIW depth will shallow in response to the ongoing warming, likely due to the overwhelming effects of enhanced stratification and shallowing mixed layer.

A Westward Shift in Tropical Cyclone Potential Intensity and Genesis Regions in the North Atlantic During the Last Interglacial

AbstractGiven the longstanding debate on geomorphological features (i.e., megaboulders, chevron‐shaped sand ridges, and runup deposits) observed around the Bahamas, it is unclear whether Earth experienced superstorms over the North Atlantic during the Last Interglacial (LIG). Based on multimodels from the Paleoclimate Modeling Intercomparison Project, we illustrate that the tropical North Atlantic exhibits an overall warming (∼0.9°C) during the storm season of the LIG relative to preindustrial. Potential intensity shows a zonal dipole anomaly over the main development region during the LIG, with positive anomalies in the west, offering an increased probability of extremely strong storms. Favorable conditions for storm formation shift from the eastern to the western main development region during the LIG, with enhanced large‐scale steering flows at higher latitudes (∼15 – 30°N). These results suggest a westward migration of storm activity during the LIG, with potentially more frequent and intense storms over the western North Atlantic and increased coastal erosions.

Rise of the Colorado Plateau: A Synthesis of Paleoelevation Constraints From the Region and a Path Forward Using Temperature-Based Elevation Proxies

The Colorado Plateau’s complex landscape has motivated over a century of debate, key to which is understanding the timing and processes of surface uplift of the greater Colorado Plateau region, and its interactions with erosion, drainage reorganization, and landscape evolution. Here, we evaluate what is known about the surface uplift history from prior paleoelevation estimates from the region by synthesizing and evaluating estimates 1) in context inferred from geologic, geomorphic, and thermochronologic constraints, and 2) in light of recent isotopic and paleobotanical proxy method advancements. Altogether, existing data and estimates suggest that half-modern surface elevations were attained by the end of the Laramide orogeny (∼40 Ma), and near-modern surface elevations by the mid-Miocene (∼16 Ma). However, our analysis of paleoelevation proxy methods highlights the need to improve proxy estimates from carbonate and floral archives including the ∼6–16 Ma Bidahochi and ∼34 Ma Florissant Formations and explore understudied (with respect to paleoelevation) Laramide basin deposits to fill knowledge gaps. We argue that there are opportunities to leverage recent advancements in temperature-based paleoaltimetry to refine the surface uplift history; for instance, via systematic comparison of clumped isotope and paleobotanical thermometry methods applied to lacustrine carbonates that span the region in both space and time, and by use of paleoclimate model mediated lapse rates in paleoelevation reconstruction.

Missing in the Middle: A Review of Equatorially Disjunct Marine Taxa

Disjunct distributions of flora and fauna have intrigued biogeographers for centuries and have been central to debates over the roles that dispersal and vicariance have in speciation and evolutionary biology. One of the most notable disjunct distributions is when a single species or closely related taxa are present in temperate or polar regions of both hemispheres, but absent near the equator. Generally referred to as an anti-tropical distribution, this pattern is prevalent in marine taxa, occurring across the Tree of Life from single-celled bacteria and radiolarians, to algae, many invertebrates, fishes and marine mammals. The abundance of different species that exhibit this distribution pattern has led to a large number of theories on how it is formed and maintained. Here, seven central hypotheses that recur throughout the literature regarding the mechanisms responsible for this pattern are described and discussed in light of both historic and contemporary data. Recent advances in paleoclimate modeling and phylogenetic approaches have revealed that three of these hypotheses lack support. However, these data also highlight that a single driver of anti-tropicality is unlikely and that multiple mechanisms are responsible for this observed distribution pattern. The importance of using a multifaceted approach in future studies that incorporates phylogenetic, physiological, life history and ecological data is discussed, as there is a gap in our knowledge regarding adult and larval behavior, as well as thermal tolerances of many anti-tropical species. While recent studies have helped reveal some commonalities between anti-tropical groups, only by using an integrative approach will we be able to more fully understand which mechanisms are most prevalent in driving this pattern across the oceans.

Hot Air, Hot Lakes, or Both? Exploring Mid‐Holocene African Temperatures Using Proxy System Modeling

AbstractClimate models predict Africa will warm by up to 5°C in the coming century, stressing African societies. To provide independent constraints on model predictions, this study compares two notable reconstructions of East African temperatures to those predicted by Paleoclimate Model Intercomparison Project (PMIP3) and transient TraCE (Transient Climate Evolution) simulations, focusing on the Mid‐Holocene (MH, 5–8 kyr B.P.). Reconstructions of tropical African temperature derived from lake sedimentary archives indicate 1–2.5°C of warming during the MH relative to the 20th century, but most climate models do not replicate the warming observed in these paleoclimate data. We investigate this discrepancy using a new lake proxy system model, with attention to the (potentially non‐stationary) relationship between lake temperature and air temperature. We find amplified lake surface temperature changes compared to air temperature during the MH due to heightened seasonality and precessional forcing. Lacustrine processes account for some of the warming, and highlight how the lake heat budget leads to a rectification of the seasonal cycle; however, the simulated lake heating bias is insufficient to reconcile the full discrepancy between the models and the proxy‐derived MH warming. We find further evidence of changes in mixing depth over time, potentially driven by changes in cloud cover and shortwave radiative fluxes penetrating the lake surface. This may confound interpretation for glycerol dialkyl glycerol tetraethers (GDGT) compounds which exist in the mixed layer, and suggests a need for independent constraints on mixed layer depth. This work provides a new interpretive framework for invaluable paleoclimate records of temperature changes over the African continent.

Assessing first‐order BQART estimates for ancient source‐to‐sink mass budget calculations

AbstractConstraining the timing and volume of sediment dispersal in an ancient sedimentary system is vital to understand a basin's infill history. One preferred method for a first‐order approximation of ancient sediment load estimates, the BQART model, is based on empirical observations of modern river systems relating basin morphology, topography, climate, run‐off and bedrock characteristics. Despite the popularity of such methods, a comprehensive assessment on the validity of using modern river observations to measure sediment load on geological timescales is lacking. Here, we investigate the uncertainties, sensitivities and practicalities surrounding the use of modern empirical observations in general and the BQART model in particular, to evaluate ancient sediment river loads. Although catchment area and relief are the least constrained parameters in an ancient sedimentary system, the temperature parameter may have an even more significant impact in the range of predicted sediment load estimates using a BQART approach. The applicability of BQART is most suitable for regional to continental scale source‐to‐sink systems that are based on robust paleogeographic and paleoclimatic models of cold (<2°C) or warm temperate (>8°C) climates. One further needs to consider the high amplitude discharge events that can dominate the stratigraphic record which are not captured by historical observations of sediment load over a 30‐year period. In addition, our limited understanding of bedload material transport and an unknown pristine environment in the Anthropocene reduce the reliability of modern sediment load estimates for the ancient. Mass budget estimates in deep time based on empirical relationships of modern river systems can thus provide first‐order estimates within an order of magnitude but need to consider the limitations imposed by extrapolating the modern to the ancient. Here, we present a framework to consider the suitability of the BQART method for ancient source‐to‐sink mass budget analyses.

Understanding the mechanisms of groundwater recharge and flow in periglacial environments: New insights from the Ledo-Paniselian aquifer in Belgium

The Ledo-Paniselian aquifer in Belgium has been proposed to offer unique opportunities to study groundwater recharge and flow in periglacial conditions during the Last Glacial Maximum (LGM), due to its location in the permanent permafrost area, south of the ice sheet at that time. A palaeoclimatic record had been set up previously for this aquifer, consisting of major ion chemistry, stable isotopes, radiocarbon and noble gases. In this paper, methane data have been used to further refine the paleoclimatic model, along with revisiting in detail the set of chemical data, focusing on the area where groundwaters, recharged around the LGM, are known to occur. It was found that the high methane concentrations corroborate the hypothesis of groundwater recharge taking place during permafrost melting, from methane-bubbling lakes that had developed to the south of an eolian sand ridge. A relict flow path, existing in the aquifer during some period as permafrost was thawing, has been established, starting from these temporary recharge areas, based on various chemical parameters, radiocarbon model ages and noble gas recharge temperatures.

Timing, provenance, and implications of two MIS 3 advances of the Laurentide Ice Sheet into the Upper Mississippi River Basin, USA

Evidence for a glacial advance into the Upper Mississippi River Basin between Marine Isotope Stage (MIS) 2 and MIS 6 has been debated for over a century. New sedimentological and chronological data provide evidence for two advances of the Laurentide Ice Sheet (LIS) into Iowa during MIS 3 to the east and west of the local glacial maximum margin. The lithology of the MIS 3 till is similar to the MIS 2 Des Moines Lobe till, which suggests both tills were sourced from the Keewatin Dome. A synthesis of new and previously collected radiocarbon ages indicate the two advances reached their terminus near 42⁰ N around 42 ka and 30 ka. A comparison of this record with published loess chronology and provenance studies suggests that these advances were likely a glacial source for the Mississippi Valley Roxana Silt and the Pisgah Formation loess in the Missouri River Valley. The apparent discrepancy between the basal ages of the Roxana and Pisgah loesses may be due to the timing of Des Moines Lobe entering the Mississippi River watershed before the Missouri River watershed. The results of this study demonstrate that the LIS advanced much farther south during MIS 3 than previously recognized, which has significant implications for ice sheet and paleoclimate modeling.

Bridging the knowledge gap on the evolution of the Asian monsoon during 26–16 Ma

SummaryThe evolution of the Asian monsoon from the Late Oligocene to the Early Miocene is poorly understood. Here, we first reconstruct the precipitation data of central Tibet during 26–16 million years ago (Ma), applying the coexistence approach to sedimentary pollen data, and detect an intensified Asian monsoon with ∼1.35 Ma and ∼0.33 Ma cycles. Paleoclimate modeling is used to show the importance of paleogeographic location in the development of the paleomonsoon. In addition, the results of spectral analysis suggest that the fluctuations in the Asian monsoon during 26–16 Ma can be attributed to the long-period cyclicities in obliquity (∼1.2 Ma). These findings provide climate data that can be used to understand the Asian monsoon evolution during the Late Oligocene to Early Miocene and highlight the effects of paleogeographic patterns and long-period orbital forcings on the tectonic-scale evolution of the Asian monsoon.

Land–sea temperature contrasts at the Last Interglacial and their impact on the hydrological cycle

Abstract. Due to different orbital configurations, high northern latitude summer insolation was higher during the Last Interglacial period (LIG; 129–116 thousand years before present, ka) than during the pre-industrial period (PI), while high southern latitude summer insolation was lower. The climatic response to these changes is studied here with focus on the Southern Hemisphere monsoons, by performing an equilibrium experiment of the LIG at 127 ka with the Australian Earth System Model, ACCESS-ESM1.5, as part of the Paleoclimate Model Intercomparison Project 4 (PMIP4). Simulated mean surface air temperature between 40 and 60∘ N over land during boreal summer is 6.5 ∘C higher at the LIG compared to PI, which leads to a northward shift of the Intertropical Convergence Zone (ITCZ) and a strengthening of the North African and Indian monsoons. Despite 0.4 ∘C cooler conditions in austral summer in the Southern Hemisphere (0–90∘ S), annual mean air temperatures are 1.2 ∘C higher at southern mid-latitudes to high latitudes (40–80∘ S). These differences in temperature are coincident with a large-scale reorganisation of the atmospheric circulation. The ITCZ shifts southward in the Atlantic and Indian sectors during the LIG austral summer compared to PI, leading to increased precipitation over the southern tropical oceans. However, weaker Southern Hemisphere insolation during LIG austral summer induces a significant cooling over land, which in turn weakens the land–sea temperature contrast, leading to an overall reduction (−20 %) in monsoonal precipitation over the Southern Hemisphere's continental regions compared to PI. The intensity and areal extent of the Australian, South American and South African monsoons are consistently reduced in LIG. This is associated with greater pressure and subsidence over land due to a strengthening of the Southern Hemisphere Hadley cell during austral summer.