Northern Hemisphere Glaciation Research Articles

Climatic and tectonic controls on sedimentation and erosion during the Pliocene-Quaternary in the Qaidam Basin (China)

The Qaidam Basin is an internally drained basin located in the northeastern Tibetan Plateau. Presently, over 50% of the basin floor exposes thick (>1 km) sections of Pliocene–Quaternary strata that are deformed by folding and faulting. We investigated this nearly continuous Pliocene–Quaternary sedimentary record for the effects of global climate change and deformation on basin sedimentation. New detailed stratigraphic, magnetostratigraphic, and stable isotope (δ 18 O, δ 13 C) data from the Pliocene Shizigou and Pleistocene Qigequan Formations along the southwestern flank of an intrabasin fold within the north-central Qaidam Basin are presented here. Strata reveal climatically controlled, meter-scale parasequences within shallow-lacustrine, marginal-lacustrine, and deltaic lithofacies. Paleocurrents shift from eastward at the base to southwestward for the majority of the section, but they abruptly shift toward the south-southeast in the upper 400 m. Twenty-two magnetozones constrain deposition between 5.2 Ma and ca. 0.8 Ma and reveal that sedimentation rates were fairly constant (474 ± 34 m/m.y.) from 5.2 to 3.0 Ma, after which time rates abruptly decreased to 154 ± 40 mm/yr before increasing again to ∼750 m/m.y. since 1.2 Ma. The δ 18 O values shift from relatively constant values (avg. −6.8‰, range −9.6‰ to −4.5‰ relative to Vienna Peedee belemnite [VPDB]) to less negative values (avg. -1.2‰, range -1.2‰ to -2.7‰ VPDB) between 3.1 and 2.6 Ma and to extremely variable values (avg. –2.9‰, range −8.3‰ to 4.0‰) after ca. 2.6 Ma. The post–2.6 Ma extreme variability in stable isotopes reflects the same timeframe of enhanced climatic cyclicity associated with Northern Hemisphere glaciation. The δ 13 C values remain relatively constant (average −4.0‰, range −5.7‰ to -1.0‰) until ca. 0.9 Ma, when the values increase to -0.3‰ (range -1.0‰ to 1.5‰) VPDB. The appearance of growth strata at 3.0 Ma, shallow-water, evaporite deposition after 2.6 Ma, and the observation of paleoyardangs (buried, wind-sculpted landforms) within lake-marginal strata at 2.4 Ma imply that emergence of the adjacent anticline was followed by the shallowing and partitioning of the lake basin and subaerial exposure and erosion of marginal lake sediments. These data reflect a significant change to a more arid climate in the Qaidam Basin between ca. 3.1 Ma and 2.6 Ma, overlapping with the onset of significant Northern Hemisphere glaciations and basin-floor deformation. Lake-level cycles were on ∼230,000 yr frequency from 2.6 Ma to 1.2 Ma, before increasing to ∼12,000 yr frequency, suggesting increased aridity and broad, subaerial exposure of lake sediments after the Pliocene-Quaternary transition. Deformation of intrabasin sediments by at least 3.0 Ma caused uplift of the basin floor, which provided an environment rich in friable material for wind deflation of the Qaidam Basin, a likely source for sediments on the Chinese Loess Plateau and nutrients for the Pacific Ocean carried by westerly winds. By 2.6 Ma, deformation of the Qaidam Basin created closed depressions that facilitated evaporite sedimentation that continues today. Coeval intrabasin deformation, combined with increasing aridity after 3.1 Ma, thus controlled both deposition and erosion within the region.

Surface water hydrography of the Kuroshio Extension during the Pliocene–Pleistocene climate transition

Abstract The Pliocene–Pleistocene climate transition offers an opportunity to study the effect of glaciation on the ocean–climate system. We present a Globigerinoides ruber δ18O record from Ocean Drilling Program Site 1208 (Kuroshio Current Extension; KCE). This exclusively (sub)tropical foraminifer, a summer/fall mixed-layer dweller at the KCE, affords the first long (3.0 Ma to 1.8 Ma) orbital-scale (2.5-kyr time step) account of the sea surface in this area. The section's temperature-corrected benthic foraminiferal δ18O record constrains global changes in ice volume, yielding a Δδ18O record that primarily reflects summer/fall KCE hydrography (temperature and salinity). A 0.3‰ decrease in Δδ18O values at 2.7 Ma coincides with the onset of Northern Hemisphere glaciation, indicating as much as 1.5 °C warming during the summer/fall to suggest that the subtropical North Pacific sea surface provided heat and moisture for expanding ice sheets. On the orbital scale, the 41-kyr cycle that dominates high-latitude climate is absent from the Δδ18O record, indicating a stable surface water hydrographic regime on this time scale. Rather, the Δδ18O record varies at and is coherent with the 19-kyr precessional component of the regional insolation curve, supporting a direct response to subtropical insolation and insensitivity to extra-regional forcing factors, such as ice sheets.

40Ar/39Ar constraints on some French landmark Late Pliocene to Early Pleistocene large mammalian paleofaunas: Paleoenvironmental and paleoecological implications

To improve the French Plio-Pleistocene biostratigraphy scheme based on mammal biozone boundaries or “Mammal Neogene/Quaternary Zones” (MNQ) we collected volcanic material that could be dated using the 40Ar/39Ar method in five exceptional mammalian paleofauna sites located in the Massif Central (France). We present 40Ar/39Ar ages that we obtained for Perrier-Les Etouaires, Roca-Neyra, Chilhac, Senèze and le Creux de Peyrolles. We show that the overall stratigraphic position of these sites based on faunal assemblages is valid from the relative point of view. However, we greatly improve both the accuracy and precision of the age of these mammalian paleofaunas. We obtained 40Ar/39Ar ages varying between 2.78 ± 0.01 Ma (1σ external) for Les Etouaires (Lower MNQ 16b) and 1.47 ± 0.01 Ma for the Creux de Peyrolles site (MNQ 19). Based on these new dates we estimate the duration of several biozones including MNQ 16b, 17a and 17b. We suggest that the first Late Villafranchian biozone (MNQ 18) starts as early as the Reunion subchron or just after. The first occurrence of Equus stenonis in Roca-Neyra (i.e. 2.60 ± 0.02 Ma) is close to or synchronous with the Gauss–Matuyama transition (i.e. 2.59 Ma) and the Pliocene/Pleistocene boundary. The chronological framework we build shows the very rapid increase of the large grazers community in French faunal assemblages at the beginning of MNQ 17 (i.e. 2.6 to 2.4 Ma). This rapid faunal turnover is probably associated with a general decrease of woodland habitat in the Massif Central contemporaneous with the onset of the Northern Hemisphere glaciations. The faunal assemblages in France, Spain, and Italy covering the period between 2.1 and 2.0 Ma suggest that favorable conditions for early hominin settlement (mainly savannah prairies, grassland with open forest patches) existed in southwestern Europe at least 200 ka before the first traces of Homo in Eurasia. This period also shows the arrival of taxa originating in Asia and Africa, suggesting dispersal events within southwestern Europe well before the Olduvai subchron and with no indication (as yet) of Homo as a “fellow traveler”.

Changes in sea-surface conditions in the Equatorial Pacific during the middle Miocene–Pliocene as inferred from coccolith geochemistry

This study presents a new alkenone-derived Sea Surface Temperature (SST) record and δ18ONoelaerhabdaceae data of the 2–5μm carbonate fractions from the IODP site U1338 located in the Eastern Equatorial Pacific (EEP), over the Miocene–Pliocene. Our data and those available from other sites of the same area show the establishment of a cold tongue during the early Pliocene (4.4–3.6Ma). SST and δ18ONoelaerhabdaceae time-series indicate periods of significant salinity variations. Comparison with the δ18Obenthic curve from sediment cores of the Equatorial Pacific Ocean allows us to distinguish between global changes and local surface salinity variations in the EEP. Ice sheet growth and evaporation–precipitation are then discussed as possible drivers of such changes, along with the role of Central American and the Indonesian seaway restriction. Our data suggest a shallowing of the thermocline in the EEP, between 6.8 and 6Ma, and its shoaling between 4.8 and 4.0Ma, suggesting the appearance of the cold tongue (Steph et al., 2010). The Pliocene climate transition would therefore not be primarily driven by Northern hemisphere glaciation.

A latest Pliocene age for the earliest and most extensive Cordilleran Ice Sheet in northwestern Canada

The Klondike gravel is a widespread glaciofluvial gravel marking the earliest and most extensive Cordilleran Ice Sheet (CIS) in NW North America. New terrestrial cosmogenic nuclide (TCN) burial ages indicate this gravel was emplaced 2.64+0.20/−0.18 Ma (1σ). Coupled with previously interpreted paleomagnetic stratigraphy, this numerical age constrains the timing of the earliest CIS to the late Gauss Chron and provides a minimum age for the Upper White Channel gravel, a significant placer gold source in the Yukon. This implies the first CIS glacial maximum pre-dates the maximum extent of the Laurentide Ice Sheet, indicating that during the initial stages of northern hemisphere glaciation, the most extensive glaciers were present in the relatively cold and high elevation northern Cordillera. Our results verify the CIS as a likely source of persistent coeval ice-rafted debris in the northern Pacific, and suggest that the first CIS formed as a response to the establishment of the northern Pacific halocline and emergence of the 41 ka obliquity cycle during the Plio-Pliocene transition.

Evolution of marine sedimentation in the Bering Sea since the Pliocene

Sediment of the Bering Sea, derived mainly from biogenic, glaciomarine, and, secondarily, riverine sources, reflects the history of oceanographic changes within the basin and climatic changes on the adjacent continents. Integrated Ocean Drilling Program (IODP) Expedition 323 recovered cores that reveal the evolution of sedimentation in the Bering Sea over the past 5 m.y., a period that includes globally significant events such as the early Pliocene warm period, the onset of extensive Northern Hemisphere glaciation, and the Pleistocene glacial-interglacial and millennial-scale climate cycles. To begin to understand the Bering Sea regional response to and role in these global climate change events, we examined the sedimentary constituents of Expedition 323 sites U1339, U1343, and U1344 on the Bering Slope, and U1340 and U1341 on Bowers Ridge. New particle size and petrographic analyses, combined with shipboard lithostratigraphic and physical property data, are used to characterize sediment types and texture and its distribution through space and time. The sediment comprises mainly two components, opaline diatom valves and siliciclastic grains (mainly clay and fine silt size). Approximately 40% of the variance in particle size can be explained by the abundance and preservation of diatom valves, a rough indicator of biogenic opal productivity. Particle size data indicate that productivity was generally higher during interglacials compared to glacials, and higher during the Pliocene warm period, decreasing as Northern Hemisphere glaciation intensified ∼3 m.y. ago. Although the abundance of diatoms in the sediment varied, diatom ooze and diatom mud are the dominant lithologies at Bowers Ridge, indicating that there was a persistent supply of diatoms to the sediment in the open Bering Sea during the past 5 m.y. This study provides a comprehensive view of sediment types and sedimentation processes; future work should be aimed at validating our interpretations of past changes in productivity and siliciclastic sedimentation mechanisms with multiple additional proxies.

Reorganization of the Kuroshio and Subtropical Gyre in the Northwest Pacific during the Northern Hemisphere Glaciation: evidences from geochemical records of the Ananai Formation drilling core

Reorganization of the Kuroshio and Subtropical Gyre in the Northwest Pacific during the Northern Hemisphere Glaciation: evidences from geochemical records of the Ananai Formation drilling core

A new method of obtaining high-resolution paleoclimate records from speleothem fluid inclusions

The Greenland ice sheet is accepted as a key factor controlling the Quaternary “glacial scenario“. However, the origin and mechanisms of major Arctic glaciation starting at 3.15 Ma and culminating at 2.74 Ma are still controversial. For this phase of intense cooling Ravelo et al. [1] [A.C. Ravelo, D.H. Andreasen, M. Lyle, A.O. Lyle, M.W. Wara, Regional climate shifts caused by gradual global cooling in the Pliocene epoch. Nature 429 (2004) 263–267.] proposed a complex gradual forcing mechanism. In contrast, our new submillennial-scale paleoceanographic records from the Pliocene North Atlantic suggest a far more precise timing and forcing for the initiation of northern hemisphere glaciation (NHG), since it was linked to a 2–3 °C surface water warming during warm stages from 2.95 to 2.82 Ma. These records support previous models [G.H. Haug, R. Tiedemann, Effect of the formation of the Isthmus of Panama on Atlantic Ocean thermohaline circulation, Nature 393 (1998) 673–676. [2]] claiming that the final closure of the Panama Isthmus (3.0– ∼2.5 Ma [J. Groeneveld S. Steph, R. Tiedemann, D. Nürnberg, D. Garbe-Schönberg, The final closure of the Central American Seaway, Geology, in prep. [3]]) induced an increased poleward salt and heat transport. Associated strengthening of North Atlantic Thermohaline Circulation and in turn, an intensified moisture supply to northern high latitudes resulted in the build-up of NHG, finally culminating in the great, irreversible “climate crash” at marine isotope stage G6 (2.74 Ma). In summary, there was a two-step threshold mechanism that marked the onset of NHG with glacial-to-interglacial cycles quasi-persistent until today.

Patterns of European vegetation changes over the last 15,000 years based on the European Pollen Database with new chronologies

Quantifying changes in seawater carbonate chemistry is crucial to deciphering of patterns and drivers of the oceanic carbon cycle and climate change. Here, we present a new deep-water carbonate ion saturation state (Δ[CO32−]) record for the Pliocene western tropical Pacific, reconstructed from the size-normalized weight of the planktonic foraminifer Trilobatus sacculifer of IODP Site U1490. A steep decline in deep-water Δ[CO32−] occurred at ∼4.6 Ma synchronous to the enhanced production of North Atlantic Deep Water (NADW) related to the closure of the Panamanian Gateway. Subsequently, at the onset of the Northern Hemisphere glaciation at ∼2.7 Ma the weakening of NADW formation resulted in a deep-water Δ[CO32−] peak. The changes in NADW production rate likely controlled a seesaw-like fluctuation in deep-water Δ[CO32−] between the Pacific and Atlantic oceans. During the late Pliocene (∼3.8–2.8 Ma), Antarctic ice-sheet/sea-ice expansions sequestered CO2 in the deep Pacific through ventilation of the deep watermass, leading to a long-term decrease in deep Pacific Δ[CO32−]. We infer that fluctuating NADW production rates at ∼4.6 Ma and ∼2.7 Ma influenced inter-basinal fractionation of deep-ocean carbon between the Atlantic and Pacific, and that deep Pacific carbon storage linked to expansions of Antarctic ice sheet/sea ice contributed to the lowering of atmospheric pCO2 and global cooling during the late Pliocene.

∼4 Ma coarsening of sediments from Baikal, Chinese Loess Plateau and South China Sea and implications for the onset of NH glaciation

Abstract Intense tectonic movement occurred along the northeastern margin of the Tibetan Plateau 3–4 Ma. During the same time period, global climate changed from a period of stability to a period of frequent and abrupt changes, which prevented fluvial and glacial systems from establishing a state of equilibrium. Thus, it is difficult to directly attribute the 3–4 Ma coarse-grained sediment accumulation in East Asia to climate change or tectonic activity. This study compares Late Cenozoic multi-proxy records from the Chinese Loess Plateau (CLP) to the low-latitude grain size, sea level and benthic δ 18 O records from the South China Sea (SCS) and the high latitude grain-size record from Lake Baikal. All records suggest a steady and persistent cooling since ~ 4 Ma, which correlates well with those cooling records from other regions around the world. This coincided in timing with the modeling results that the closure of the Central American Seaway (CAS) initiated strengthening of Atlantic meridional overturning circulation between 4.8 and 4.0 Ma which led to both warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere (SH). Cooling of the SH would induce a marked development of the Antarctic ice sheets at ~ 4 Ma, pushing the Intertropical Convergence Zone (ITCZ) northward. This was superimposed on warming of the NH and brought more precipitation to the middle latitudes of the NH, resulting in increases in coarse-grained sediments in the Sikouzi section from the western CLP since 4.2 Ma. Notably, the absence of coarse-grained sedimentation in the Sikouzi section during 3.0-2.1 Ma and the gradual decrease in coarse-grained peaks since 2.1 Ma reflected by the Sikouzi grain-size record probably resulted from stepwise increase in the NH ice volume pushing the ITCZ southward. On the other hand, development of the Antarctic ice sheets would induce global cooling and enhancement of physical weathering, initiating increases in sedimentation rates as well as increases in grain size from Lake Baikal to the CLP to the SCS. Therefore the closure of the CAS during 4.8–4.0 Ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 Ma. A persistent and steady cooling during 4–3 Ma probably made a significant contribution to the establishment of the NH ice sheets at 2.75 Ma ago.

Abrupt landscape change post–6 Ma on the central Great Plains, USA

The principal control on landscape evolution in the central Great Plains of the United States over the past 10 m.y. is a contentious subject. New sedimentary data collected from Late Miocene Ogallala Group and Pliocene Broadwater Formation of the Nebraskan Great Plains demonstrates a twofold increase in the median grain size (from 20 mm to >40 mm) exported from the Rocky Mountains across the Miocene-Pliocene boundary. Paleoslope reconstructions derived from these data support the tilting of the Miocene Ogallala Group after 6 Ma, but demonstrate that the transport slope of the lower part of the unconformably overlying Pliocene succession is identical to the present-day slope. These data allow us to constrain the timing of differential uplift in the Great Plains to between 6 and 3.7 m.y.; the wavelength and short duration of this tilting are best explained by the initiation of localized dynamic topography. Our results also suggest a threefold to fourfold increase in specific stream power at this time, meaning that Pliocene rivers draining the central Rockies were considerably more competent than their Miocene predecessors. Incision during this period was not continuous. A significant episode of aggradation from 3.7 to 2.5 Ma is best explained by high rates of sediment supply relating to the warm, wet mid-Pliocene climate optimum. The modern pattern of incision on the Great Plains occurred from 2.5 Ma, and not from the end of the Miocene as is sometimes supposed, reflecting the onset of major Northern Hemisphere glaciation.

Enhanced stratification and seasonality in the Subarctic Pacific upon Northern Hemisphere Glaciation–New evidence from diatom-bound nitrogen isotopes, alkenones and archaeal tetraethers

Coincident with the intensification of Northern Hemisphere Glaciation (NHG) around 2.73 million years (Ma) ago, sediment cores from both the open subarctic North Pacific and the Antarctic indicate a rapid decline in diatom opal accumulation flux to the seabed, representing one of the most abrupt and dramatic changes in the marine sediment record associated with the development of Pleistocene glacial cycles. In the North Pacific, bulk sediment nitrogen isotope data and alkenone-derived sea surface temperature (SST) estimates suggest that the productivity decline was driven by reduced exchange between surface and deep water, due to weaker wind-driven upwelling and/or a strengthening of the halocline (i.e. “stratification”). In this study of the 2.73Ma transition at Ocean Drilling Program (ODP) Site 882 in the western subarctic North Pacific, diatom-bound nitrogen isotopes (δ15Ndb), alkenone mass accumulation rate, and alkenone- and archaeal tetraether-based SST reconstructions support the stratification hypothesis, indicating perennially lower export production, generally higher nitrate consumption, and greater inter-seasonal variation in SST after the 2.73Ma transition. In addition, the δ15Ndb of large and small size fractions of Coscinodiscus spp. suggest that these diatoms grew mostly during the spring bloom during the late Pliocene, switching to their current fall-to-winter growth period at the 2.73Ma transition; this view is consistent with their decline in dominance and provides further evidence for increased stratification (reduced vertical exchange) in the North Pacific after 2.73Ma. The δ15Ndb data indicate that, over the ∼100kyr period after the 2.73Ma transition studied here, nitrate consumption did not reach late Pleistocene ice age levels and that nitrate consumption in post-2.73Ma warm stages was similar to that before the transition, even though productivity was greatly reduced. We tentatively attribute this to relatively weak dust-borne iron inputs in the early post-2.73Ma period.

Paleoceanographic significance of deep-sea benthic foraminiferal species diversity at southeastern Indian Ocean Hole 752A during the Neogene

Diversity parameters of Neogene deep-sea benthic foraminifera were measured at Ocean Drilling Program (ODP) Hole 752A, southeastern Indian Ocean (water depth of 1086.3m) using Information Function (H), Equitability (E), number of species (S) and Sander's rarefaction values. These parameters combined with population abundance of dominant benthic foraminifera (Bulimina macilenta, Nuttallides umbonifera, Cibicides wuellerstorfi, Cibicides lobatulus, Bolivina pusilla, Ehrenbergina carinata, Gavelinopsis lobatulus, Cassidulina laevigata, Globocassiulina subglobosa) reveal significant paleoceanographic changes in the southeastern Indian Ocean during the Neogene. The values of all the diversity parameters show a decrease from 25 to 23Ma and thereafter an increase with peak values at ~13.5Ma. The Late Oligocene to Earliest Miocene was an interval of more unstable conditions at Hole 752A dominated by species characteristic of low organic carbon, well-ventilated, carbonate corrosive high energy conditions. The highest values of diversity parameters coincide with the early Middle Miocene climatic optimum. All these parameters show a declining trend and gradual decrease from 13.5 to 4.5Ma coinciding with the major build up of ice sheets in the Antarctic region. Major increase in B. pusilla and E. carinata population during this time suggests high nutrient levels and low oxygen conditions at Hole 752A. This interval corresponds with the so-called “biogenic bloom” and an intense Oxygen Minimum Zone as observed throughout the Indo-Pacific region. Deep waters were warmer from 4.5 to 3Ma marked by an increase in species diversity values, coinciding with the early middle Pliocene warmth. The species diversity values abruptly decreased in the younger interval, contemporaneous with the major Northern Hemisphere glaciation. During this time species characteristics of high-energy bottom currents and relatively cold deep water were dominant.

Tectonic and climatic controls on long‐term silicate weathering in Asia since 5 Ma

Determining the interplay between tectonic deformation, climate, atmospheric CO2concentrations and continental weathering and erosion is key to understanding the mechanisms that forced Cenozoic global cooling. In contrast with studies of paleo‐climate and pCO2, the history of long‐term silicate weathering in the Himalaya and Tibetan Plateau (HTP) during the late Cenozoic remains unclear. We reconstruct 5 m.y. of silicate sedimentary records at Ocean Drilling Program (ODP) Site 1143 in the South China Sea to explore the weathering history of the Mekong River basin that supplied the sediment. Coherent variation of weathering proxies from the South China Sea, Bay of Bengal, Loess Plateau, as well as the Yangtze and Yellow Rivers, indicates weakening chemical weathering intensity since the late Pliocene, as the climate cooled. This cooling, coupled with tectonic activity, shifted the dominant weathering regime from more transport‐limited to more weathering‐limited, causing less chemical depletion of silicate minerals. While silicate weathering rates became strongly correlated to erosion rates, they were decoupled from chemical weathering intensity. Physical denudation and associated silicate weathering rates in the HTP area increased in the Pliocene, driven by both rock uplift and stronger monsoon precipitation, decreasing atmospheric CO2 concentrations, and so contributing to Northern Hemisphere Glaciation (NHG).

Flux and provenance of ice-rafted debris in the earliest Pleistocene sub-polar North Atlantic Ocean comparable to the last glacial maximum

Relatively little is known in detail about the locations of the early Pleistocene ice-sheets responsible for ice-rafted debris (IRD) inputs to the sub-polar North Atlantic Ocean during intensification of northern hemisphere glaciation (iNHG). To shed new light on this problem, we present the first combined in-depth analysis of IRD flux and geochemical provenance of individual sand-sized IRD deposited in the sub-polar North Atlantic Ocean during the earliest large amplitude Pleistocene glacial, marine isotope stage (MIS) 100 (∼2.52Ma), arguably the key glacial during iNHG. IRD provenance is assessed using laser ablation lead (Pb) isotope analyses of single feldspar grains. We find that the Pb-isotope composition (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb) of individual ice-rafted (>150μm) feldspars deposited at DSDP Site 611A, ODP Site 981 and IODP Site U1308 during MIS 100 records a shift from predominantly Archaean-aged circum-North Atlantic Ocean continental sources during early glacial ice-rafting events to dominantly Palaeozoic and Proterozoic-aged sources during full glacial conditions. The distribution of feldspars in Pb–Pb space for full glacial MIS 100 more closely resembles that documented for feldspars deposited at the centre of the last glacial IRD belt (at IODP/DSDP Site U1308/609) during ambient (non-Heinrich-event) ice-rafting episodes of MIS 2 (∼23.8ka) than that documented for MIS 5d (∼106ka). Comparison of our early Pleistocene and last glacial cycle datasets suggests that MIS 100 was characterised by abundant iceberg calving from large ice-sheets on multiple continents in the high northern latitudes (not just on Greenland).

Impact of Pleistocene aridity oscillations on the population history of a widespread, vagile Australian mammal, Macropus fuliginosus

AbstractAim Climatic fluctuations during the Pleistocene have shaped the population structure of many extant taxa. However, few studies have examined widespread species inhabiting the Australian continent, where periods of increased aridity characterized the Pleistocene. Here we investigate the phylogeography and population history of a widespread and vagile southern Australian marsupial, the western grey kangaroo (Macropus fuliginosus).Location Southern Australia.Methods We examined the variation of the mitochondrial DNA (mtDNA) control region from 511 individuals of M. fuliginosus sampled throughout their transcontinental distribution. Maximum likelihood and Bayesian analyses were used to investigate the phylogeography and coalescence analyses were then used to test hypothesized biogeographical scenarios.Results The combined results of the phylogeographical and coalescence analyses revealed a complex evolutionary history. Macropus fuliginosus originated in the south‐west of the continent, with north‐western and south‐western populations subsequently diverging as a result of vicariance events during the mid‐Pleistocene. Subsequent arid phases affected these populations differently. In the north‐west, the expansion and contraction of the arid zone resulted in repeated vicariance events and multiple divergent north‐western mtDNA subclades. In contrast, the south‐western population was less impacted by climatic oscillations but gave rise to a major transcontinental eastward expansion.Main conclusions Macropus fuliginosus exhibits the genetic signature of divergence due to unidentified barriers in south‐western Western Australia, while previously identified barriers across southern Australia appear to have had little impact despite evidence of a broad‐scale range expansion prior to the Last Glacial Maximum (LGM). This pattern of localized expansion and contraction is comparable to unglaciated regions in both the Northern and Southern Hemispheres. Furthermore, this study indicates that despite the potential similarities between Northern Hemisphere glaciation and the activation of dune systems in the Australian arid zone, both of which rendered large areas inhospitable, the biotic responses and resultant phylogeographical signatures are dissimilar. Whereas a limited number of major geographically concordant refugia are observed in glaciated areas, the Southern Hemisphere arid zone appears to be associated with multiple species‐specific idiosyncratic refugia.

Arc‐continent collision and orocline formation: Closing of the Central American seaway

Closure of the Central American seaway was a local tectonic event with potentially global biotic and environmental repercussions. We report geochronological (six U/Pb LA‐ICP‐MS zircon ages) and geochemical (19 XRF and ICP‐MS analyses) data from the Isthmus of Panama that allow definition of a distinctive succession of plateau sequences to subduction‐related protoarc to arc volcaniclastic rocks intruded by Late Cretaceous to middle Eocene intermediate plutonic rocks (67.6 ± 1.4 Ma to 41.1 ± 0.7 Ma). Paleomagnetic analyses (24 sites, 192 cores) in this same belt reveal large counterclockwise vertical‐axis rotations (70.9° ± 6.7°), and moderate clockwise rotations (between 40° ± 4.1° and 56.2° ± 11.1°) on either side of an east‐west trending fault at the apex of the Isthmus (Rio Gatun Fault), consistent with Isthmus curvature. An Oligocene‐Miocene arc crosscuts the older, deformed and segmented arc sequences, and shows no significant vertical‐axis rotation or deformation. There are three main stages of deformation: 1) left‐lateral, strike‐slip offset of the arc (∼100 km), and counterclockwise vertical‐axis rotation of western arc segments between 38 and 28 Ma; 2) clockwise rotation of central arc segments between 28 and 25 Ma; and 3) orocline tightening after 25 Ma. When this reconstruction is placed in a global plate tectonic framework, and published exhumation data is added, the Central American seaway disappears at 15 Ma, suggesting that by the time of northern hemisphere glaciation, deep‐water circulation had long been severed in Central America.

Benthic foraminiferal assemblage turnover during intensification of the Northern Hemisphere glaciation in the Piacenzian Punta Piccola section (Southern Italy)

We present the results of a high-resolution analysis of the benthic foraminiferal assemblages at the Punta Piccola section (Stratotype for the Piacenzian Stage), which spans the gradual climate transition of the intensification of the Northern Hemisphere glaciation (3.6–2.6Ma). This study highlighted a major benthic foraminiferal fauna turnover, which started at about 3.05Ma with LO of Cibicidoides italicus, registered the gradual decline of Stilostomella spp. and culminated at about 2.7–2.75Ma, when the dominant Siphonina reticulata was replaced by Cibicidoides pachyderma, costate Bulimine, spinose Bulimine, spinose Uvigerine and the Bolivina dilatata group. Four compositional zones, identified by constrained cluster analysis, are indicative of palaeoenvironmental changes and document the transition from stable and mainly oligotrophic conditions to unstable, mesotrophic and more fluctuating bottom conditions.Two intervals of deposition of sapropelite clusters coded as O and A, investigated using both benthic and planktic assemblages, reveal that anoxia conditions were never reached. Planktic foraminifera in the sapropelite layers of cluster O indicate deep mixing and benthic foraminifera suggest an increase in export productivity to the sea floor. By contrast, planktic foraminifera in the uppermost sapropelite layers of cluster A indicate oligotrophic surface water conditions, stratification and more eutrophic subsurface water conditions, without deep mixing. The benthic foraminifera indicate hypoxic and eutrophic bottom conditions, and evidence the first episode of down-slope transport, which testifies to a strengthening of runoff and stratification of superficial waters, but not true stagnation.

Aeolian construction and alluvial dismantling of a fault‐bounded intracontinental aeolian dune field (Teruel Basin, Spain); a continental perspective on Late Pliocene climate change and variability

AbstractAn aeolian dune field migrating to the east encroached on the toes of alluvial fans in the Teruel Basin (eastern Spain) during a short interval in the Late Pliocene (ca 2·9 to 2·6 Ma), when Northern Hemisphere glaciation and strong glacial–interglacial cycles began. Preservation of the dune field was controlled by syn‐sedimentary activity of a normal fault. Ephemeral water discharge eroded aeolian sands and formed V‐shaped channels in which aeolian sandstone blocks accumulated. The incorporation of loose aeolian sand in wadi waters modified the sediment/water ratio, changing the physical properties of the flows as they penetrated the aeolian dune field. The erosion and cover of aeolian dune foresets by sheetflood deposits suggest that dune‐damming caused the intermittent ponding of water behind the dunes and its flashy release. The arid climate in the Late Pliocene western Mediterranean realm favoured the transport of windblown sediments from northern Africa and western Mediterranean land masses into the Mediterranean. The formation of the studied aeolian dune field (2·9 to 2·6 Ma) and possibly others (for example, the Atacama, Namib and Sahara deserts) correlates with a strong increase of the influence of obliquity, which can be attributed to the combination of a regional expression related to the reduced effect of precession due to a minimum in the long‐period (2·3 Ma) eccentricity cycle and a remote expression of the onset of the Northern Hemisphere glaciation.

Stable-isotope stratigraphy of the Pliocene–Pleistocene climate transition in the northwestern subtropical Pacific

From Ocean Drilling Program (ODP) Site 1208 on Shatsky Rise below the Kuroshio Current Extension, we present the North Pacific's first orbital-scale benthic-foraminiferal δ18O and δ13C time series to span the Pliocene–Pleistocene climate transition. Excellent agreement between the Site 1208 δ18O record and the global δ18O stack of Lisiecki and Raymo (2005) provides orbital-scale age control and confirms continuous stratigraphy from 3.7 to 1.8Ma at the single-hole site. Cross-spectral analysis of the δ18O and δ13C time series reveals that these are coherent to 80% confidence at the 41-kyr obliquity band prior to 3.3Ma (glacial isotope stage M2) and increase to the 95% level thereafter. Throughout, δ18O cycles consistently lead δ13C cycles by ~3kyr. This suggests that global-ocean δ13C variations, as produced by terrestrial–marine 12C transfers, were responsive to obliquity-induced climate changes before the Northern Hemisphere glaciations (NHG) reached mid latitudes at 2.7Ma. In contrast, 41-kyr carbonate sedimentation (as derived from sediment reflectance) cycles, maxima tightly coupled to (>95% confidence) and in phase with minima in the δ18O record, do not emerge until 2.7Ma. Foraminiferal fragmentation counts indicate that carbonate preservation is not the primary process behind enhanced carbonate deposition during interglacials. Thus, we surmise that hydrography-related changes in biogenic opal and carbonate production in surface water best explain glacial–interglacial carbonate cycles beginning with significant NHG. Firm establishment of orbital-scale age control on the stratigraphically complete Site 1208 section now provides a platform for high-resolution paleoceanographic reconstruction of the relatively understudied North Pacific.