Growth Of Ice Sheets Research Articles

The Effect of Obliquity‐Driven Changes on Paleoclimate Sensitivity During the Late Pleistocene

AbstractWe reanalyze existing paleodata of global mean surface temperature ΔTg and radiative forcing ΔR of CO2 and land ice albedo for the last 800,000 years to show that a state‐dependency in paleoclimate sensitivity S, as previously suggested, is only found if ΔTg is based on reconstructions, and not when ΔTg is based on model simulations. Furthermore, during times of decreasing obliquity (periods of land ice sheet growth and sea level fall) the multimillennial component of reconstructed ΔTg diverges from CO2, while in simulations both variables vary more synchronously, suggesting that the differences during these times are due to relatively low rates of simulated land ice growth and associated cooling. To produce a reconstruction‐based extrapolation of S for the future, we exclude intervals with strong ΔTg‐CO2 divergence and find that S is less state‐dependent, or even constant state‐independent), yielding a mean equilibrium warming of 2–4 K for a doubling of CO2.

Glacial isostatic adjustment deflects the path of the ancestral Hudson River

Quantifying the pace of ice-sheet growth is critical to understanding ice-age climate and dynamics. Here, we show that the diversion of the Hudson River (northeastern North America) late in the last glaciation phase (ca. 30 ka), which some previous studies have speculated was due to glacial isostatic adjustment (GIA), can be used to infer the timing of the Laurentide Ice Sheet’s growth to its maximum extent. Landscapes in the vicinity of glaciated regions have likely responded to crustal deformation produced by ice-sheet growth and decay through river drainage reorganization, given that rates of uplift and subsidence are on the order of tens of meters per thousand years. We perform global, gravitationally self-consistent simulations of GIA and input the predicted crustal deformation field into a landscape evolution model. Our calculations indicate that the eastward diversion of the Hudson River at 30 ka is consistent with exceptionally rapid growth of the Laurentide Ice Sheet late in the glaciation phase, beginning at 50–35 ka.

Glacial isostatic adjustment modelling: historical perspectives, recent advances, and future directions

Abstract. Glacial isostatic adjustment (GIA) describes the response of the solid Earth, the gravitational field, and the oceans to the growth and decay of the global ice sheets. A commonly studied component of GIA is “postglacial rebound”, which specifically relates to uplift of the land surface following ice melt. GIA is a relatively rapid process, triggering 100 m scale changes in sea level and solid Earth deformation over just a few tens of thousands of years. Indeed, the first-order effects of GIA could already be quantified several hundred years ago without reliance on precise measurement techniques and scientists have been developing a unifying theory for the observations for over 200 years. Progress towards this goal required a number of significant breakthroughs to be made, including the recognition that ice sheets were once more extensive, the solid Earth changes shape over time, and gravity plays a central role in determining the pattern of sea-level change. This article describes the historical development of the field of GIA and provides an overview of the processes involved. Significant recent progress has been made as concepts associated with GIA have begun to be incorporated into parallel fields of research; these advances are discussed, along with the role that GIA is likely to play in addressing outstanding research questions within the field of Earth system modelling.

Spatial variation in Late Ordovician glacioeustatic sea-level change

Mass extinction of Late Ordovician marine fauna closely coincided with southern hemisphere glaciation. The sequence stratigraphic architecture of shallow marine deposits informs estimates of glacioeustatic sea-level change at sites both proximal and distal to the reconstructed Ordovician ice sheet(s) and contemporaneous changes in ice volume. A recent correlation framework for the stratigraphic architectures of one near and one far field Late Ordovician margin concluded that the Late Ordovician glaciation encompassed multiple long-term cycles of ice volume growth and retreat with superimposed higher frequency cycles. Here we posit that—similar to Cenozoic glacial cycles—glacial isostatic adjustment can preclude synchronous and similar magnitude (or directional) changes in Late Ordovician sea level between ice proximal and ice distal locations and, hence, distort a globally correlative sequence stratigraphy. We explored whether long-duration (i.e., million year) Late Ordovician glacial cycles should produce a globally coherent, eustatic record of sea-level change between ice proximal and ice distal margins using a gravitationally self-consistent theory that accounts for the deformational, gravitational and rotational perturbations to sea level on a viscoelastic Earth model. We adopted a Late Ordovician paleogeography and a synthetic continental ice-sheet distribution and volume informed by the areal extent of glaciogenic deposits and geochemical records, respectively. We demonstrate that modeled million year Late Ordovician glacial cycles produce sea-level histories on near and far field margins that differ from eustasy, and from one another, due primarily to elastic flexure and associated gravitational effects. While predicted far-field sea-level histories faithfully preserve the temporal structure of modeled glacioeustasy, their amplitude may differ from eustasy by as much as 30–40%. The impact of glacial isostatic adjustment is largest at the margins of glaciated continents, and these effects can be of the same order of magnitude as the eustatic, and even induce a local sea-level rise during an episode of ice growth and eustatic sea-level fall, and vice versa. In this regard, stratal surfaces of maximum regression and flooding expressed at near-field margins need not reflect global (‘eustatic’) trends in ice sheet growth and decay, respectively, and thus may not provide chronostratigraphic horizons for correlation with far-field sequence stratigraphic architectures.

The rise and fall of the Cretaceous Hot Greenhouse climate

Abstract A compilation of foraminiferal stable isotope measurements from southern high latitude (SHL) deep-sea sites provides a novel perspective important for understanding Earth's paleotemperature and paleoceanographic changes across the rise and fall of the Cretaceous Hot Greenhouse climate and the subsequent Paleogene climatic optimum. Both new and previously published results are placed within an improved chronostratigraphic framework for southern South Atlantic and southern Indian Ocean sites. Sites studied were located between 58° and 65°S paleolatitude and were deposited at middle to upper bathyal paleodepths. Oxygen isotope records suggest similar trends in both bottom and surface water temperatures in the southern sectors of the South Atlantic and in the Indian Ocean basins. Warm conditions were present throughout the Albian, extreme warmth existed during the Cretaceous Thermal Maximum (early-mid-Turonian) through late Santonian, and long-term cooling began in the Campanian and culminated in Cretaceous temperature minima during the Maastrichtian. Gradients between surface and seafloor δ18O and δ13C values were unusually high throughout the 11.5 m.y. of extreme warmth during the Turonian-early Campanian, but these vertical gradients nearly disappeared by the early Maastrichtian. In absolute terms, paleotemperature estimates that use standard assumptions for pre-glacial seawater suggest sub-Antarctic bottom waters were ≥21 °C and sub-Antarctic surface waters were ≥27 °C during the Turonian, values warmer than published climate models support. Alternatively, estimated temperatures can be reduced to the upper limits of model results through freshening of high latitude waters but only if there were enhanced precipitation of water with quite low δ18O values. Regardless, Turonian planktonic δ18O values are ~1.5‰ lower than minimum values reported for the Paleocene-Eocene Thermal Maximum (PETM) from the same region, a difference which corresponds to Turonian surface temperatures ~6 °C warmer than peak PETM temperatures if Turonian and Paleocene temperatures are estimated using the same assumptions. It is likely that warm oceans surrounding and penetrating interior Antarctica (given higher relative sea level) prevented growth of Antarctic ice sheets at all but the highest elevations from the late Aptian through late Campanian; however, Maastrichtian temperatures may have been cool enough to allow growth of small, ephemeral ice sheets. The standard explanation for the sustained warmth during Cretaceous Hot Greenhouse climate invokes higher atmospheric CO2 levels from volcanic outgassing, but correlation among temperature estimates, proxy estimates of pCO2, and intervals of high fluxes of both mafic and silicic volcanism are mostly poor. This comparison demonstrates that the relative timing between events and their putative consequences need to be better constrained to test and more fully understand relationships among volcanism, pCO2, temperature ocean circulation, Earth's biota and the carbon cycle.

Land–sea coupling of early Pleistocene glacial cycles in the southern North Sea exhibit dominant Northern Hemisphere forcing

Abstract. We assess the disputed phase relations between forcing and climatic response in the early Pleistocene with a spliced Gelasian (∼2.6–1.8 Ma) multi-proxy record from the southern North Sea basin. The cored sections couple climate evolution on both land and sea during the intensification of Northern Hemisphere glaciation (NHG) in NW Europe, providing the first well-constrained stratigraphic sequence of the classic terrestrial Praetiglian stage. Terrestrial signals were derived from the Eridanos paleoriver, a major fluvial system that contributed a large amount of freshwater to the northeast Atlantic. Due to its latitudinal position, the Eridanos catchment was likely affected by early Pleistocene NHG, leading to intermittent shutdown and reactivation of river flow and sediment transport. Here we apply organic geochemistry, palynology, carbonate isotope geochemistry, and seismostratigraphy to document both vegetation changes in the Eridanos catchment and regional surface water conditions and relate them to early Pleistocene glacial–interglacial cycles and relative sea level changes. Paleomagnetic and palynological data provide a solid integrated timeframe that ties the obliquity cycles, expressed in the borehole geophysical logs, to Marine Isotope Stages (MIS) 103 to 92, independently confirmed by a local benthic oxygen isotope record. Marine and terrestrial palynological and organic geochemical records provide high-resolution reconstructions of relative terrestrial and sea surface temperature (TT and SST), vegetation, relative sea level, and coastal influence. During the prominent cold stages MIS 98 and 96, as well as 94, the record indicates increased non-arboreal vegetation, low SST and TT, and low relative sea level. During the warm stages MIS 99, 97, and 95 we infer increased stratification of the water column together with a higher percentage of arboreal vegetation, high SST, and relative sea level maxima. The early Pleistocene distinct warm–cold alterations are synchronous between land and sea, but lead the relative sea level change by 3000–8000 years. The record provides evidence for a dominantly Northern Hemisphere-driven cooling that leads the glacial buildup and varies on the obliquity timescale. Southward migration of Arctic surface water masses during glacials, indicated by cool-water dinoflagellate cyst assemblages, is furthermore relevant for the discussion on the relation between the intensity of the Atlantic meridional overturning circulation and ice sheet growth.

Export of nutrient rich Northern Component Water preceded early Oligocene Antarctic glaciation

The onset of the North Atlantic Deep Water formation is thought to have coincided with Antarctic ice-sheet growth about 34 million years ago (Ma). However, this timing is debated, in part due to questions over the geochemical signature of the ancient Northern Component Water (NCW) formed in the deep North Atlantic. Here we present detailed geochemical records from North Atlantic sediment cores located close to sites of deep-water formation. We find that prior to 36 Ma, the northwestern Atlantic was stratified, with nutrient-rich, low-salinity bottom waters. This restricted basin transitioned into a conduit for NCW that began flowing southwards approximately one million years before the initial Antarctic glaciation. The probable trigger was tectonic adjustments in subarctic seas that enabled an increased exchange across the Greenland–Scotland Ridge. The increasing surface salinity and density strengthened the production of NCW. The late Eocene deep-water mass differed in its carbon isotopic signature from modern values as a result of the leakage of fossil carbon from the Arctic Ocean. Export of this nutrient-laden water provided a transient pulse of CO2 to the Earth system, which perhaps caused short-term warming, whereas the long-term effect of enhanced NCW formation was a greater northward heat transport that cooled Antarctica.

Cordilleran ice-sheet growth fueled primary productivity in the Gulf of Alaska, northeast Pacific Ocean

Fertilization of the ocean by eolian dust and icebergs is an effective mechanism to enhance primary productivity. In particular, high-nutrient, low-chlorophyll (HNLC) areas where phytoplankton growth is critically iron-limited, such as the subarctic Pacific Ocean and the Southern Ocean, are proposed to respond to increases in bioavailable Fe supply with enhanced phytoplankton productivity and carbon export to the seafloor. While Fe-fertilization from dust is widely acknowledged to explain a higher export production during glacial periods in the Southern Ocean, paleoceanographic records supporting links between productivity and eolian dust and/or icebergs in the North Pacific are scarce. By combining independent proxies indicative of ice-sheet dynamics and ocean productivity from a single marine sedimentary record (Integrated Ocean Drilling Program [IODP] Site U1417), we present a comprehensive data set of phytoplankton response to different fertilization mechanisms in the subarctic northeast Pacific between 1.5 and 0.5 Ma, including the Mid Pleistocene Transition. Importantly, the timing of the fertilization events is more strongly controlled by local ice-sheet extent than by glacial-interglacial climate variability. Our findings indicate that fertilization by glacigenic debris results in productivity events in HNLC areas adjacent to ice sheets, and that this mechanism may represent an important, yet rarely considered, driver of phytoplankton growth.

Crustal heat production and estimate of terrestrial heat flow in central East Antarctica, with implications for thermal input to the East Antarctic ice sheet

Abstract. Terrestrial heat flow is a critical first-order factor governing the thermal condition and, therefore, mechanical stability of Antarctic ice sheets, yet heat flow across Antarctica is poorly known. Previous estimates of terrestrial heat flow in East Antarctica come from inversion of seismic and magnetic geophysical data, by modeling temperature profiles in ice boreholes, and by calculation from heat production values reported for exposed bedrock. Although accurate estimates of surface heat flow are important as an input parameter for ice-sheet growth and stability models, there are no direct measurements of terrestrial heat flow in East Antarctica coupled to either subglacial sediment or bedrock. As has been done with bedrock exposed along coastal margins and in rare inland outcrops, valuable estimates of heat flow in central East Antarctica can be extrapolated from heat production determined by the geochemical composition of glacial rock clasts eroded from the continental interior. In this study, U, Th, and K concentrations in a suite of Proterozoic (1.2–2.0 Ga) granitoids sourced within the Byrd and Nimrod glacial drainages of central East Antarctica indicate average upper crustal heat production (Ho) of about 2.6 ± 1.9 µW m−3. Assuming typical mantle and lower crustal heat flux for stable continental shields, and a length scale for the distribution of heat production in the upper crust, the heat production values determined for individual samples yield estimates of surface heat flow (qo) ranging from 33 to 84 mW m−2 and an average of 48.0 ± 13.6 mW m−2. Estimates of heat production obtained for this suite of glacially sourced granitoids therefore indicate that the interior of the East Antarctic ice sheet is underlain in part by Proterozoic continental lithosphere with an average surface heat flow, providing constraints on both geodynamic history and ice-sheet stability. The ages and geothermal characteristics of the granites indicate that crust in central East Antarctica resembles that in the Proterozoic Arunta and Tennant Creek inliers of Australia but is dissimilar to other areas like the Central Australian Heat Flow Province that are characterized by anomalously high heat flow. Age variation within the sample suite indicates that central East Antarctic lithosphere is heterogeneous, yet the average heat production and heat flow of four age subgroups cluster around the group mean, indicating minor variation in the thermal contribution to the overlying ice sheet from upper crustal heat production. Despite these minor differences, ice-sheet models may favor a geologically realistic input of crustal heat flow represented by the distribution of ages and geothermal characteristics found in these glacial clasts.

Oceanic heat pulses fueling moisture transport towards continental Europe across the mid-Pleistocene transition

The mid-Pleistocene Transition (MPT; approx. 1.2–0.7 Ma), is characterized by growing Northern Hemisphere ice sheets and the shift from a 41 kyr to a 100 kyr glacial-interglacial cyclicity. Concomitant to the growth of large ice sheets, atmospheric and oceanic circulation pattern have changed. One key feature of the North Atlantic is the wind-driven Subtropical Gyre, a major provider of heat and moisture for continental Europe. Here, we investigate changes in the strength and spatial configuration of the Subtropical Gyre during the MPT and its impact on the continental moisture balance.To reconstruct Subtropical Gyre dynamics, we conducted paired δ18O and Mg/Ca analyses on the deep-dwelling foraminifera Globorotalia inflata from Iberian Margin Site U1385 yielding thermocline temperature (Ttherm) variability between 1400 and 500 ka at the eastern boundary of the Subtropical Gyre.Long-term trends of Ttherm at Site U1385 oppose the North Atlantic climatic evolution of progressively intensified glacials during the MPT. Particularly, glacials MIS 20 and 18 were marked by warm thermocline waters off Iberia. We infer that a southward shift of the (sub)polar front displaced the source region of thermocline waters within the Subtropical Gyre from high to mid-latitudes. In addition, a strong Mediterranean Outflow Water production during the MPT caused the advection of warm waters to Iberia. Humid conditions during MIS 20 and 18 in SE Europe indicate that atmospheric moisture derived from this warm water might have been advected deep into continental Europe and contributed to enhanced growth of Alpine glaciers.

Jurassic high heat production granites associated with the Weddell Sea rift system, Antarctica

The distribution of heat flow in Antarctic continental crust is critical to understanding continental tectonics, ice sheet growth and subglacial hydrology. We identify a group of High Heat Production granites, intruded into upper crustal Palaeozoic metasedimentary sequences, which may contribute to locally high heat flow beneath the West Antarctic Ice Sheet. Four of the granite plutons are exposed above ice sheet level at Pagano Nunatak, Pirrit Hills, Nash Hills and Whitmore Mountains. A new UPb zircon age from Pirrit Hills of 178.0±3.5Ma confirms earlier RbSr and UPb dating and that the granites were emplaced approximately coincident with the first stage of Gondwana break-up and the developing Weddell rift, and ~5m.y. after eruption of the Karoo-Ferrar large igneous province. Aerogeophysical data indicate that the plutons are distributed unevenly over 40,000km2 with one intruded into the transtensional Pagano Shear Zone, while the others were emplaced within the more stable Ellsworth-Whitmore mountains continental block. The granites are weakly peraluminous A-types and have Th and U abundances up to 60.7 and 28.6ppm respectively. Measured heat production of the granite samples is 2.96–9.06μW/m3 (mean 5.35W/m3), significantly higher than average upper continental crust and contemporaneous silicic rocks in the Antarctic Peninsula. Heat flow associated with the granite intrusions is predicted to be in the range 70–95mW/m2 depending on the thickness of the high heat production granite layer and the regional heat flow value. Analysis of detrital zircon compositions and ages indicates that the high Th and U abundances are related to enrichment of the lower-mid crust that dates back to 200–299Ma at the time of the formation of the Gondwanide fold belt and its post-orogenic collapse and extension.

Silicate Weathering and Carbon Cycle Controls on the Oligocene‐Miocene Transition Glaciation

AbstractChanges in both silicate weathering rates and organic carbon burial have been proposed as drivers of the transient “Mi‐1” glaciation event at the Oligocene‐Miocene transition (OMT; ~23 Ma). However, detailed geochemical proxy data are required to test these hypotheses. Here we present records of Li/Ca, Mg/Ca, Cd/Ca, U/Ca, δ18O, δ13C, and shell weight in planktonic foraminifera from marine sediments spanning the OMT in the equatorial Atlantic Ocean. Li/Ca values increase by 1 μmol/mol across this interval. We interpret this to indicate an ~20% increase in silicate weathering rates, which would have lowered atmospheric CO2, potentially forcing the Antarctic glaciation ~ 23 Ma. δ13C of thermocline dwelling planktonic foraminifera track the global increase in seawater δ13C across the OMT and during the Mi‐1 event, hence supporting a hypothesized global increase in organic carbon burial rates. High δ13C previously measured in epipelagic planktonic foraminifera and high Cd/Ca ratios during Mi‐1 are interpreted to represent locally enhanced primary productivity, stimulated by increased nutrients supply to surface waters. The fingerprint of high export production and associated organic carbon burial at this site is found in reduced bottom water oxygenation (inferred from high foraminiferal U/Ca) and enhanced respiratory dissolution of carbonates, characterized by reduced foraminiferal shell weight. Replication of our results elsewhere would strengthen the case that weathering‐induced CO2 sequestration preconditioned climate for Antarctic ice sheet growth across the OMT, and increased burial of organic carbon acted as a feedback that intensified cooling at this time.

Earliest accumulation history of the north polar layered deposits, Mars from SHARAD

The approximately 2km thick north polar layered deposits (NPLD) are often considered to contain the most complete and detailed stratigraphic records of recent climate of Mars. Exposures of the dense layering within troughs and scarps allowed detailed reconstructions of the latest accumulation history of these water ice deposits, but we lack knowledge of their initial emplacement. The Shallow Radar (SHARAD) onboard Mars Reconnaissance Orbiter (MRO) penetrates the NPLD to their base and detects their internal layering, overcoming the limitation of scarce and scattered visible outcrops of the lowermost sequences.In this study, we map reflectors in SHARAD data that result from discrete stratigraphic horizons in order to delineate the three-dimensional stratigraphy of the lowermost ∼500m NPLD sequence and reconstruct their accumulation history. We confirm the large-scale lateral continuity and thickness uniformity of the deposits previously detected within the lowermost NPLD. However, stratigraphic complexity—in the form of pinch-outs and significant thickness variations—arises when we examine single radar units. We find evidence of an initially limited geographic stability of water ice within two deposits that are centered at the North Pole and present-day Gemina Lingula. A period of lateral ice sheet growth followed, interrupted only once by a retreat episode. Lower net accumulation is observed on pre-existing slopes, suggesting a reduction of water ice stability due to increased solar radiation incidence and/or transport by katabatic winds. Lateral transport of water ice by wind is also suggested by thickness undulations toward the top of the sequence, resembling cyclic steps.Water ice accumulation models based on orbital forcing predict a sequence of deposition and retreat events that is generally compatible with our reconstructed accumulation history. Therefore, we interpret the stratigraphic complexity that we observe as regional and, possibly global, climate change induced by orbital forcing. We also find that at least two units are completely buried within the NPLD and do not outcrop, and that NPLD deposition in some places was contemporaneous with deposition of the stratigraphically underlying cavi unit in other places. Both of these findings show that radar reflector mapping is a necessary complement to any stratigraphic reconstruction based on visible exposures.

A new volcanic province: an inventory of subglacial volcanoes in West Antarctica

Abstract The West Antarctic Ice Sheet overlies the West Antarctic Rift System about which, due to the comprehensive ice cover, we have only limited and sporadic knowledge of volcanic activity and its extent. Improving our understanding of subglacial volcanic activity across the province is important both for helping to constrain how volcanism and rifting may have influenced ice-sheet growth and decay over previous glacial cycles, and in light of concerns over whether enhanced geothermal heat fluxes and subglacial melting may contribute to instability of the West Antarctic Ice Sheet. Here, we use ice-sheet bed-elevation data to locate individual conical edifices protruding upwards into the ice across West Antarctica, and we propose that these edifices represent subglacial volcanoes. We used aeromagnetic, aerogravity, satellite imagery and databases of confirmed volcanoes to support this interpretation. The overall result presented here constitutes a first inventory of West Antarctica's subglacial volcanism. We identified 138 volcanoes, 91 of which have not previously been identified, and which are widely distributed throughout the deep basins of West Antarctica, but are especially concentrated and orientated along the >3000 km central axis of the West Antarctic Rift System.

Carbon isotope evidence for a northern source of deep water in the glacial western North Atlantic

The prevailing view of western Atlantic hydrography during the Last Glacial Maximum (LGM) calls for transport and intermixing of deep southern and intermediate northern end members. However, δ13C and Δ14C results on foraminifera from a sediment core at 5.0 km in the northern subtropics show that there may have also been a northern source of relatively young, very dense, nutrient-depleted water during the LGM (18 ky to 21 ky ago). These results, when integrated with data from other western North Atlantic locations, indicate that the ocean was poorly ventilated at 4.2 km, with better ventilation above and below that depth. If this is a signal of water mass source and not nutrient storage, it would indicate that a previously unrecognized deep water end member originated along the western margin of the Labrador Sea, analogous to dense water formation today around Antarctica and in the Okhotsk Sea.

Enhanced weathering and CO2 drawdown caused by latest Eocene strengthening of the Atlantic meridional overturning circulation

On timescales significantly greater than 105 years, atmospheric pCO2 is controlled by the rate of mantle outgassing relative to the set-point of the silicate weathering feedback. The weathering set-point has been shown to depend on the distribution and characteristics of rocks exposed at the Earth’s surface, vegetation types and topography. Here we argue that large-scale climate impacts caused by changes in ocean circulation can also modify the weathering set-point and show evidence suggesting that this played a role in the establishment of the Antarctic ice sheet at the Eocene–Oligocene boundary. In our simulations, tectonic deepening of the Drake Passage causes freshening and stratification of the Southern Ocean, strengthening the Atlantic meridional overturning circulation and consequently raising temperatures and intensifying rainfall over land. These simulated changes are consistent with late Eocene tectonic reconstructions that show Drake Passage deepening, and with sediment records that reveal Southern Ocean stratification, the emergence of North Atlantic Deep Water, and a hemispherically asymmetric temperature change. These factors would have driven intensified silicate weathering and can thereby explain the drawdown of carbon dioxide that has been linked with Antarctic ice sheet growth. We suggest that this mechanism illustrates another way in which ocean–atmosphere climate dynamics can introduce nonlinear threshold behaviour through interaction with the geologic carbon cycle. During the latest Eocene, declining atmospheric CO2 levels led to the inception of the Antarctic ice sheet. Simulations suggest that the deepening of the Drake Passage caused climate changes that enhanced continental weathering and CO2 drawdown.

Midlatitude land surface temperature impacts the timing and structure of glacial maxima

AbstractOrbitally modulated insolation is thought to be among the key mechanisms driving ice sheet change. Yet, late Pleistocene ice sheets attained maximum marginal extents prior to summer insolation minima, suggesting that additional mechanisms influenced ice growth and decay. Continuous orbital‐scale terrestrial temperature records can help define the role that temperature near ice sheet margins plays on the timing and structure of glacial maxima. We hypothesize, based on a 360 kyr long air temperature record from the Chinese Loess Plateau and shorter records from North America and Europe, that midlatitude terrestrial temperature influenced the rate of ice sheet growth prior to and during glacial maxima; cold conditions prior to glacial maxima enhanced ice sheet growth, while warming during glacial maxima inhibited further growth, despite low summer insolation. Thus, the midlatitude surface energy budget may be an important component of understanding and modeling ice volume, particularly for intervals prior to and during glacial maxima, when ice sheet margins reached midlatitudes.

Ice–Atmosphere Feedbacks Dominate the Response of the Climate System to Drake Passage Closure

AbstractThe response of the global climate system to Drake Passage (DP) closure is examined using a fully coupled ocean–atmosphere–ice model. Unlike most previous studies, a full three-dimensional atmospheric general circulation model is included with a complete hydrological cycle and a freely evolving wind field, as well as a coupled dynamic–thermodynamic sea ice module. Upon DP closure the initial response is found to be consistent with previous ocean-only and intermediate-complexity climate model studies, with an expansion and invigoration of the Antarctic meridional overturning, along with a slowdown in North Atlantic Deep Water (NADW) production. This results in a dominance of Southern Ocean poleward geostrophic flow and Antarctic sinking when DP is closed. However, within just a decade of DP closure, the increased southward heat transport has melted back a substantial fraction of Antarctic sea ice. At the same time the polar oceans warm by 4°–6°C on the zonal mean, and the maximum strength of the Southern Hemisphere westerlies weakens by ≃10%. These effects, not captured in models without ice and atmosphere feedbacks, combine to force Antarctic Bottom Water (AABW) to warm and freshen, to the point that this water mass becomes less dense than NADW. This leads to a marked contraction of the Antarctic overturning, allowing NADW to ventilate the abyssal ocean once more. Poleward heat transport settles back to very similar values as seen in the unperturbed DP open case. Yet remarkably, the equilibrium climate in the closed DP configuration retains a strong Southern Hemisphere warming, similar to past studies with no dynamic atmosphere. However, here it is ocean–atmosphere–ice feedbacks, primarily the ice-albedo feedback and partly the weakened midlatitude jet, not a vigorous southern sinking, which maintain the warm polar oceans. This demonstrates that DP closure can drive a hemisphere-scale warming with polar amplification, without the presence of any vigorous Southern Hemisphere overturning circulation. Indeed, DP closure leads to warming that is sufficient over the West Antarctic Ice Sheet region to inhibit ice-sheet growth. This highlights the importance of the DP gap, Antarctic sea ice, and the associated ice-albedo feedback in maintaining the present-day glacial state over Antarctica.

Anatomy of a eustatic event during the Turonian (Late Cretaceous) hot greenhouse climate

Sequence stratigraphic studies consider relative change in sea level (as regulated by eustasy, local tectonics and sediment supply) as the main builder of the stratigraphic record. Eustasy has generally been considered as a consequence of the growth and decay of continental ice sheets that would explain large, rapid changes in sea level, even during periods of relative global climatic warmth. However, such a mechanism has become increasingly difficult to envision during times of extreme global warmth such as the Turonian, when the equator-to-pole temperature gradient was very low and the presence of polar ice seems improbable. This paper investigates the timing and extent of sea level falls during the late Cenomanian through Turonian, especially the largest of those events, sequence boundary KTu4, which occurred during the middle to late Turonian peak of the Cretaceous hot greenhouse climate. We conclude that the amplitude of the widespread third-order sea level fall in the middle Turonian that is centered at ~91.8 Ma varies at different locations depending on the influence of dynamic topography on local tectonics and regional climatic conditions. Ice volume variations seem unlikely as a mechanism for controlling sea level at this time. However, this causal factor cannot be ruled out completely since Antarctic highlands (if they existed in the Late Cretaceous) could sequester enough water as ice to cause eustatic falls. To ascertain this requires detailed tomographic imaging of Antarctica, followed by geodynamic modeling, to determine whether high plateaus could have existed to accumulate ephemeral ice sheets. Other mechanisms for sea level change, such as transference between ground water (a small amplitude shorter time scale effect) and the ocean and entrainment and release of water from the mantle to the oceanic reservoir (a potentially large amplitude and longer time scale process), are intriguing and need to be explored further to prove their efficacy at third-order time scales.

Toward understanding Cretaceous climate—An updated review

New data and ideas are changing our view of conditions during the Cretaceous. Paleotopography of the continents was lower than originally thought, eliminating the ‘cold continental interior paradox’ of fossils of plants that could not tolerate freezing occurring in regions indicated by climate models to be well below freezing in winter. The controversy over the height of Cretaceous sea levels has been resolved by knowledge of the effects of passage of the subducted slab of the Farallon Plate beneath the North American crust. The cause of shorter term sea level changes of the order of 30 to 50 meters is not because of growth and decay of ice sheets, but more likely the filling and release of water from groundwater reservoirs and lakes although there may have been some ice in the Early and latest Cretaceous. Carbon dioxide was not the only significant greenhouse gas; methane contributed significantly to the warmer climate. Suggestions of very warm tropical ocean temperatures (> 40°C) have implications for the nature of plant life on land limited by Rubisco activase. The land surfaces were much wetter than has been thought, with meandering rivers and many oxbow lakes providing habitat for large dinosaurs. A major rethinking of the nature of conditions on a warmer Earth is underway, and a new suite of paleoclimate simulations for the Cretaceous is needed.