Changes In Continental Ice Volume Research Articles

Quantitative organic-walled dinoflagellate cyst stratigraphy across the Eocene-Oligocene Transition in the Gulf of Mexico: A record of climate- and sea level change during the onset of Antarctic glaciation

The Eocene – Oligocene Transition (EOT, 34–33.5 Ma) marks a major transition in Cenozoic climate evolution through the relatively rapid establishment of a continental-scale ice sheet on Antarctica. The EOT is characterized by two 200 kyr spaced shifts (termed EOT-1 and Oi-1) in the oxygen isotopic composition (δ18O) of benthic foraminifera, representing both changes in continental ice-volume and temperature. Estimates of the timing and magnitude of these changes during this critical phase in Earth’s climatic evolution are controversial. Here we present marine palynological assemblage data, in particular of organic-walled dinoflagellate cysts (dinocysts), across a classic upper Eocene to lower Oligocene neritic succession cored in Alabama, USA; the Saint Stephens Quarry (SSQ) borehole. These palynological data combined with lithological information allow the identification of three sequence boundaries across the EOT. Critically, we identify a sequence boundary at the level corresponding to the EOT-1. Integrated sea level and paleotemperature records show that EOT-1 primarily represents cooling with some minor and transient continental ice sheet expansion. Furthermore, we identify a significant hiatus, likely caused by major sea level fall at the base of Magnetochron C13n that corresponds to the Oi-1 shift. This clarifies the δ18O records from SSQ that essentially lack the expected pronounced shift to positive δ18O values so characteristic for Oi-1. Furthermore, we document originations and extinctions of potentially temperature-sensitive dinocysts associated with the EOT-1. In contrast, the Oi-1 does not stand out as period of substantial dinoflagellate turnover. The combined results illustrate that major cooling, limited and transient ice growth and major biotic change were occurring before the full-size expansion of the Antarctic cryosphere. © 2018 The Authors.

Plio-Pleistocene climate sensitivity evaluated using high-resolution CO2 records

Theory and climate modelling suggest that the sensitivity of Earth's climate to changes in radiative forcing could depend on the background climate. However, palaeoclimate data have thus far been insufficient to provide a conclusive test of this prediction. Here we present atmospheric carbon dioxide (CO2) reconstructions based on multi-site boron-isotope records from the late Pliocene epoch (3.3 to 2.3 million years ago). We find that Earth's climate sensitivity to CO2-based radiative forcing (Earth system sensitivity) was half as strong during the warm Pliocene as during the cold late Pleistocene epoch (0.8 to 0.01million years ago). We attribute this difference to the radiative impacts of continental ice-volume changes (the ice-albedo feedback) during the late Pleistocene, because equilibrium climate sensitivity is identical for the two intervals when we account for such impacts using sea-level reconstructions. We conclude that, on a global scale, no unexpected climate feedbacks operated during the warm Pliocene, and that predictions of equilibrium climate sensitivity (excluding long-term ice-albedo feedbacks) for our Pliocene-like future (with CO2 levels up to maximum Pliocene levels of 450parts per million) are well described by the currently accepted range of an increase of 1.5K to 4.5K per doubling of CO2.

Long-term vegetation change and controlling factors in Donghae area, Korea, over the past 40,000 years

Determining the effect of climate change since the last glacial maximum (LGM) in the East Sea (Sea of Japan) coastal region of Korea has been difficult owing to a dearth of suitable paleoclimate proxies. A 40,000-year carbon isotope ( δ 13C) record from cave deposits collected from Cheongok Cave, Korea reveals both long-term change and superimposed shorter-term isotopic excursions. The δ 13C value of the sedimentary organic matter, a proxy for past vegetation change, varied from − 25‰ to − 21‰, with lower values during the LGM and higher values during the Holocene, a pattern that is generally consistent with trends in total organic carbon content (TOC%) and magnetic susceptibility. From the LGM to the late Holocene, the C 4 plant abundance (mainly grasses), estimated using a simple binary mixing model, increased from 15% to 45%. This long-term change resembles trends in biogenic opal production in the East Sea, sea-surface temperature, and global sea level more than it does Asian summer monsoon strength. Long-term changes in C 4 plant abundance in eastern coastal areas of Korea over the last 40,000 years were probably affected most strongly by sea level-driven ocean circulation changes in the East Sea coupled with continental ice-volume changes; low pCO2 during the LGM was not sufficient to cause the expansion of C 4 plant coverage without favorable climatic conditions.

Stable isotope clue to episodic sea ice formation in the glacial North Atlantic

Sea ice is a determinant parameter of the climate system, which acts as amplifier through positive ice-albedo feedbacks and induces deep-water formation through brine production. It is also one of the most elusive parameters in the paleoclimate record. Most proxies of sea ice provide information on its presence and extent of (e.g., diatoms or dinocysts), but do not permit assessment on its rate of formation. However, seemingly off-equilibrium, low δ18O values in mesopelagic planktic foraminifer species, as observed today in the Arctic Ocean, are thought to relate to the production of isotopically light brines during sea ice formation, and might thus provide a clue on the rate of sea ice growth. With reference to the isotopic properties of modern planktic foraminifers in the Arctic Ocean and using multi-proxy data sets from the last glacial stage in the northwest North Atlantic, we re-examined some of the current interpretations of planktic isotope records in the glacial NW North Atlantic. The large amplitude light isotopic excursions recorded in Neogloboquadrina pachyderma left coiling during Heinrich events 1, for example, correspond to extensive sea ice cover as reconstructed from dinocysts, and do not seem unequivocally linked to low-salinity pulses. Such light isotopic excursion more likely responded to enhanced rates of sea ice formation resulting in the production and sinking of isotopically light brines. On the contrary, the isotopically heavy planktic foraminifers of the last glacial maximum (LGM) interval stricto sensu would rather suggest relatively low rates of brine production, thus low sea ice growth rates in the area. This would imply that the LGM distribution of sea ice in the North Atlantic was primarily linked to spreading and drifting from marginal and Nordic source areas and not from enhanced in situ production. At the scale of glacial–interglacial cycles, isotopic distillation processes relating to sea ice production at high latitudes could be one of the factors controlling the 18O–salinity relationship in deep ocean water masses that might deserve closer examination, as it would be independent of continental ice volume changes.

Paleoclimate history of Galápagos surface waters over the last 135,000 yr

Records of planktonic and benthic foraminiferal δ 18O and planktonic Mg/Ca from core TR163-22, just northwest of the Galápagos Islands, reveal a detailed (250–450 year resolution) climate history of the region over the last 135 thousand years (kyr). Sea surface temperatures (SSTs), reconstructed from G. ruber Mg/Ca, averaged 24.3±0.4 °C during the Holocene, 22.6±0.6 °C during marine isotope stage (MIS) 2, 3 and 4, and 26.0±0.9 °C during MIS 5e. Changes in SST lead changes in both planktonic and benthic δ 18O by an average ∼3 kyr, suggesting that SST changes in this region predated continental ice volume changes. Changes in SST display clear millennial scale variability, especially in marine isotope stage 3, with behavior somewhat similar to Antarctic proxy air temperature and South Pacific SST records. Removal of the temperature component from the planktonic δ 18O record demonstrates that glacial-interglacial δ 18O-water changes at this site were 1.0±0.2‰, similar to estimates for mean ocean shifts, implying that salinity changes due to regional hydrological variation between the Last Glacial Maximum and Holocene in the Galápagos region were minimal. Comparison between the TR163-22 SST record and an SST record from core TR163-19 North of the Equatorial Front reveals a largely similar broad-scale climate history, suggesting that changes in the Galápagos region were caused by large scale forcing rather than by local or regional dynamical changes. Changes in atmospheric greenhouse forcing are the most plausible explanation for the observed large-scale climate changes in the eastern equatorial Pacific.

The Mid-Pleistocene Transition in the Tropical Pacific

A sea surface temperature (SST) record based on planktonic foraminiferal magnesium/calcium ratios from a site in the western equatorial Pacific warm pool reveals that glacial-interglacial oscillations in SST shifted from a period of 41,000 to 100,000 years at the mid-Pleistocene transition, 950,000 years before the present. SST changes at both periodicities were synchronous with eastern Pacific cold-tongue SSTs but preceded changes in continental ice volume. The timing and nature of tropical Pacific SST changes over the mid-Pleistocene transition implicate a shift in the periodicity of radiative forcing by atmospheric carbon dioxide as the cause of the switch in climate periodicities at this time.

Forcing of tropical Atlantic sea surface temperatures during the mid‐Pleistocene transition

We compare a new mid‐Pleistocene sea surface temperature (SST) record from the eastern tropical Atlantic to changes in continental ice volume, orbital insolation, Atlantic deepwater ventilation, and Southern Ocean front positions to resolve forcing mechanisms of tropical Atlantic SST during the mid‐Pleistocene transition (MPT). At the onset of the MPT, a strong tropical cooling occurred. The change from a obliquity‐ to a eccentricity‐dominated cyclicity in the tropical SST took place at about 650 kyr BP. In orbital cycles, tropical SST changes significantly preceded continental ice‐volume changes but were in phase with movements of Southern Ocean fronts. After the onset of large‐amplitude 100‐kyr variations, additional late glacial warming in the eastern tropical Atlantic was caused by enhanced return flow of warm waters from the western Atlantic driven by strong trade winds. Pronounced 80‐kyr variations in tropical SST occurred during the MPT, in phase with and likely directly forced by transitional continental ice‐volume variations. During the MPT, a prominent anomalous long‐term tropical warming occurred, likely generated by extremely northward displaced Southern Ocean fronts. While the overall pattern of global climate variability during the MPT was determined by changes in mean state and frequency of continental ice volume variations, tropical Atlantic SST variations were primarily driven by early changes in Subantarctic sea‐ice extent and coupled Southern Ocean frontal positions.

Climate impact of late quaternary equatorial pacific sea surface temperature variations

Magnesium/calcium data from planktonic foraminifera in equatorial Pacific sediment cores demonstrate that tropical Pacific sea surface temperatures (SSTs) were 2.8 degrees +/- 0.7 degrees C colder than the present at the last glacial maximum. Glacial-interglacial temperature differences as great as 5 degrees C are observed over the last 450 thousand years. Changes in SST coincide with changes in Antarctic air temperature and precede changes in continental ice volume by about 3 thousand years, suggesting that tropical cooling played a major role in driving ice-age climate. Comparison of SST estimates from eastern and western sites indicates that the equatorial Pacific zonal SST gradient was similar or somewhat larger during glacial episodes. Extraction of a salinity proxy from the magnesium/calcium and oxygen isotope data indicates that transport of water vapor into the western Pacific was enhanced during glacial episodes.

Introduction

Understanding Earth's climatic history has lead researchers to explore the glacial history of Antarctica and its role in the Earth's climate system. Scientists have used the information recorded both on the continent and from the surrounding marine sediments. Attention to the deep sea record has focused on the use of proxy indicators such as ice-rafted detritus, sedimentological variations, stable-isotope records and changes in faunal and floral assemblages to identify fluctuating ice sheets and changes in continental ice volume. Dating of specific geological events has relied on microfossil biostratigraphy with calibration of specific biostratigraphical datums (e.g. first and last occurrences) to polarity stratigraphies. In the absence of polarity records, dating of biostratigraphical events was based on known calibrations and/or stratigraphical successions of specific species from regions other than the Southern Ocean. For example, Miocene diatom events in the Southern Ocean were typically assigned ages based on the known ranges of the species in the North Pacific. The chronological framework, based on imported microfossil datums, was used to calibrate the ages of datums specific to the Southern Ocean, i.e. a process involving secondary correlation. These approaches assumed isochroneity of events between oceans. The biochronological framework in this paper is based on the results of the numerous shipboard and shorebased scientists who completed detailed analyses of the sediments recovered at the 59 Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) Southern Ocean sites (Fig. 1; Table 1). Their contributions have made significant advances in the understanding of species biogeography, evolutionary succession

The Atmosphere’s Response to the Ice Sheets of the Last Glacial Maximum

This paper discusses some modeling results that indicate how the atmospheric response to the topography of the continental ice of the Last Glacial Maximum (LGM) may be related to the cold North Atlantic Ocean of that time. Broccoli and Manabe (1987) used a three-dimensional general circulation model (GCM) of the atmosphere coupled with a fixed-depth, static ocean mixed-layer model with ice-age boundary conditions to investigate the individual influences of the CLIMAP ice sheets, snow-free land albedos, and reduced atmospheric CO2 concentrations. They found that the ice sheets are the most influential of the ice-age boundary conditions in modifying the northern hemisphere climate, and that the presence of continental ice sheets alone leads to cooling over the North Atlantic Ocean.One approach for extending these GCM results is to consider the stationary waves generated by the ice sheets. Cook and Held (1988) showed that a linearized, steady-state, primitive equation model can give a reasonable simulation of the GCM’s stationary waves forced by the Laurentide ice sheet. The linear model analysis suggests that the mechanical effect of the changed slope of the surface, and not changes in the diabatic heating (e.g. the high surface albedos) or time-dependent transports that necessarily accompany the ice sheet in the GCM, is largely responsible for the ice sheet’s influence. To obtain the ice-age stationary-wave simulation, the linear model must be linearized about the zonal mean fields from the GCM’s ice-age climate. This is the case because the proximity of the cold polar air to the region of adiabatic heating on the downslope of the Laurentide ice sheet is an important factor in determining the stationary waves. During the ice age, cold air can be transported southward to balance this downslope heating by small perturbations in the meridional wind, consistent with linear theory. Since the meridional temperature gradient is more closely related to the surface albedo (ice extent) than to the ice volume, this suggests a mechanism by which changes in the stationary waves and, therefore, their cooling influence at low levels over the North Atlantic Ocean, can occur on time scales faster than those associated with large changes in continental ice volume.

The Atmosphere’s Response to the Ice Sheets of the Last Glacial Maximum

This paper discusses some modeling results that indicate how the atmospheric response to the topography of the continental ice of the Last Glacial Maximum (LGM) may be related to the cold North Atlantic Ocean of that time. Broccoli and Manabe (1987) used a three-dimensional general circulation model (GCM) of the atmosphere coupled with a fixed-depth, static ocean mixed-layer model with ice-age boundary conditions to investigate the individual influences of the CLIMAP ice sheets, snow-free land albedos, and reduced atmospheric CO2 concentrations. They found that the ice sheets are the most influential of the ice-age boundary conditions in modifying the northern hemisphere climate, and that the presence of continental ice sheets alone leads to cooling over the North Atlantic Ocean. One approach for extending these GCM results is to consider the stationary waves generated by the ice sheets. Cook and Held (1988) showed that a linearized, steady-state, primitive equation model can give a reasonable simulation of the GCM’s stationary waves forced by the Laurentide ice sheet. The linear model analysis suggests that the mechanical effect of the changed slope of the surface, and not changes in the diabatic heating (e.g. the high surface albedos) or time-dependent transports that necessarily accompany the ice sheet in the GCM, is largely responsible for the ice sheet’s influence. To obtain the ice-age stationary-wave simulation, the linear model must be linearized about the zonal mean fields from the GCM’s ice-age climate. This is the case because the proximity of the cold polar air to the region of adiabatic heating on the downslope of the Laurentide ice sheet is an important factor in determining the stationary waves. During the ice age, cold air can be transported southward to balance this downslope heating by small perturbations in the meridional wind, consistent with linear theory. Since the meridional temperature gradient is more closely related to the surface albedo (ice extent) than to the ice volume, this suggests a mechanism by which changes in the stationary waves and, therefore, their cooling influence at low levels over the North Atlantic Ocean, can occur on time scales faster than those associated with large changes in continental ice volume.

Climate studies with a coupled atmosphere-upper-ocean-ice-sheet model

A two-dimensional zonally averaged model has been developed for simulating the seasonal cycle of the climate of the Northern Hemisphere. The atmospheric component of the model is based on the two-level quasi-geostrophic potential vorticity system of equations. At the surface, the model has land—sea resolution and incorporates detailed snow and sea-ice mass budgets. The upper ocean is represented by an integral mixed-layer model that takes into account the meridional advection and turbulent diffusion of heat. Comparisons between the computed present-day climate and climatological data show that the model does reasonably well in simulating the seasonal cycle of the temperature field. In response to a projected CO 2 trend based on the scenario of Wuebbles et al. (DOE/ NBB-0066 Technical Report 15 (1984)), the modelled annual hemispheric mean surface temperature increases by 2 °C between 1983 and 2063. In the high latitudes, the response undergoes significant seasonal variations. The largest surface warmings occur during autumn and spring. The model is then asynchronously coupled to a model that simulates the dynamics of the Greenland, the Eurasian and the North American ice sheets in order to investigate the transient response of the climate to the long-term insolation anomalies caused by orbital perturbations. Over the last interglacial-glacial cycle, the coupled model produces continental ice-volume changes that are in general agreement with the low-frequency part of palaeoclimatic records.

Stable isotope stratigraphy from the Antarctic continental margin during the last one million years

A stable isotope record from the eastern Weddell Sea from 69°S is presented. For the first time, a 250, 000-yr record from the Southern Ocean can be correlated in detail to the global isotope stratigraphy. Together with magnetostratigraphic, sedimentological and micropalaeontological data, the stratigraphic control of this record can be extended back to 910,000 yrs B.P. A time scale is constructed by linear interpolation between confirmed stratigraphic data points. The benthic δ 18O record ( Epistominella exigua) reflects global continental ice volume changes during the Brunhes and late Matuyama chrons, whereas the planktonic isotopic record ( Neogloboquadrina pachyderma) may be influenced by a meltwater lid caused by the nearby Antarctic ice shelf and icebergs. The worldwide climatic improvement during deglaciations is documented in the eastern Weddell Sea by an increase in production of siliceous plankton followed, with a time lag of approximately 10,000 yrs, by planktonic foraminifera production. Peak values in the difference between planktonic and benthic δ 13C records, which are 0.5‰ higher during warm climatic periods than during times with expanded continental ice sheets, also suggest increased surface productivity during interglacials in the Southern Ocean.

Direct dating of the oxygen-isotope record of the last deglaciation by 14C accelerator mass spectrometry

Major trends of Quaternary global climate are reflected in the continental ice volume changes which have been reconstructed by oxygen-isotope analysis1,2. δ18O records from deep-sea sediments show that the net glacial build-up occurs relatively slowly3, but that the end of an ice age occurs quickly, in less than 10,000 yr, implying a nonlinear response4,5 to simple Milankovitch theory6. The latter observation suggests that the cause of the most recent deglaciation was the maximum in summer calorific radiation at the upper latitudes of the Northern Hemisphere centred around 11,000 yr ago, a view supported by early studies7. Later work has produced conflicting dates, the main source of confusion being problems with obtaining accurate and reliable dates. Here, by using accelerator mass spectrometry, we have measured 14C for various species of foraminifera to produce a reliable timescale for the oxygen-isotope record. Our results show that, at the end of the last ice age, continental ice sheets began to melt more than 4,000 yr before the Northern Hemisphere maximum of summer calorific radiation.