Mark Siddall

Sea-level fluctuations during the last glacial cycle

The last glacial cycle was characterized by substantial millennial-scale climate fluctuations, but the extent of any associated changes in global sea level (or, equivalently, ice volume) remains elusive. Highstands of sea level can be reconstructed from dated fossil coral reef terraces, and these data are complemented by a compilation of global sea-level estimates based on deep-sea oxygen isotope ratios at millennial-scale resolution or higher. Records based on oxygen isotopes, however, contain uncertainties in the range of ±30u2009m, or ±1u2009°C in deep sea temperature. Here we analyse oxygen isotope records from Red Sea sediment cores to reconstruct the history of water residence times in the Red Sea. We then use a hydraulic model of the water exchange between the Red Sea and the world ocean to derive the sill depth—and hence global sea level—over the past 470,000 years (470u2009kyr). Our reconstruction is accurate to within ±12u2009m, and gives a centennial-scale resolution from 70 to 25u2009kyr before present. We find that sea-level changes of up to 35u2009m, at rates of up to 2u2009cmu2009yr-1, occurred, coincident with abrupt changes in climate.

Atmospheric Carbon Dioxide Concentration Across the Mid-Pleistocene Transition

A Change in the Air? Between around 1.2 million and 500,000 years ago, Earths glacial cycle changed from one with a period of roughly 40,000 years to one with a period of about 100,000 years. Although there has been much speculation about why this transition may have occurred, no potential explanation seemed more likely than that it was caused by decreasing concentrations of atmospheric CO2. Hönisch et al. (p. 1551) present a record of atmospheric pCO2 for the past 2.1 million years, derived from the boron isotopic composition of planktonic foraminifera, and show that the amount of CO2 in the atmosphere has remained relatively constant over that period. While pCO2 was approximately 30 ppm higher before the start of the mid-Pleistocene transition than after the transition, atmospheric CO2 did not decrease gradually as would be expected were it to be the driver of the transition. The concentration of CO2 in the atmosphere has been relatively stable over the past 2.1 million years and never as high as it is today. The dominant period of Pleistocene glacial cycles changed during the mid-Pleistocene from 40,000 years to 100,000 years, for as yet unknown reasons. Here we present a 2.1-million-year record of sea surface partial pressure of CO2 (Pco2), based on boron isotopes in planktic foraminifer shells, which suggests that the atmospheric partial pressure of CO2 (pco2) was relatively stable before the mid-Pleistocene climate transition. Glacial Pco2 was ~31 microatmospheres higher before the transition (more than 1 million years ago), but interglacial Pco2 was similar to that of late Pleistocene interglacial cycles (<450,000 years ago). These estimates are consistent with a close linkage between atmospheric CO2 concentration and global climate, but the lack of a gradual decrease in interglacial Pco2 does not support the suggestion that a long-term drawdown of atmospheric CO2 was the main cause of the climate transition.

7. Eustatic sea level during past interglacials

Eustatic sea-level variation is the primary index of global ice volume over glacial cycles. Here, we review recent studies of eustatic sea level during interglacial periods. This review includes in-depth discussion of the variability, magnitude and duration of the last five interglacial periods and a summary of evidence for the last nine interglacial periods. The last nine interglacial periods differ not only in height and variability of sea level, but also in timing relative to northern summer insolation peaks. Some interglacials have a single peak and others have several, while MIS 11 persisted with little variation for at least 30 kyr. Estimates of interglacial sea levels remain subject to uncertainties. There is an outstanding need for glacio-hydro-isostatic modelling for all glacial cycles of interest, as well as improvements in dating and dating-correction techniques.

Making sense of palaeoclimate sensitivity

Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in Ku2009W−1u2009m2) of 0.3–1.9 or 0.6–1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2–4.8u2009K per doubling of atmospheric CO2, which agrees with IPCC estimates.

Similar meltwater contributions to glacial sea level changes from Antarctic and northern ice sheets

The period between 75,000 and 20,000 years ago was characterized by high variability in climate and sea level. Southern Ocean records of ice-rafted debris suggest a significant contribution to the sea level changes from melt water of Antarctic origin, in addition to likely contributions from northern ice sheets, but the relative volumes of melt water from northern and southern sources have yet to be established. Here we simulate the first-order impact of a range of relative meltwater releases from the two polar regions on the distribution of marine oxygen isotopes, using an intermediate complexity model. By comparing our simulations with oxygen isotope data from sediment cores, we infer that the contributions from Antarctica and the northern ice sheets to the documented sea level rises between 65,000 and 35,000 years ago were approximately equal, each accounting for a rise of about 15u2009m. The reductions in Antarctic ice volume implied by our analysis are comparable to that inferred previously for the Antarctic contribution to meltwater pulse 1A (refs 16, 17), which occurred about 14,200 years ago, during the last deglaciation.

Sea-level reversal during Termination II

The termination of the penultimate glacial period (TII) shows both similarities and differences to the last termination (TI). Both terminations show significant cold reversals in the postglacial warming trend. TI consists of a continuously increasing sea-level trend, whereas TII may demonstrate a sea-level reduction midway through the termination. We present a new, continuous sea-level record for TII derived from Red Sea δ 18 O records that supports the existence of the TII sea-level reversal. The record gives an unprecedented look at the structure of the TII sea-level reversal, which consists of an early highstand lasting several millennia in duration, followed by a 30 ± 12 m to 40 ± 12 m reduction in sea level and a stillstand of several millennia, before the final sea-level rise to the marine oxygen isotope stage (MIS) 5e interglacial. We suggest that there is a link between the sea-level reversal and internal, millennial-scale variability in the climate system.

Modeling the relationship between 231Pa/230Th distribution in North Atlantic sediment and Atlantic meridional overturning circulation

Down-core variations in North Atlantic 231Paxs/230Thxs have been interpreted as changes in the strength of nthe Atlantic meridional overturning circulation (AMOC). This modeling study confirms that hypothetical nchanges in the AMOC would indeed be recorded as changes in the distribution of sedimentary 231Paxs/230Thxs. nAt different sites in the North Atlantic the changes in sedimentary 231Pa/230Th that we simulate are diverse and ndo not reflect a simple tendency for 231Paxs/230Thxs to increase toward the production ratio (0.093) when the nAMOC strength reduces but instead are moderated by the particle flux. In its collapsed or reduced state the nAMOC does not remove 231Pa from the North Atlantic: Instead, 231Pa is scavenged to the North Atlantic nsediment in areas of high particle flux. In this way the North Atlantic 231Paxs/230Thxs during AMOC shutdown nfollows the same pattern as 231Paxs/230Thxs in modern ocean basins with reduced rates of meridional overturning n(i.e., Pacific or Indian oceans). We suggest that mapping the spatial distribution of 231Paxs/230Thxs across several nkey points in the North Atlantic is an achievable and practical qualitative indicator of the AMOC strength in the nshort term. Our results indicate that additional North Atlantic sites where down-core observations of n231Paxs/230Thxs would be useful coincide with locations which were maxima in the vertical particle flux during nthese periods. Reliable estimates of the North Atlantic mean 231Paxs/230Thxs should remain a goal in the longer nterm. Our results hint at a possible ‘‘seesaw-like’’ behavior in 231Pa/230Th in the South Atlantic.

Modelling the seasonal cycle of the exchange flow in Bab El Mandab (Red Sea)

A minimum complexity, three-layer hydraulic model has been further developed to simulate the seasonal cycle of the exchange flow in Bab el Mandab (Red Sea). Unlike earlier versions our model incorporates a realistic channel cross-section. To a good approximation the model simulates observed fluxes through the strait and the layer depths at Hanish Sill. The model results indicate that the summertime intrusion of Gulf of Aden Intermediate Water into the Red Sea has been a robust feature of the exchange for the last 10 500 years. The modern intrusion acts as a dynamic barrier to the exchange in the upper and lower layers effectively, reducing the mean annual flux in each layer by 26% and 33% compared to model runs without the intrusion.

Marine Isotope Stage (MIS) 8 millennial variability stratigraphically identical to MIS 3

The Marine Isotope Stage (MIS) 3 stratigraphy is highly robust and was reproduced during another period: MIS 8.6 global ice volume was similar during MIS 8.6 to MIS 3 (60 to 90 m sea level equivalent), but the Milankovitch insolation forcing was different, implying that Earths predisposition to millennial internal variability is controlled by the configuration of the major ice sheets. The involvement of additional factors cannot be ruled out but by identifying several such periods using new deep ice cores from Dome Concordia and Dome Fuji (Antarctica) as well as the marine record we may isolate the factors predisposing Earth to these highly significant modes of climate variability.

Sea Surface and High-Latitude Temperature Sensitivity to Radiative Forcing of Climate over Several Glacial Cycles

AbstractA compilation is presented of global sea surface temperature (SST) records that span around one glacial cycle or more, and it is compared with changes in the earth’s radiative balance over the last 520 000 years, as determined from greenhouse gas concentrations, albedo changes related to ice sheet area and atmospheric dust fluctuations, and insolation changes. A first scenario uses global mean values for the radiative changes, and a second scenario uses zonal means for 10° latitude bands for a more regionally specific perspective. On the orbital time scales studied here, a smooth increase of SST response from the equator to high latitudes is found when comparison is made to global mean radiative forcing, but a sharply “stepped” increase at 20°–30° latitude when comparing with the more regionally specific forcings. The mean global SST sensitivities to radiative change are within similar limits for both scenarios, around 0.8 ± 0.4°C (W m−2)−1. Combined with previous estimates of 1.3–1.5 times strong...