N.J. Shackleton

On the structure and origin of major glaciation cycles 2. The 100,000‐year cycle

Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000-year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000-year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000-year cycle requires a source of climatic inertia having a time constant (similar to 15,000 years) much larger than the other cycles (similar to 5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity-driven modulations in the 23,000-year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000-year oscillations. Whether a glacier or ice sheet influences the climate depends very much on the scale....The interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger, thereby gradually increasing its radius of influence.

Dansgaard‐Oeschger and Heinrich event imprints in Alboran Sea paleotemperatures

Past sea surface temperature (SST) evolution in the Alboran Sea (western Mediterranean) during the last 50,000 years has been inferred from the study of C37 alkenones in International Marine Global Change Studies MD952043 core. This record has a time resolution of ∼200 years allowing the study of millennial-scale and even shorter climatic changes. The observed SST curve displays characteristic sequences of extremely rapid warming and cooling events along the glacial period. Comparison of this Alboran record with δ18O from Greenland ice (Greenland Ice Sheet Project 2 core) shows a strong parallelism between these SST oscillations and the Dansgaard-Oeschger events. Five prominent cooling episodes standing out in the SST profile are accompanied by an anomalous high abundance of Neogloboquadrina pachyderma sinistral which is confined to the duration of these cold intervals. These features and the isotopic record reflect drastic changes in the surface hydrography of the Alboran Sea in association with Heinrich events Hl–5.

Variability of the western Mediterranean Sea surface temperature during the last 25,000 years and its connection with the Northern Hemisphere climatic changes

Sea surface temperature (SST) profiles over the last 25 kyr derived from alkenone measurements are studied in four cores from a W-E latitudinal transect encompassing the Gulf of Cadiz (Atlantic Ocean), the Alboran Sea, and the southern Tyrrhenian Sea (western Mediterranean). The results document the sensitivity of the Mediterranean region to the short climatic changes of the North Atlantic Ocean, particularly those involving the latitudinal position of the polar front. The amplitude of the SST oscillations increases toward the Tyrrhenian Sea, indicating an amplification effect of the Atlantic signal by the climatic regime of the Mediterranean region. All studied cores show a shorter cooling phase (700 years) for the Younger Dryas (YD) than that observed in the North Atlantic region (1200 years). This time diachroneity is related to an intra-YD climatic change documented in the European continent. Minor oscillations in the southward displacement of the North Atlantic polar front may also have driven this early warming in the studied area. During the Holocene a regional diachroneity propagating west to east is observed for the SST maxima, 11.5–10.2 kyr B.P. in the Gulf of Cadiz, 10–9 kyr B.P. in the Alboran Sea, and 8.9–8.4 kyr B.P. in the Thyrrenian Sea. A general cooling trend from these SST maxima to present day is observed during this stage, which is marked by short cooling oscillations with a periodicity of 730±40 years and its harmonics.

Oxygen and carbon isotope record of East Pacific core V19-30: implications for the formation of deep water in the late Pleistocene North Atlantic

Abstract A detailed oxygen and carbon isotope record has been obtained from benthic Foraminifera in core V19-30 from the Carnegie Ridge on the south side of the Panama Basin. Expressing these records and the oxygen and carbon isotope records previously published for Atlantic core M-12392 on a common timescale, it is apparent that the oxygen isotope records are very similar but that the carbon isotope records are quite different. By obtaining the carbon isotope gradient between the two sites as a function of time we show that the production of North Atlantic Deep Water has varied over a wide range during the late Pleistocene, and that the pattern of variation is not simply related to the well known oxygen isotope record. Although the two oxygen isotope records are very similar, changes in the interoceanic gradient are detectable and support the hypothesis that in the glacial mode the North Atlantic was colder, and less oxygenated, than it is today. Shackletons [1] 1977 interpretation whereby the carbon isotope record from the Atlantic core reflects changes in the terrestrial biomass, is an over-simplification. However, the record from the Pacific core V19-30 probably can be explained in these terms since it probably approximates the carbon isotope record of global mean oceanic dissolved CO 2 .

High resolution palynological record off the Iberian margin: direct land-sea correlation for the Last Interglacial complex

Abstract We present high resolution pollen, dinocyst and isotopic data for the Last Interglacial complex from marine core MD952042 (southwestern margin of the Iberian Peninsula; 37°48′N; 10°10′W; 3148 m). Direct land-sea correlation from this core indicates that during this period, North Atlantic sea surface temperatures were in phase with Iberian climate. Our palynological analysis suggests a Younger Dryas-like event at the Marine Isotope Stage (MIS)-6/5 transition. The analysis also indicates that the Eemian spans from the lightest isotopic values of MIS-5e (ca. 126 ky BP) to the heavier isotopic values towards the MIS-5e/5d transition. Therefore, the Eemian is not entirely equivalent to MIS-5e. Pollen analysis identifies four climatic phases of low amplitude during the Eemian. A Mediterranean climate in southwestern Europe is gradually replaced by oceanic conditions. The middle of the Eemian is characterized by an increase in precipitation on the land and ocean, associated with a slight cooling. This seems to be the result of a displacement of the Polar Front as far south as southern Europe during this period. After the Eemian, three relatively short climatic phases on land (Melisey I, St. Germain Ia and Montaigu cold event) occurred contemporaneously with three shifts of sea surface temperatures. The Montaigu event, first identified in terrestrial pollen sequences, is, therefore, also recorded in core MD952042 on the basis of pollen, dinocyst and planktonic isotopic data. Our results also show that the warm periods of MIS-5 are not characterized by similar climatic conditions on land.

Carbonate Dissolution Fluctuations in the Western Equatorial Pacific During the Late Quaternary

Planktonic foraminiferal test fragmentation in three cores along a depth transect from the western equatorial Pacific (ERDC-93P, 1619 m; RC17-177, 2600 m; V28-238, 3120 m [Thompson, 1976]) were examined for the last 500 kyr at sample intervals from 2.5 to 5 kyr to study the fluctuations of dissolution in the western equatorial Pacific. The age models were constructed by correlating the δ18O records with the SPECMAP stack [Imbrie et al., 1984]. Results showed that intermediate and deep waters experienced the same patterns of dissolution through climatic cycles. Fragmentation varied with a greater amplitude, and the carbonate ion concentration changed less, in the deep than in the intermediate water. Dissolution has significant variance distributions and coherencies with δ18O over the 100, 41, and 23 kyr periods of orbital variations; dissolution maxima lag ice volume minima by 6 to 20 kyr. The dissolution variability was consistent with recent geochemical models which seek to explain the reduction of atmospheric CO2 concentration at the last glacial maximum [Broecker, 1982; Boyle, 1988].

Comparison of terrestrial and marine records of changing climate of the last 500,000 years

A broad correspondence between long pollen sequences and the deep-sea oxygen isotope record has been noted for some time, but there has been little effort to explore just how similar the two types of evidence are in terms of their overall structure on glacial-interglacial timescales and also how they may differ. These questions have profound importance both for how we view the stratigraphic record of changing climate in different regions and for our understanding of the climate system. Here we link the four longest European pollen records and derive a terrestrial sequence of vegetation events and a coherent stratigraphic scheme for the last 500,000 years. Comparison of the terrestrial and marine records shows good agreement, but it also reveals that the pollen sequences contain a higher degree of climate sensitivity than the oxygen isotope record. In addition, it suggests that neither an oxygen isotope record nor a Milankovitch-forced ice volume model may provide an appropriate template for fine-tuning the terrestrial record and that better chronologies will depend on an improved understanding of controls on sedimentation rates in individual sedimentary basins

Evidence for enhanced Mediterranean thermohaline circulation during rapid climatic coolings

Molecular biomarkers (C37 alkenones, n-nonacosane and n-hexacosanol) and TOC are used together with benthic δ18O and δ13C data to document the hydrographic response of the western Mediterranean Sea to rapid climatic variability. These proxies are recorded in core MD 95-2043 (Alboran Sea) affording the study of the Dansgaard–Oeschger (D–O) and Heinrich (HE) variability during the last glacial period. The results suggest that rapid changes in the western Mediterranean thermohaline circulation occurred in parallel to sea surface temperature oscillations. Enhanced deep water ventilation occurred during cold intervals (HE and D–O Stadials) probably driven by a strengthening of north-westerly wind over the north-western Mediterranean Sea. In contrast, decreased intensity of the thermohaline circulation is detected during warm intervals (D–O Interstadials) which led to low oxygenated deep water masses and better preservation of the organic matter in the sediment.

High resolution stratigraphic correlation of benthic oxygen isotopic records spanning the last 300,000 years

Abstract In order to compare the response of different oceanographic regions to global climate change, very detailed stratigraphic techniques are required. The global signal of ice volume changes recorded in the oxygen isotopic composition of foraminifera can provide the tool for developing the necessary high resolution stratigraphy. In order to evaluate the resolution of a stratigraphy based on detailed isotopic records, two techniques are used to correlate a set of benthic oxygen isotope records from seven piston cores taken in the North and South Atlantic, the Indian, and the equatorial and North Pacific oceans. The first technique was modified from the graphic correlation procedure of Shaw (1964). This procedure requires the identification of isotopic events that are correlated from core to core. Detailed correlations for intervals of the cores between events are provided by a series of straight line segments connecting all common events. The second technique developed by Martinson et al. (1982) uses inverse procedures to define a continuous non-linear mapping function that correlates the isotopic records. The mapping function maximizes the correlation coefficient between data sets being compared. The techniques are independent in that they rely on different criteria for correlating the data series. Stratigraphic correlations obtained by these procedures are in excellent agreement. The mean difference between the correlations is on the order of the sampling intervals of each core and, when corrected for sedimentation rates, suggests that benthic isotope records from the suite of seven cores can be correlated to a resolution of 2000 to 4000 yrs.

Surface water temperature, salinity, and density changes in the northeast Atlantic during the last 45,000 years: Heinrich events, deep water formation, and climatic rebounds

We developed a new method to calculate sea surface salinities (SSS) and densities (SSD) from planktonic foraminiferal δ18O and sea surface temperatures (SST) as determined from planktonic foraminiferal species abundances. SST, SSS, and SSD records were calculated for the last 45,000 years for Biogeochemical Oceanic Flux Study (BOFS) cores 5K and 8K recovered from the northeast Atlantic. The strongest feature is the dramatic drop in all three parameters during the Heinrich “ice-rafting” events. We modelled the possibility of deepwater formation in die northeast Atlantic from the SSD records, by assuming that the surface waters at our sites cooled as they flowed further north. Comparison with modelled North Atlantic deepwater densities indicates that there could have been periods of deepwater formation between 45,000 and 30,000 14C years B.P. (interrupted by iceberg meltwater input of Heinrich event 3 and 4, at 27,000 and 38,000 14C years B.P.) and during the Holocene. No amount of cooling in the northeast Atlantic between 30,000 and 13,000 years could cause deep water to form, because of the low salinities resulting from the high meltwater inputs from icebergs. Our records indicate that after each Heinrich event there were periods of climatic rebound, with milder conditions persisting for up to 2000 years, as indicated by the presence of warmer and more saline water masses. After these warm periods conditions returned to average glacial levels. These short term cold and warm episodes in the northeast Atlantic are superimposed on the general trend towards colder conditions of the Last Glacial Maximum (LGM). Heinrich event 1 appears to be unique as it occurs as insolation rose and was coeval with the initial melting of the Fennoscandian ice sheet. We propose that meltwater input of Heinrich event 1 significantly reduced North Atlantic Deep Water formation, reducing the heat exchange between the low and high latitudes, thus delaying deglaciation by about 1500 radiocarbon years (2000 calendar years).