André Paul

Modeling the water masses of the Atlantic Ocean at the Last Glacial Maximum

We produced gridded monthly sea-surface boundary conditions for the Atlantic Ocean at the Last Glacial Maximum (LGM) based on the sea-surface temperature reconstruction of the GLAMAP project. We used an ocean general circulation model (OGCM), subject to these sea-surface boundary conditions and a corresponding wind stress field from an atmospheric general circulation model, to study the differences in the distribution of the main water masses between the LGM and the present. Our global OGCM is characterized by high vertical resolution, low vertical diffusion, and isopycnal mixing and hence allows for a realistic representation of the hydrology and circulation of the modern Atlantic Ocean. According to a series of LGM experiments with an increasing sea-surface salinity anomaly in the Weddell Sea, the ventilated thermocline was colder than today by 2�3°C in the North Atlantic Ocean and, in the experiment with the largest anomaly (1.0 beyond the global anomaly), by 4�5°C in the South Atlantic Ocean. Its depth was reduced by 50 m on average, most notably in the tropics. In the North Atlantic Ocean the outcrop locations of the thermocline isopycnal surfaces migrated southward by 5°�10°, and the ventilation increased. In the South Atlantic Ocean the mixed layer and thermocline water masses were dominated by cold water originating from Drake Passage, and the import of warm water from the Indian Ocean was reduced to about 4 Sv or 40% of its modern value. Antarctic Intermediate Water was colder by 3�4°C and could be traced as far as 10°N. The meridional overturning rates of North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) in the Atlantic Ocean were similar to those of the present-day experiment (9�10 Sv and 4 Sv, respectively). However, NADW cooled by 2.5°C and AABW by 1°C. AABW was near the freezing point of seawater at the surface and the saltiest water mass in the Atlantic Ocean, even saltier than NADW. We show that the differences between the LGM and the present-day experiments can be traced back to the changes in the subpolar and interhemispheric sea-surface density gradients.

Temperature:δ18O relationships of planktonic foraminifera collected from surface waters

Abstract Most of the isotopic paleotemperature equations used for paleoceanographic reconstructions have been derived from culture experiments or inorganic precipitates of calcium carbonate. To test these equations in the modern ocean, we measured the oxygen isotope composition of planktonic foraminifera (Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma) collected from Atlantic and Southern Ocean surface waters, and added published plankton tow data from the Pacific, Indian and Arctic Oceans. The resulting species-specific regression equations of the temperature:δ18O relationships for G. ruber, G. sacculifer and G. bulloides are statistically indistinguishable. The equations derived for G. sacculifer and G. bulloides agree with relationships obtained from laboratory experiments, in which these species were cultured at pH values close to modern surface waters. The equation derived from N. pachyderma has a significantly lower slope and offset than the other three species but produces a regression equation that is nearly identical to the one for the epifaunal benthic foraminifer Cibicides sp. Our work on plankton tow and pumped samples indicates that culture-derived equations appear to be more appropriate for predicting the absolute δ18O of the species examined compared to equations derived from inorganic precipitates. However, over the oceanic temperature range, the slopes of the equations we derive for living species agree with the slopes obtained from inorganic precipitates.

Holocene Climate Variability on Centennial-to-Millennial Time Scales: 1. Climate Records from the North-Atlantic Realm

Holocene oxygen isotope data from the GISP2 ice core reveal temperature oscillations in Greenland with a periodicity of ~900 y, which can be correlated to climate perturbations in northern and central Europe. We suggest that the 900-y climate fluctuations are generated within the climate system, and are probably triggered by negative salinity anomalies in the North Atlantic. A simple template is used to show that two such triggering events centered at ~8.3 and 4.7 ky BP are required to explain temporal evolution of 900-y climate cycles between ~3.5–8.5 ky BP as sequence of damped oscillations. Although pacing of the 900-y cycles by changes in the Earth’s orbit cannot be ruled out, we regard this scenario as unlikely. We show that the existing paleoceanographic evidence for ~1400–1500-y climate oscillations during the Holocene is questionable. Instead we suggest that deep-sea records from the North Atlantic may be reconciled with 900-y climate oscillations during this period.

Influence of vertical mixing on the thermohaline hysteresis: Analyses of an OGCM

The thermohaline hysteresis response to varying North Atlantic freshwater forcing is studied by means of a three-dimensional global ocean general circulation model (OGCM). The influence of vertical diffusivity is examined using a wide range of mixing coefficients. For sufficiently large vertical diffusivity the model shows a pronounced hysteresis behavior, so that two equilibrium states of the thermohaline circulation are found: one mode with intense deep-water formation in the North Atlantic (‘‘on’’ mode) and one mode with strongly reduced convective activity (‘‘off’’ mode). By decreasing the diffusivity, the two branches of the hysteresis merge. In addition, the effect of numerical diffusion is analyzed by applying different advection schemes. A positive feedback between overturning strength and the numerical diffusivity of upstream advection is found, resulting in a reduced stability of the ‘‘on’’ mode. Comparing the results with those from zonally averaged models exhibits substantial differences regarding the stability properties of the thermohaline circulation. Freshwater transports by horizontal gyres have an important effect on the overturning circulation in the OGCM.

The Atlantic Ocean at the Last Glacial Maximum: 1. Objective Mapping of the GLAMAP Sea-Surface Conditions

Recent efforts of the German paleoceanographic community have resulted in a unique data set of reconstructed sea-surface temperature for the Atlantic Ocean during the Last Glacial Maximum, plus estimates for the extents of glacial sea ice. Unlike prior attempts, the contributing research groups based their data on a common definition of the Last Glacial Maximum chronozone and used the same modern reference data for calibrating the different transfer techniques. Furthermore, the number of processed sediment cores was vastly increased. Thus the new data is a significant advance not only with respect to quality, but also to quantity. We integrate these new data and provide monthly data sets of global sea-surface temperature and ice cover, objectively interpolated onto a regular 1°×1° grid, suitable for forcing or validating numerical ocean and atmosphere models. This set is compared to an existing subjective interpolation of the same base data, in part by employing an ocean circulation model. For the latter purpose, we reconstruct sea surface salinity from the new temperature data and the available oxygen isotope measurements.

The South Atlantic Oxygen Isotope Record of Planktic Foraminifera

This paper reviews the recording of oxygen isotope ratios in planktic foraminifera and summarizes recent results of the application of oxygen isotopes in paleoceanographic studies of the South Atlantic. The most important factors controlling the δ18O of planktic foraminifera are temperature, the δ18O and the pH of ambient seawater. Seasonal and vertical calcification weight the mean δ18O of a foraminiferal population towards the hydrographic conditions in the preferred ecological niche. After deposition, the δ18O signal is affected by bioturbation and dissolution. Despite many influence factors, the composition of oxygen isotopes in fossil tests of planktic foraminifera provides important constraints on variations of the surface water hydrography of the South Atlantic and the Southern Ocean throughout the past 20,000 years. During the last glacial maximum, the Polar Front remained close to its modem position or shifted only slightly towards the north. In the tropics, oxygen isotopes indicate only a moderate glacial cooling of 2–3°C. During deglaciation, oxygen isotope ratios in the eastern boundary currents of the subtropical South Atlantic decreased asynchronously relative to those in the eastern North Atlantic, with the highest interhemispheric contrasts during the Younger Dryas and the Heinrich Event 1. This pattern is consistent with a redistribution of heat within the Atlantic Ocean in response to a weakening of the thermohaline circulation. The slowdown of deglacial overturning was associated with a southward displacement of the thermal equator and the Intertropical Convergence.

Simulation of Oxygen Isotopes in a Global Ocean Model

We incorporate the oxygen isotope composition of seawater δ18Ow into a global ocean model that is based on the Modular Ocean Model (MOM, version 2) of the Geophysical Fluid Dynamics Laboratory (GFDL). In a first experiment, this model is run to equilibrium to simulate the present-day ocean; in a second experiment, the oxygen isotope composition of Antarctic Surface Water (AAS W) is set to a constant value to indirectly account for the effect of sea-ice. We check the depth distribution of δ18Ow against observations. Furthermore, we computed the equilibrium fractionation of the oxygen isotope composition of calcite δ18Oc from a paleotemperature equation and compared it with benthic foraminiferal δ18O. The simulated δ18Ow distribution compares fairly well with the GEOSECS data. We show that the δ18Ow values can be used to characterize different water masses. However, a warm bias of the global ocean model yields δ18Oc values that are too light by about 0.5 %o above 2 km depth and exhibit a false vertical gradient below 2 km depth. Our ultimate goal is to interpret the wealth of foraminiferal δ18O data in terms of water mass changes in the paleocean, e.g. at the Last Glacial Maximum (LGM). This requires the warm bias of the global ocean model to be corrected. Furthermore the model must probably be coupled to simple atmosphere and sea-ice models such that neither sea-surface salinity (SSS) nor surface δ18Ow need to be prescribed and the use of present-day δ18Ow-salinity relationships can be avoided.

Modeling the oxygen‐isotopic composition of the North American Ice Sheet and its effect on the isotopic composition of the ocean during the last glacial cycle

contribution to the change of seawater d 18 O( dw) between the Last Glacial Maximum (LGM) and the Holocene and to evaluate the effect of nonequilibrium isotopic composition of the NAIS on the relationship between ice-volume variations and the ocean isotopic enrichment. The enrichment due to the NAIS at the LGM was 0.63%, corresponding to � 60% of the LGM sea-level lowstand and to a mean d 18 O of the NAIS of approximately � 31%. The modeled NAIS volume variations and the induced dw changes over the past 120,000 years indicated no significant time lag. The inaccuracy associated with linearly inferring ice-volume variations from dw changes was generally less than 10%. Citation: Sima, A., A. Paul, M. Schulz, and J. Oerlemans (2006), Modeling the oxygen-isotopic composition of the North American Ice Sheet and its effect on the isotopic composition of the ocean during the last glacial cycle, Geophys. Res. Lett., 33, L15706, doi:10.1029/2006GL026923.

South Atlantic interocean exchange as the trigger for the Bølling warm event

The North Atlantic Ocean underwent an abrupt temperature increase of 9 °C at high latitudes within a couple of decades during the transition from Heinrich event 1 (H1) to the Bolling warm event, but the mechanism responsible for this warming remains uncertain. Here we address this issue, presenting high-resolution last deglaciation planktic and benthic foraminiferal records of temperature and oxygen isotopic composition of seawater (δ 18 Osw) for the subtropical South Atlantic. We identify a warming of ~6.5 °C and an increase in δ 18 O sw of 1.2‰ at the permanent thermocline during the transition, and a simultaneous warming of ~3.5 °C with no significant change in δ 18 O sw at intermediate depths. Most of the warming can be explained by tilting the South Atlantic east-west isopycnals from a flattened toward a steepened position associated with a collapsed (H1) and strong (Bolling) Atlantic meridional overturning circulation (AMOC). However, this zonal seesaw explains an increase of just 0.3‰ in permanent thermocline δ 18 O sw . Considering that δ 18 O sw at the South Atlantic permanent thermocline is strongly influenced by the inflow of salty Indian Ocean upper waters, we suggest that a strengthening in the Agulhas leakage took place at the transition from H1 to the Bolling, and was responsible for the change in δ 18 O sw recorded in our site. Our records high-light the important role played by Indian-Atlantic interocean exchange as the trigger for the resumption of the AMOC and the Bolling warm event.

Simulating the sea level imprint on marine oxygen isotope records during the middle Miocene using an ice sheet-climate model

[1] Oxygen isotopic ratios are implemented in an ice sheet–climate model in order to directly compare the modeled isotopic ratio of the seawater to the high‐resolution isotopic records from deep‐sea sediment cores in the middle Miocene. The isotopic depletion resulting from the modeled ice sheet expansion explains the mean oxygen isotope step found in deep‐sea sedimentary records of ∼0.5‰. Approximately 85% of the modeled increase in the isotopic composition of seawater is caused by an increase in ice volume; the remainder is due to a stronger depletion in oxygen isotopes in the large ice sheet. Furthermore, we also investigated the relation between sea level (or global ice volume) and the isotopic composition of seawater. Our experiments confirm the validity of the relation of approximately 1‰ enrichment per 100 m sea level lowering. We further show that this relationship is r estricted by the mean ocean depth and the assumed oxygen isotopic composition of the ice sheet. Small deviations (±10%) from this general relationship occur depending on the size and the mean isotopic content of the ice sheet. Large continental ice sheets are more depleted in heavy oxygen isotopes and therefore reach a slightly higher ratio. In contrast, small ice sheets have a less depleted isotopic composition and correspondingly have a smaller effect on the isotopic composition of the ocean.