Elisabeth Michel

EVIDENCE FOR CHANGES IN THE NORTH ATLANTIC DEEP WATER LINKED TO MELTWATER SURGES DURING THE HEINRICH EVENTS

Abstract Deep sea sediment records from North Atlantic cores (40°N–55°N) provide evidence of several massive iceberg discharges, known as Heinrich events, during the last glacial period. High resolution benthic δ 18 O and δ 13 C records from North Atlantic sediment cores were used to monitor the impact of Heinrich events on thermohaline circulation and to estimate the sensitivity of deep oceanic circulation to changes in freshwater input to the North Atlantic surface waters. Our data indicate that major rearrangements of deep-water masses were directly associated with these massive iceberg discharges. To trace in detail the deep water conditions in the North Atlantic, benthic δ 13 C values in several cores were used to generate time slices before, during and after Heinrich event 4 dated at ∼ 35 ka BP. Although North Atlantic Deep Water continued to form during the oxygen isotope stage 3 at 37 ka BP, deep circulation was characterized by an increased incursion of deep waters of southern origin, which reduced the δ 13 C composition of North Atlantic deep waters, particularly in the eastern Atlantic basin. North Atlantic Deep Water production was reduced during Heinrich Layer 4 (HL4 at ∼ 35 ka BP) synchronously with the changes in the surface water hydrology. Deep convection processes may have occurred in areas not affected by the salinity decrease. Soon after HL4 ( ∼ 33 ka BP) the δ 13 C distribution was similar to that before the event. Similarly, a rapid return to initial δ 13 C values was observed at the end of the most clearly defined Heinrich events (HL5, HL4 and HL1). Comparison between deep circulation patterns corresponding to HL4, the Last Glacial Maximum and Heinrich event 1 indicates that each of these periods was characterized by a different circulation state associated with changes in convection sites.

The timing of the last deglaciation in North Atlantic climate records

To determine the mechanisms governing the last deglaciation and the sequence of events that lead to deglaciation, it is important to obtain a temporal framework that applies to both continental and marine climate records. Radiocarbon dating has been widely used to derive calendar dates for marine sediments, but it rests on the assumption that the ‘apparent age’ of surface water (the age of surface water relative to the atmosphere) has remained constant over time. Here we present new evidence for variation in the apparent age of surface water (or reservoir age) in the North Atlantic ocean north of 40° N over the past 20,000 years. In two cores we found apparent surface-water ages to be larger than those of today by 1,230 ± 600 and 1,940 ± 750 years at the end of the Heinrich 1 surge event (15,000 years BP) and by 820 ± 430 to 1,010 ± 340 years at the end of the Younger Dryas cold episode. During the warm Bølling–Allerød period, between these two periods of large reservoir ages, apparent surface-water ages were comparable to present values. Our results allow us to reconcile the chronologies from ice cores and the North Atlantic marine records over the entire deglaciation period. Moreover, the data imply that marine carbon dates from the North Atlantic north of 40° N will need to be corrected for these highly variable effects.

Geomagnetic field intensity, North Atlantic Deep Water circulation and atmospheric Δ14C during the last 50 kyr

We present simulated records of past changes in the atmospheric Δ14C for the last 50 kyr due to changes in geomagnetic field intensity and in the strength of the North Atlantic Deep Water (NADW). A new geomagnetic record was used, largely based on the NAPIS-75 record [Laj et al., Phil. Trans. R. Soc. London A 358 (2000) 1009–1025] which has been extended for the 0–20 kyr interval using archeomagnetic and volcanic data. Past changes of the NADW were derived from a mineral magnetic study of the cores used in the construction of NAPIS-75. Two box models of different complexity (4 and 17 boxes) were used to simulate the carbon cycle. Calculated records of Δ14C are consistent with experimental determinations for the last 24 kyr. For older ages, the records calculated with variable oceanic circulation conditions reach values as high as 600‰ (with an average of 500‰) between 20 and 40 kyr with maxima around 21, 30 and 38 kyr (GISP2 age model), while low values are observed prior to 42 kyr. Although large inconsistencies in experimental data preclude precise comparison, the average record simulated with the 17-box model is overall consistent with the Icelandic Sea record [Voelker et al., Radiocarbon 40 (1998) 517–534; 42 (2000) 437–452], except for the extremely high peak observed in this record at 40.5 kyr. On the other hand, the results recently reported from a stalagmite recovered from a submerged cave in the Bahamas [Beck et al., Science 292 (2001) 2453–2458] are inconsistent with all our model simulations. In the 20–45 kyr interval, the improved geomagnetic record combined with the new NADW profile allows us to give a modeled evaluation of the relative contribution of these factors to changes in atmospheric Δ14C. The average simulation provides a first order modeled correction for conventional radiocarbon ages older than 25 kyr for which no calibration curve is available as yet.

COULD DEEP SUBANTARCTIC CONVECTION FEED THE WORLD DEEP BASINS DURING THE LAST GLACIAL MAXIMUM

Simple box model calculations are used to simulate the oceanic circulation during the last glacial maximum (LGM). These experiments show that the main features of the δ13C and Δ14C distributions and of the lysocline depth may be explained by a circulation pattern very different from the modem one. Intermediate and upper deep waters were produced in the North Atlantic Ocean, whereas deep waters of Subantarctic Mode type, forming at the northern edge of the Subantarctic convergence, invaded the main oceanic basins. The Southern Ocean, mainly self ventilated, had a reduced deep component that flew southward along the East Pacific Ridge and the Australian west cost. The thermodynamic fractionation that occurs during air-sea exchange has only contributed slightly to the glacial deep δ13C distribution through surface water temperature variations.

Carbon isotope records reveal precise timing of enhanced Southern Ocean upwelling during the last deglaciation.

The Southern Ocean plays a prominent role in the Earths climate and carbon cycle. Changes in the Southern Ocean circulation may have regulated the release of CO₂ to the atmosphere from a deep-ocean reservoir during the last deglaciation. However, the path and exact timing of this deglacial CO₂ release are still under debate. Here we present measurements of deglacial surface reservoir ¹⁴C age changes in the eastern Pacific sector of the Southern Ocean, obtained by ¹⁴C dating of tephra deposited over the marine and terrestrial regions. These results, along with records of foraminifera benthic-planktic ¹⁴C age and δ¹³C difference, provide evidence for three periods of enhanced upwelling in the Southern Ocean during the last deglaciation, supporting the hypothesis that Southern Ocean upwelling contributed to the deglacial rise in atmospheric CO₂. These independently dated marine records suggest synchronous changes in the Southern Ocean circulation and Antarctic climate during the last deglaciation.

Calibration of the carbon isotope composition (δ13C) of benthic foraminifera

The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (−2.6 ± 0.4) × 10−3‰/(μmol kg−1) [CO32−] and pressure (−4.9 ± 1.7) × 10−5‰ m−1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.

Carbon isotope offsets between benthic foraminifer species of the genus Cibicides (Cibicidoides) in the glacial sub-Antarctic Atlantic

Epibenthic foraminifer δ13C measurements are valuable for reconstructing past bottom water dissolved inorganic carbon δ13C (δ13CDIC), which are used to infer global ocean circulation patterns. Epibenthic δ13C, however, may also reflect the influence of 13C-depleted phytodetritus, microhabitat changes, and/or variations in carbonate ion concentrations. Here, we compare the δ13C of two benthic foraminifer species, Cibicides kullenbergi and Cibicides wuellerstorfi, and their morphotypes, in three sub-Antarctic Atlantic sediment cores over several glacial-interglacial transitions. These species are commonly assumed to be epibenthic, living above or directly below the sediment-water interface. While this might be consistent with the small δ13C offset that we observe between these species during late Pleistocene interglacial periods (Δδ13C = -0.19 ±0.31 ‰, N =63), it is more difficult to reconcile with the significant δ13C offset that is found between these species during glacial periods (Δδ13C =-0.76 ±0.44‰, N =44). We test possible scenarios by analysing Uvigerina spp. δ13C and benthic foraminifer abundances: 1) C. kullenbergi δ13C is biased to light values, either due to microhabitat shifts or phytodetritus effects; and 2) C. wuellerstorfi δ13C is biased to heavy values, relative to long-term average conditions, for instance by recording the sporadic occurrence of less depleted deep water δ13CDIC. Neither of these scenarios can be ruled out unequivocally. However, our findings emphasize that supposedly epibenthic foraminifer δ13C in the sub-Antarctic Atlantic may reflect several factors rather than being a sole function of bottom water δ13CDIC, which directly bear on the interpretation of extremely light South Atlantic δ13C values at the last glacial maximum.

Mg/Ca thermometry in planktic foraminifera: Improving paleotemperature estimations for G. bulloides and N. pachyderma left

Planktic foraminiferal Mg/Ca ratios have become a fundamental seawater temperature proxy in past climate reconstructions, due to the temperature dependence of Mg uptake into foraminiferal calcite. However, empirical calibrations for single species from methodologically consistent data are still lacking. Here we present species-specific calibrations of Mg/Ca versus calcification temperature for two commonly used species of planktic foraminifera: Globigerina bulloides and Neogloboquadrina pachyderma left, based on a series of Southern Ocean and North Atlantic core tops. Combining these new data with previously published data, we derive an integrated G. bulloides Mg/Ca-temperature calibration for mid and high latitudes of both hemispheres between 2 and 18°C, where Mg/Ca = 1.006 ± 0.032 * e0.065 ± 0.003*Tiso (R2 = 0.82). G. bulloides is found to calcify deeper in the Southern Ocean (∼ 200 m) than in the North Atlantic (top 50 m). We also propose a Mg/Ca temperature calibration to describe the temperature response in N. pachyderma left that calcified away from the influence of sea ice in the Southern Ocean, valid between ∼ −1 and 9°C, of the form Mg/Ca = 0.580 ± 0.016 * e0.084 ± 0.006*Tiso (R2 = 0.70). These calibrations account for uncertainties on Mg/Ca measurements and calcification temperature that were carefully estimated and propagated using Monte Carlo iterations. The 1σ propagated error in Mg/Ca-derived temperatures is 1.1°C for G. bulloides and 0.9°C for N. pachyderma left for the presented data sets. Geographical extension of genotypes must be assessed when choosing to develop regional or global calibrations.

Teleconnection between the Intertropical Convergence Zone and southern westerly winds throughout the last deglaciation

Comparison of environmental changes between northeastern Brazil and western Patagonia during the last deglaciation reveals concomitant trends in moisture from the Intertropical Convergence Zone (ITCZ) and southern westerly winds (SWW). The data confirm an atmospheric teleconnection between the ITCZ and SWW, associated with Atlantic Meridional Overturning Circulation (AMOC) variations. When the AMOC decreases, both the ITCZ and the SWW shift southward; they shift northward when the AMOC increases. Climate simulations in which the AMOC is made to vary agree with this general pattern. Additional experiments performed with an atmosphere-only model show that the tropical Atlantic is a key area in promoting relationships between the AMOC, ITCZ, and SWW. Our data show that this mechanism, which transfers climate changes between low and middle latitudes to high latitudes in the Southern Hemisphere, acted throughout the abrupt climatic events of the last deglaciation.

correction: The timing of the last deglaciation in North Atlantic climate records

This corrects the article DOI: 35089060