Multiproxy comparison of oceanographic temperature during Heinrich Events in the eastern subtropical Atlantic
Highlights
► Mg/Ca-paleothermometry during Heinrich Events. ► Regional Mg/Ca context. ► Effects of salinity and ΔCO32− are not significant on regional Mg/Ca–temperature. ► Cd/Ca constrains upwelling to be no more intense than in modern. ► SST response during Heinrich Events may depend on background climate state.
Introduction
During the last glacial and deglacial periods, Greenland experienced rapid changes in surface air temperature (Dansgaard–Oeschger or “D–O” Events), characterized by an abrupt, decadal scale warming followed by a gradual, millennial scale cooling (Dansgaard et al., 1993, Grootes and Stuiver, 1997). The coldest stadials of these events were punctuated by catastrophic ice sheet calvings, especially of the Laurentide Ice Sheet, approximately every 7.2 kyr (Sarnthein et al., 2000). These massive calving episodes, Heinrich Events (HE), are identified by layers of ice rafted debris as far south as the northern subtropics and are associated with strong sea surface temperature fluctuations in the North Atlantic (Cortijo et al., 1997, Van Kreveld et al., 2000). The climatic significance of HE is also evidenced by far-reaching geographic locations that show correlative events along the coasts of Brazil and Florida and in the Mediterranean Sea (Arz et al., 1998, Broecker, 2006, Cacho et al., 1999, Grimm et al., 2006).
SST for HE have been derived mostly from faunal assemblage data with some additional data from alkenone U37K′ and Mg/Ca. SST estimates derived from planktonic foraminiferal assemblage data suggest that during HE, summer SSTs dropped to 3–6 °C in the subarctic North Atlantic (Chapman and Maslin, 1999, Cortijo et al., 1997) and to 5–10 °C along the Iberian Margin in the subtropical North Atlantic (de Abreu et al., 2003, McManus et al., 2004, Salgueiro et al., 2010, Schoenfeld and Zahn, 2000). A curious feature of the subtropical SSTs implied by the faunal assemblage data along the Iberian Margin is that the data imply nearly the same large cooling during each HE, despite disparate thicknesses of ice-rafted debris (IRD) layers in the North Atlantic for different HE. In contrast, the dramatic shifts in subtropical SSTs of as much as 10 °C, specifically during HE1 (~ 17–15 ka), HE4 (~ 40–38 ka) and HE5 (~ 47–45 ka) (de Abreu et al., 2003, Voelker et al., 2006), are not consistent with SSTs derived from alkenone U37K′ which imply little cooling during the glacial HE in this region (Bard et al., 2000, Pailler and Bard, 2002).
Lower SSTs throughout the North Atlantic during HE are consistent, however, with modeling experiments in which freshwater released into areas of deep water formation in the North Atlantic results in a weakening, or complete shutdown of North Atlantic Deep Water (NADW) circulation (Ganopolski and Rahmstorf, 2001). Model runs that explore a range of freshwater forcing scenarios predict lower SSTs throughout the North Atlantic that vary with both the amplitude and timing of the freshwater input. The degree of North Atlantic cooling during these events and the effects on the Atlantic Meridional Overturning Circulation (AMOC) system are likely also to depend on the background climate state (Prange et al., 2004). Proxy data can help in evaluating the environmental changes associated with HE during both the glacial and deglacial states and may help to constrain sensitivity of the AMOC to the background climate.
Here, we use Mg/Ca data from planktonic foraminifera to further explore SST changes during HE. Mg/Ca has been widely applied in paleotemperature reconstructions, particularly over the last glacial cycle, but have been only looked at during HE in a limited number of studies (Peck et al., 2008, Skinner and Elderfield, 2005). Complications in applying Mg-paleothermometry, in general, are the potential influence of dissolution and salinity on shell Mg/Ca as well as the necessity for species-specific Mg–temperature equations. Dissolution is unlikely to be a major influence in the cores in this study given that both cores are located above the regional lysocline and contain well-preserved foraminifer throughout. In these two cores, shell weights per individual are high (10–14 μg), indicating well preserved specimens, however there is variation in normalized shell weight during HE1 in core MD95-2040 (see Section 4.1.1). Any potential salinity effect, however, could have a larger impact in temperature reconstructions given that HE could be accompanied by significant local salinity changes due to the melting of ice; yet, the magnitude of the salinity effect on Mg/Ca is not well defined. Here we deal with uncertainties in the Mg–temperature relationship, including salinity, by using multiple calibration equations to constrain a range of temperatures and compare the range to other temperature proxies. We derive one of the calibration equations from published core top data for the North Atlantic.
Comparing new foraminiferal minor and trace element data to the suite of existing paleoceanographic proxy data including foraminiferal δ18O, assemblage data and alkenone U37K′ during HE1, HE4 and HE5 in this region helps to constrain the climate fluctuations during HE. We use a published ~ 15 ka record of Mg/Ca variability from the same region to put the temperature changes during HE into the context of climate variability during the last glacial and deglacial (Skinner and Elderfield, 2005). We also compile SST proxy data for HE1 and HE4 to develop a better picture of the impact of these two events in the broader North Atlantic. Finally, we present the first measurements of planktonic Cd/Ca from HE. We investigate whether planktonic Cd/Ca, a proxy for nutrient concentration, shows evidence for changes in upwelling, which is one mechanism by which SSTs could change among the coastal records as well as in comparison to changes in the North Atlantic, more generally, during HE.
We show that, in this region, each temperature proxy implies different amplitudes of change during Heinrich Events. Despite the differences among the proxies, the dominant trend among the proxy data suggests that the climate changes in the North Atlantic were larger during HE1, which occurs over the transition from glacial to interglacial, than during HE4 and HE5, which are solidly in the glacial. Temperature changes in the coastal region we examine here appear to have been larger than changes in the open ocean during both HE1 and HE4, but are unlikely to be attributed to more intense coastal upwelling.
Section snippets
Core locations, age models and sampling
We obtained samples from two cores, MD95-2040 (40.58°N, 9.86°W; 2465 m) along the western Iberian Margin (the northwest margin) and MD99-2339 (35.88°N, 7.53°W; 1170 m) along the southern Iberian Margin, in the Gulf of Cadiz (Fig. 3). Analysis of these IMAGES project sediment cores reveals IRD during each of the five most recent HE, indicating the presence of icebergs in this region (de Abreu et al., 2003, Voelker et al., 2006). The presence of IRD allows tight correlation of regional
Mg/Ca
Mg/Ca of G. bulloides during all three HE are lower than Holocene values at the two sites investigated. The lowest values are found at the northernmost site (MD95-2040) during HE1 (Fig. 1). HE1 shows the most well-defined and structured cooling of HE1, HE4, and HE5 in this core. Near the site of MD95-2040, Mg/Ca of surface G. bulloides samples are 2.6 mmol/mol; just prior to HE1 (~ 17.6 ka) Mg/Ca values are 2.0 mmol/mol (Fig. 1A). There is a distinct drop in the Mg/Ca at the onset of HE1 that
Calculating a range of temperature estimates from Mg/Ca
We begin our discussion with an examination of the set of Mg-temperature equations that we use to constrain temperatures during HE. Several Mg/Ca–temperature equations have been developed for G. bulloides using core top, sediment trap and culturing data (Cléroux et al., 2008, Elderfield and Ganssen, 2000, Lea et al., 1999, McConnell and Thunell, 2005). Calibrations for G. bulloides yield a Mg–T response of ~ 6–10% increase per °C, with most of the equations defining a Mg–T response of ~ 8–10%
Conclusion
A comparison of temperature proxies outside of HE4 and HE5 shows that Mg/Ca-derived temperatures and alkenone (U37K′)-derived SSTs agree very well. Both of these reconstructed temperatures are lower than assemblage-derived SSTs and the amplitudes of HE are larger than the difference of the proxy comparison. During HE, alkenone and Mg/Ca proxies reveal similar amplitudes of change (relative to core top reconstructions) while assemblage data shows very large temperature fluctuations. HE1 is the
Acknowledgments
G.P. wrote the manuscript with substantial input from P.M. and A.V. A.V. also provided samples. E.S. provided regional surface sediment data and help with speciation as well as participating in discussion. We would like to thank Georges Paradis and Kate Steger for analysis of our cleaned samples by magnetic sector ICP-MS. Samples were analyzed at the University of California, Santa Barbara; we thank D. Lea for making the analytical facility available. Discussion with Tobias Friedrich was very
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