Amplitude and timing of temperature and salinity variability in the subpolar North Atlantic over the past 10 k.y. Rosemarie E. Came* Massachusetts Institute of Technology–Woods Hole Oceanographic Institution Joint Program in Oceanography, Woods Hole, Massachusetts 02543, USA
Delia W. Oppo Jerry F. McManus Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA
ABSTRACT Paired planktic foraminiferal δ18O and Mg/Ca data reveal trends of increasing temperatures (~3 °C) and salinities in the subpolar North Atlantic over the course of the Holocene, which were punctuated by abrupt events. The trends likely reflect an insolation-forced northward retreat of the boundary between polar and North Atlantic subsurface waters. The superimposed variability does not appear to be periodic, but tends to recur within a broad millennial band. The records provide convincing evidence of open-ocean cooling (nearly 2 °C) and freshening during the 8.2 ka event, and suggest similar conditions at 9.3 ka. However, the two largest temperature oscillations in our record (~2 °C) occurred during the past 4 k.y., suggesting a recent increase in temperature variability relative to the mid-Holocene, perhaps in response to neoglaciation, which began at about this time. Keywords: Ocean Drilling Program Site 984, paleotemperature, Mg/Ca, Björn Drift, Neogloboquadrina pachyderma dextral.
INTRODUCTION The Holocene Epoch is a time of relative climate stability when viewed within the context of the large-amplitude, millennial-scale fluctuations observed in the colder sections of the Greenland ice core records (Dansgaard et al., 1993; Grootes and Stuiver, 1997). Recent studies (Bianchi and McCave, 1999; Bond et al., 1997; deMenocal et al., 2000; O’Brien et al., 1995), however, confirm an earlier work (Denton and Karlén, 1973) suggesting that smaller, suborbital-scale variability has occurred throughout the past 10 k.y., but the nature and forcing mechanisms of this variability are not well constrained. It has been argued that the same mechanisms that drive millennial-scale variability during glacial periods also drive Holocene variability, causing a pervasive 1500 yr cyclicity in sea surface temperature, sea surface salinity, and subsurface processes (Bianchi and McCave, 1999; Bond et al., 1997). It has also been argued that solar forcing underlies the Holocene portion of the 1500 yr cycle (Bond et al., 2001), although recent work (Marchal, 2005) indicates that a solar forcing mechanism for millennial-scale variability is unlikely. Of the Holocene climate events, the 8.2 ka event is the largest excursion in the Greenland Ice Sheet Project 2 (GISP2) δ18O record (Alley et al., 1997). Multiple proxies in Greenland ice reveal a pattern at 8.2 ka of reduced air temperatures, drier conditions, stronger winds over the North Atlantic, and low atmospheric methane (Alley et al., 1997). Evidence of climatic excursions at 8.2 ka is also present in records from various regions of the circum-Atlantic, suggesting a widespread climatic event, i.e., European lake sediments (von Grafenstein et al., 1998) and tree rings (Klitgaard-Kristensen et al., 1998), high-latitude foraminiferal abundances (Ellison et al., 2006; Klitgaard-Kristensen et al., 1998; Labeyrie et al., 1999), African lake level records (Gasse, 2000), and Cariaco Basin sediments (Hughen et al., 1996). The spatial pattern of climate variability ca. 8.2 ka is similar to that of the Younger Dryas (Alley et al., 1997), and * Current address: Department of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA.
is consistent with model responses to a reduction in the North Atlantic meridional overturning circulation (MOC) (Manabe and Stouffer, 1988), perhaps caused by an increase in freshwater supply to the sea surface in the high-latitude North Atlantic (Barber et al., 1999; Ellison et al., 2006; Keigwin et al., 2005). Oxygen isotopes in planktic foraminifera have been used as tracers of freshwater in the surface North Atlantic (Bond et al., 2001; Keigwin et al., 2005). However, both seawater δ18O (δ18Osw) and calcification temperature influence the δ18O of foraminifera. The Mg/Ca of planktic foraminifera allows the determination of temperature independently of δ18O (Nürnberg et al., 1996). Using Mg/Ca-derived temperatures, the foraminiferal δ18O, and a correction for ice volume, the δ18Osw can be calculated, and salinity can be estimated if the regional δ18O-salinity relationship is known. Here we present the first open-ocean, subpolar North Atlantic records of Mg/Ca-derived near-surface temperature and salinity variability during the Holocene. Our paired measurements of foraminiferal Mg/Ca and δ18O were generated using the planktic species Neogloboquadrina pachyderma dextral [N. pachyderma (d.)]. Sediment trap data collected north of Iceland suggest that N. pachyderma (d.) calcifies throughout the year, at a depth of 30–40 m (Ostermann et al., 2001), although it may calcify at depths below 75 m, as suggested by data from the northeast Atlantic (Ottens, 1992) and the Santa Barbara Basin (Pak et al., 2004). Our records were generated using sediment from Ocean Drilling Program Site 984 (Fig. 1), located on the Björn Drift, on the eastern flank of the Reykjanes Ridge (61°N, 25°W, 1648 m). A near constant sedimentation rate of ~29 cm/k.y. in this core during the past 10 k.y. (see GSA Data Repository Fig. DR11) makes it suitable for the study of suborbital climate variability. Today, surface waters at Site 984 1 GSA Data Repository item 2007078, Figure DR1 (Site 984 data vs. depth), Figure DR2 (modern δ18O-salinity relationship), and Figure DR3 (multitaper coherence for Mg/Ca temperatures and atmospheric Δ14C), is available online at www.geosociety.org/pubs/ft2007.htm, or on request from editing@geosociety. org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
© 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. GEOLOGY, April 2007 Geology, April 2007; v. 35; no. 4; p. 315–318; doi: 10.1130/G23455A.1; 3 figures; Data Repository item 2007078.
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are dominated by the warm salty North Atlantic Current; however, colder fresher water may enter the study area from the northwest. Seasonal temperatures at 30 m depth range from 7.3 °C to 11.2 °C (Levitus and Boyer, 1994), and salinities are nearly constant at 35.1–35.2 (Levitus et al., 1994). The modern thermocline δ18Osw in this region of the North Atlantic is between 0‰ and 0.3‰ (Schmidt et al., 1999), in agreement with core top calculations. A conservative estimate using available nearsurface data (Schmidt et al., 1999) indicates that today a change of 0.1‰ in δ18Osw is equivalent to a salinity change of ~0.13 (Data Repository Fig. DR2). The presence of glacial meltwater would cause a similar relationship: a change of 0.1‰ in δ18Osw would be equivalent to a salinity change of ~0.1. RESULTS AND DISCUSSION The Mg/Ca and δ18O records reveal trends of increasing temperatures (~3 °C) and increasing δ18Osw (~1.0‰) from the early Holocene to 4 ka (Figs. 2B–2D). Suborbital-scale oscillations are superimposed on the Holocene trend. At 9.3 ka, Mg/Ca-derived near-surface temperatures decreased by 1–2 °C, and δ18Osw decreased by ~0.3‰. Similarly, at 8.2 ka, near-surface temperatures decreased by ~2 °C, and δ18Osw decreased by ~0.3‰. The records do not reveal significant suborbital variability from 8 to 4 ka, but they do reveal variability in the later Holocene. After 4 ka, two 2 °C temperature oscillations occurred, with cold events at 3.7, 1.8, and 0.9 ka, and relative warming at 3 and 1.5 ka. The late Holocene temperature component of foraminiferal δ18O variability dominates over the δ18Osw influence, and our estimates suggest relatively little change in δ18Osw after 3.7 ka. The warming trend observed in the Mg/Ca data from Site 984 is consistent with the Holocene trend observed in high-latitude North Atlantic foraminiferal-based proxies (Marchal et al., 2002; Risebrobakken et al., 2003), but contradicts many alkenone- and diatom-based records (Keigwin et al., 2005; Koç and Jansen, 1994; Marchal et al., 2002; Moros et al., 2004), which reveal an early to middle Holocene climatic optimum followed by a cooling trend. This discrepancy may be explained by differences in depth habitat and season (Moros et al., 2004). Diatoms and alkenones record summer temperatures in the shallow euphotic zone. Foraminifera, often living deeper in the water column, may record thermocline temperatures, which are set by ventilation during the winter months. Thus, alkenones
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Figure 2. Planktic data from Ocean Drilling Program Site 984. A: Greenland Ice Sheet Project 2 (GISP2) δ18O (Grootes et al., 1993) (purple). B: Average Mg/Ca-derived temperature estimates (blue), with 3-point running mean (black). Mg/Ca converted to temperature using von Langen et al. (2005) relationship. C: Average δ18O (red), with 3-point running mean (black). D: Seawater δ18O (black), calculated using 3-point smoothed Mg/Ca temperatures, 3-point smoothed foraminiferal δ18O, ice volume correction (Waelbroeck et al., 2002), and δ18O temperature equation for Neogloboquadrina pachyderma (d.) (von Langen, 2001). Accelerator mass spectrometer radiocarbon dates converted to calendar age (green) using CALIB 5.01 (Stuiver and Reimer, 1993) and calibration data set (Hughen et al., 2004). Yellow shading denotes suborbital oscillations discussed in text.
and diatoms may record the oceanographic response to changes in summer insolation, which decreased over the course of the Holocene, while foraminifera may record the response to changes in winter insolation, which increased over the course of the Holocene. This interpretation is supported by model simulations (Liu et al., 2003) that predict a Holocene cooling trend in the surface North Atlantic and a warming trend in the subsurface due to the changes in summer and winter insolation. However, we view this as a working hypothesis, as the winter insolation change is small, and the seasonality and depth habitat of N. pachyderma (d.) in our study area is not known. Nevertheless, the timing of a mid-Holocene climatic optimum, where it occurs, differs among North Atlantic sites. Furthermore, many other sites exhibit trends of late Holocene climate amelioration (Helama et al., 2002; Marchal et al., 2002; Risebrobakken et al., 2003), suggesting either considerable spatial variability or differences in the winter and summer response, as we suggest. The δ18Osw record suggests that wintertime salinities increased steadily at Site 984, from a minimum value at 8.2 ka to a maximum at 4 ka. The low salinities in the earlier part of the Holocene may partly reflect the presence of light deglacial surface waters. Because deglaciation
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was largely over by 8 ka, the subsequent gradual increase in salinity may reflect a trend of increasing advection of high-salinity waters relative to fresher polar waters, perhaps related to intensification of the MOC, or a change in local evaporation minus precipitation. Alternatively, a gradual redistribution of freshwater within the Atlantic, as has occurred in recent decades (Curry et al., 2003), and on millennial time scales (Weldeab et al., 2006), may have led to increasing salinities at high latitudes and associated freshening at low latitudes. Or an increase in the AtlanticPacific freshwater contrast may have accompanied a southward migration of the Intertropical Convergence Zone (Haug et al., 2001). The δ18Osw data suggest that salinities rapidly decreased by as much as 0.5 after 4 ka, and remained at or near modern values for the remainder of the record. The Mg/Ca-derived temperature data also reveal well-defined suborbital variability. At 8.2 ka temperatures decreased by ~2 °C, coincident with the cooling recorded in the GISP2 ice core, and confirming the cooling recorded at the Gardar Drift (Ellison et al., 2006). Concurrent with that cooling there was no significant change in the planktic foraminiferal δ18O, resulting in an estimated 0.3‰ δ18Osw decrease. Using our modern estimate, a decrease of 0.3‰ in δ18O equals a salinity decrease of ~0.4‰; using a glacial meltwater estimate, a decrease of 0.3‰ in δ18O equals a salinity decrease of ~0.3‰. The data strongly suggest the presence of freshwater along with the 2 °C cooling at 8.2 ka, although we cannot rule out the possibility of a change in the δ18O of precipitation. A freshwater-induced reduction in MOC has been proposed as a mechanism of high-latitude cooling during the 8.2 ka event (Barber et al., 1999). Barber et al. (1999) suggested that a catastrophic drainage of Laurentide lakes into the Labrador Sea disrupted the formation of Labrador Sea Water, and caused the cooling over central Greenland (Barber et al., 1999). However, planktic foraminiferal δ18O records from the active convection region of the Labrador Sea do not record a freshening during this interval, suggesting that the freshwater did not directly enter the Labrador Sea (HillaireMarcel et al., 1994). However, records from the Laurentian Fan (Keigwin et al., 2005), located south of the Labrador Sea, and from the Gardar Drift (Ellison et al., 2006) do show freshening at 8.2 ka, and there is evidence of ice rafting in the subpolar North Atlantic (Bond et al., 1997), consistent with an increased delivery of freshwater to the North Atlantic during this event. Our data suggest that the region south of Iceland freshened during the event. If the freshwater had a southern source, it may have mixed with waters of the North Atlantic Current, carrying the freshwater signal to high latitudes. Alternatively, the freshwater signal may have arrived at Site 984 from the north, competing with the North Atlantic Current, in association with a decrease in the MOC. Our results suggest that the event at 8.2 ka was not the only such event during the Holocene. Similar temperature and salinity decreases occurred at 9.3 ka, although the magnitude of the temperature change was smaller (1–2 °C), and the evidence for a contemporaneous excursion in the GISP2 δ18O record is less evident. However, the cooling and freshening occurred concurrently with an excursion in the GISP2 terrestrial dust concentration, indicating increased windiness over Greenland (O’Brien et al., 1995), and is consistent with detrital evidence of an ice-rafting event in the North Atlantic at 9.3 ka (Bond et al., 2001). Suborbital variability also occurred in the later Holocene. The ~2 °C temperature oscillations recorded in the most recent 4 k.y. of the record occurred without large episodic increases and decreases in freshwater; only at 8.2 and 9.3 ka were temperature oscillations associated with large changes in local salinity. However, the temperature minima and maxima of the past 4 k.y. may have coincided with neoglacial advances and retreats. Maximum advances on Baffin Island at ~1000, 1900, and 3000 14C yr B.P. (Davis, 1985), or at ~0.9, 1.8, and 3.2 k.y. B.P., roughly correspond to the temperature minima in the most recent 4 k.y. of our record, suggesting the possibility that these temperature changes reflect regional high-latitude North Atlantic climate variations.
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Figure 3. Multitaper spectral analysis for Mg/Ca temperatures, 554–10,423 yr B.P. Mean time step = 107.3 yr, with linear interpolation of temperature data. Time-bandwidth product = 3; number of tapers = 5. Temperature data were not detrended. Shading denotes 95% confidence interval. Matlab code courtesy of Peter Huybers.
It has been suggested that synchronous surface cooling and freshening events occurred throughout the Holocene with an event spacing of 1–2 k.y. (Bond et al., 1997). Our records suggest suborbital variability in the early Holocene, relative stability in the mid-Holocene, and renewed variability in the late Holocene. Multitaper spectral analysis of our Mg/Ca temperatures (Fig. 3) suggests variance in a broad millennial band spanning frequencies from ~1/0.5 k.y. to 1/1.5 k.y., indicating a preference for millennial variability. No sharp spectral peaks were found. We also generated multitaper coherence estimates using our Mg/Ca data set and the atmospheric Δ14C data set of Stuiver et al. (1998) in order to assess the influence of solar variability (Data Repository Fig. DR3; see footnote 1). However, the results are inconclusive because the coherence changes significantly when age control points are altered within the 2σ range of calibrated radiocarbon ages. In summary, our new Mg/Ca and δ18O records from Site 984 reveal long-term Holocene trends of increasing temperatures and salinities in the near-surface subpolar North Atlantic. Northward retreat of the boundary between polar and North Atlantic waters or changes in the hydrologic cycle may have caused these changes. Spectral analysis of our temperature record does not show any sharp spectral peaks, but does suggest a preference for millennial variability. The new records provide the first Mg/Ca-derived evidence of temperature and salinity decreases during the 8.2 ka event, confirming previously obtained foraminiferal assemblage data (Ellison et al., 2006), and consistent with the hypothesis of a reduced MOC. However, the 8.2 ka event is not the only large temperature event of the Holocene; our new records reveal temperature excursions during the past 4 k.y. that are the largest oscillations of the Holocene, underscoring the need to better understand the forcing mechanisms for millennial climate change. ACKNOWLEDGMENTS We thank D. Schneider, S. Birdwhistell, D. Ostermann, M. Jeglinski, and L. Zou for laboratory assistance and L. Keigwin, J. Sachs, Y. Rosenthal, E. Boyle, S. Thorrold, W. Curry, P. Huybers, G. Ganssen, and two anonymous reviewers for comments and suggestions. Work was supported by a Massachusetts Institute of Technology John Lyons Fellowship, the Woods Hole Oceanographic Institution Ocean and Climate Change Institute, and National Science Foundation grants OCE-0220776, OCE-80256500, and OCE-0215905.
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