PALEOCEANOGRAPHY, VOL. 14, NO. 3, PAGES , JUNE 1999

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1 PALEOCEANOGRAPHY, VOL. 14, NO. 3, PAGES , JUNE 1999 North Atlantic Intermediate Waters in the late Pliocene to early Pleistocene K. Mc Intyre, A. C. Ravelo, and M. L. Delaney Institute of Marine Sciences, University of California, Santa Cruz Abstract. We generated benthic isotope records from Ocean Drilling Program (ODP) site 981 on the Feni drift (2173 rn water depth) and from ODP site 983 on the Gardar drift (1983 rn water depth) to examine the interaction between North Atlantic Deep Water (NADW) and Glacial North Atlantic Intermediate Water (GNAIW) formation from 2.0 to 1.4 Ma. We find NADW at both sites during interglacial periods, and a mix of NADW and Southem Ocean water at the Feini drift during most glacial periods. Prior to 1.7 Ma we find no evidence ofr GNAIW at the Gardar drift site. Instead, glacial Gardar drift 6 3C values are as low or lower than values for all other sites in the North Atlantic and reflect continued glacial overflow from the Nordic seas. After 1.7 Ma Gardar drift 6 3C values increase and suggest that there was GNAIW at the Gardar drift site during some glacial intervals. Overall, we find that NADW and GNAIW production changed around 1.7 Ma in concert with changes in sea surface temperature and salinity and in the Earth's obliquity cycle. 1. Introduction In the modem ocean, North Atlantic Deep Water (NADW) is formed when saline surface water in the Greenland and Iceland Seas cool and sink, flow over the gateway ridges into the North Atlantic, and combine with other dense waters formed in the Labrador sea [Worthington, 1976]. It can be distinguished from other deep water masses by its low nutrient concentration and relatively high b 3C values of dissolved inorganic carbon, a result of its formation from nutrientdepleted surface waters [Kroopnick, 1980]. Benthic foraminiferal b 3C data from sites throughout the North Atlantic indicate that NADW formation was less than modem during the last glacial maximum [e.g., Broecker, 1997], while waters with a high b 3C signature filled the North Atlantic to inter- mediate depths [Boyle and Keigwin, 1987; Oppo and Lehman, 1993; Oppo et al., 1997; Marchitto et al., 1998]. The switch between NADW formation and Glacial North Atlantic Intermediate Water (GNAIW) formation is an important feature linking circulation to climate during late Pleistocene gla- cial cycles [e.g., Boyle, 1988; de Menocal et ai., 1992; Imbrie et al., 1995]. A number of studies have demonstrated that the b 3C value of glacial deep waters in the North Atlantic decreased after 1.6 Ma [Raytoo et al., 1989, 1990; Bickert et al., 1997]. This change has been suggested to reflect decreased NADW formation, allowingreater amounts of low bl3c Southern Ocean waters into the North Atlantic [Raymo et al., 1989; Bickert et al., 1997]. Relative to this evolutionary change in glacial NADW production, the history of GNAIW formation is less well known. To date the only continuous record of North Now at Marine Science Institute, University of California, Santa Barbara. Copyright 1999 by the American Geophysical Union. Paper number 1998PA /99/1998PA Atlantic intermediate waters over the last 2.6 m.y. is from Ocean Drilling Program (ODP) site 502 in the Caribbean Sea (Table 1) [de Menocal et al., 1992; Oppo et al., 1995], which monitors NAIW at sill depths of m. Benthic foraminiferal 5 3C data indicate that waters with high 5 3C values (GNAIW) have occupied this site during glacial periods for the last 2.6 m.y. and that this effect was greater in the past 1 m.y. [de Menocal et al., 1992; Oppo et al., 1995]. To understand better the relative strength of NAIW and NADW formation across the 1.6 Ma transition, we examined deep waters at two middepth sites in the high-latitude, eastern North Atlantic adjacento the source of dense overflow waters that contribute to NADW. We examined surface water characteristics at one of our sites to understand changes in the surface waters that become North Atlantic intermediate and deep waters. Our data cover an interval between 2.0 and 1.4 Ma, including the last few thousand years of the Pliocene and the beginning of the Pleistocenepoch. During this interval the amplitude of glacial-interglacial 5 80 variations as recorded by benthic foraminifera is about half that for the late Pleistocene, with warmer temperatures and/or lower global ice volume during glacial periods and cooler temperatures and/or greater global ice volume during interglacial periods relative to the late Pleistocene [Raymo et al., 1989; Ruddiman et al., 1989; Mix et al., 1995a, b]. In this time period, ice volume variations occur dominantly at a period of 41 kyr in contrasto the 100 kyr period dominant in the late Pleistocene [Raytoo et al., 1989; Ruddiman et al., 1989]. Our sites are located on the Feni and Gardar drifts (Table 1 and Figure 1). In the modem ocean, deep water at the Feni drift site (ODP site 981, 2173 m water depth) is a combination of lower NADW, recirculated from the southern part of the North Atlantic, and a small component of water overflowing the Wyville-Thompson ridge [Schmitz and McCartney, 1993] (Figure 1). Deep water at the Gardar drift site (ODP site 983, 1983 m water depth), separated from the Feni drift by the Rockall plateau, is a combination of Iceland- Scotland ridge and Wyville-Thompson ridge overflows [Schmitz and McCartney, 1993] (Figure 1). During the last 324

2 MCINTYRE ET AL.: NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS ø N 55ø 45 ø %1... e ø', <'! _ 0.eSite9' _, ge.. i i ß -:-,,_.._...,.:,'- -" t, C.00. ' '.... ß. :.?..'.,...'-.?:; '. -.- ß '. '*- - : ;, ' : '/,. a..:-: itc ß _ ' -' ' :... --o.. ";-,-' O : xl (, ka Polar e' i..-"' LDW 40øW 20 ø 0 Figure 1. Map showing the locations of the Feni drift site 981 (55ø28'N, 14ø39'W, 2173 m water depth) and Gardar drift site 983 (60ø24'N, 23ø38'W, 1983 m water depth). The arrows show the direction and strength of bottom water currents at these locations in the modem ocean. The figure is from Jansen et al. [1996] as modified from Robinson and McCave [ 1994]. glacial maximum a low/513c water mass was present at the Feni drift, reflecting the increased influence of Southern Ocean Deep Waters in the North Atlantic [Jansen and Yeum, 1990; Oppo and Lehman, 1993], while the Gardar drift was on the interface between this low b 3C water mass and high /513C GNAIW [Oppo et al., 1997]. Thus, during the late Pleistocene the Feni drift site monitors the upper limit of recirculated deep waters while the Gardar drift monitors dense overflow water on its way to mix with other deep waters in the North Atlantic during interglacials and monitors nutrientdepleted or high-b]3c intermediate waters during glacial in- tervals. We used several proxies in this study to examine the deep waters at the Feni and Gardar drift sites. We measured/5 80 values in benthie foraminifer Cibicidoides wuellerstorfi to reconstruct glacial to interglacial variations in ice volume and bottom water temperatures. We measured/5 3C values in C. wuellerstorfi to determine the ventilation history in this region. We measured the percent coarse fraction (percent sediment >63 lam) to assess the balance between drift deposition and surface productivity. For Feni drift site 981 we measured the percent of the planktonic species Neogloboquadrina pachyderma sinistral in the foraminiferal assemblage to reconstruct variations in surface water conditions. We measured the amount of ice-rafted debris at this site to monitor iceberg melt within glacia14nterglacial cycles. We have compared our benthie foraminiferal isotopic data to data for other sites (Table 1) to examine how gradients in /5 3C and/5 80 values in the North Atlantic have varied over time. We use Caribbean site 502 [de Menocal et al., 1992; Oppo et al., 1995] to monitor the history of shallow intermediate waters. We use Ceara Rise site 929 (4369 m), which is at the depth of the interface between NADW and Antarctic Bottom Water in the modem ocean and was bathed in Southern Ocean waters during the last glacial maximum [Bickert et al., 1997], to monitor the influence of Southern Ocean bottom waters. We use site 607 (3427 m) in the deep western Atlantic basin [Raymo et al., 1989] to monitor values in the deep North Atlantic. We use Iceland basin site 552 (2311 m) [Shackleton and Hall, 1984], which has been used as a monitor of the NADW and GNAIW in the late Pleistocene [Raymo et al., 1990; de Menocal et al., 1992; Oppo et al., 1995], to monitor the source of middepth waters in the North Atlantic. 2. Methods For the Feni drift site (ODP site 981) we sampled 10 cm 3 every 15 cm. For the Gardar drift site (ODP site 983) we sampled 20 cm 3 every 30 cm. The samples were dried for 24 hours at 50øC and then weighed. We washed samples in a sodiu metaphosphate solution and wet sieved ihem to separate the >63 gm fraction of sediment. The >63 gm fraction was dried for 24 hours at 50øC and then weighed. We calculated the percent coarse fraction as the ratio of the dry weight of the >63 gm fraction to the dry weight of the bulk sample. Table 1. Locations and Depths of North Atlantic Sites Discussed in This Study Location Site Latitude, N Longitude, W Depth, rn Reference Gardar drift ODP ø24 ' 23ø38 ' 1983 Iceland basin V ø00 ' 20ø00 ' 2063 Iceland basin DSDP ø03 ' 23ø14 ' 2311 Feni drift ODP ø28 ' 14ø39 ' 2173 Feni drift V ø00 ' 17ø00 ' 2393 Deep western North ODP ø00 ' 33ø58 ' 3427 Atlantic Cape Verde Plateau, ODP ø05 ' 21 ø02' 3070 eastern Atlantic Caribbean Sea ODP ø00' 80o00 ' 3051 ( ) a Deep Ceara Rise ODP 929 6ø00 ' 43ø44 ' 4369 Sill depths defining the depth of Caribbean source waters. This study Oppo and Lehman [ 1993] Shackleton and Hall [ 1984] This study Oppo and Lehman [ 1993] Ruddiman et al. [ 1989]; Raymo et al. [ 1989] Tiedemann et al. [ 1994] de Menocal et al. [1992]; Oppo et al. [ 1995] Bickert eta/. [ 1997]

3 326 MCINTYRE ET AL.: NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS We measured b so and 513C values of 1-3 C. wuellerstorfi tests (>150 gm fraction) using a FISONS Prism III mass spectrometer. Errors (ls) on replicate internal lab standards were +0.05%0 for 5 3C and +0.08%o for b so measurements. Results are reported in per mil relative to Vienna Peedee Belemnite (VPDB) calibrated via National Bureau of Standards (NBS)-19. We made a quantitative split of the original sample >150 gm and then counted the percent of planktonic foraminifera N. pachyderma sinistral out of 300 specimens. We counted the percent ice rafted debris (%IRD) by counting rock fragments relative to the total number of entities in our sample. We counted the IRD per gram by counting the total number of fragments >150 gm in our quantitative split. In some samples, there were pyritized burrows and mud clasts, which we considered to be formed in situ and did not include in our counts of ice rafted debris. We used the Analysedes program [Paillard et al., 1996] to apply the astronomically derived timescale from ODP site 659 [Tiedemann et al., 1994] to our records by correlating the C. wuellerstorfi b so records for the Feni and Gardar drift sites to the C. wuellerstorfi b so record for site 659. We then established a timescale for both sites by linearly interpolating between tie points derived from the site 659 timescale, the magnetostragraphic datums, and the biostratigraphic datums within this interval at both sites [Jansen et al., 1996]. We generated spectral power frequency plots using the Blackman-Tukey method [Hays et al., 1976; Irnbrie et al., 1984]. Sedimentation rates for Feni drift site 981 vary from 2 to 10 cm/kyr with an average sedimentation rate of 6 cm/kyr, yielding a-1600 year sampling resolution. Sedimentation rates for Gardar drift site 983 vary from 2 to 72 cm/kyr and average 22 cm/kyr, yielding a- 500 year resolution. In order to compare records at Feni drift site 981 with those at western Atlantic site 607 we independently tied the Feni drift site timescale to the western Atlantic site 607 timescale for this comparison using the benthic foraminiferal b 80 records at these sites. In order to examine gradients within given glacial and interglacial time slices we selected maximum and minimum b 80 values within our data and within published data sets (Table 1).l 3. Results Oxygen isotopic values for the Feni drift (ODP site 981) range from 2.45%0 to 4.01%0 (Figure 2b), about half the range I Supporting material is available electronically at World Data Center- A for Paleoclimatology, NOAA/NGDC, 325 Broadway, Boulder, Colorado ( paleo mail.ngdc.noaa.gov; URL: http :// gdc.noaa. go v/pal eo ). O _ o 30 l 20 -n 10 g o 0 = Age (Ma) Figure 2. (a) The b 80 values ofbenthic foraminifera Cibicidoides wuellerstorfi from site 659 [Tiedemann et al., 1994], (b) the b 80 values of C. wuellerstorfi from the Feni drift site, (c) the b 3C values of C. wuellerstorfi from the Feni drift site, and (d) the percent coarse fraction from the Feni drift site. The shaded bars indicate glacial intervals with numbers designatinglacial marine isotopic stages [after Raymo et al., 1989].

4 MCINTYRE ET AL.' NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS 327 of the late Pleistocene at North Atlantic site 607 [Ruddiman et al., 1989] and of the SPECMAP stack [Martinson et al., 1987]. There are 12 glacial-interglacial cycles present (Figure 2b), and the amplitude of these glacial-interglacial cycles is relatively constant over this interval. The Gardar drift (ODP site 983) benthic foraminiferal b 80 record, covering isotope stages 45-72, is less continuous than the record from the Feni drift site. There are gaps up to 2.4 m long in the Gardar drift site record, primarily during glacial intervals, because of the absence of C. wuellerstorfi (Figure 3b). For the Gardar drift site the total range of C. wuellerstorfi b 80 values is 2.30%0 to 5.00%0, changing significantly over the course of the interval from 2.0 to 1.4 Ma (Figure 3b). In the interval prior to 1.85 Ma, there are two glacial-interglacial cycles (stages 70-72) with a-1.0%o amplitude (Figure 3b). Between 1.6 and 1.85 Ma the amplitude of the b ao signal is low, making it difficult to identify glacial-interglacial cycles (Figure 3b). Younger than 1.6 Ma, the amplitude increases to -2.7%0, exceeding the glacial-interglacial amplitude for the Feni drift site (Figure 3b). C. wuellerstorfi b13c values for Feni drift site 981 range from -0.08%0 to 1.33%o and are more variable than 5 80 values for this site (Figure 2c), but the amplitude does not vary significantly. C. wuellerstorfi 5 3C values for Gardar drift site 983 range from-0.83%0 to 1.40%o, more variable than those for the Feni drift, and the average glacial values increased from 2.0 to 1.4 Ma (Figure 3c). Before 1.7 Ma, there are five glacial intervals with unusually low benthic foraminiferal 5 3C values (Figure 3c). Younger than 1.7 Ma, glacial benthic foraminiferal b C values increase, approaching the values for the Feni drift site (Figure 3c). Spectral analyses of the benthic foraminiferal b C and b 80 records from the Feni drift site verify that most spectral power is concentrated within the 41 kyr obliquity band (spectral results not shown). Cross-spectral analysis indicates that the relative phasing of maximum b 80 values (glacial) to minimum b 3C values is near zero (3+2 kyr), similar to the phasing of these two properties in the 41 kyr band during the late Pleistocene [Imbrie et al., 1995]. We did not attempt to apply spectral analysis to the Gardar drift site benthie foraminiferal stable isotope data because of the large gaps and the nonstationary nature of the record. In a peak to peak comparison of the Gardar drift 15 so and 15 3C records we find that maximum 15 O values and minimum b 3C values essentially coincide, similar to our observations for the Feni drift site. The Feni drift percent coarse fraction averages 13%+9% and varies from 1% to 44%, with high values often associated with glacial maxima in the interval younger than 1.7 Ma (Figure 2d). The Gardar drift percent coarse fraction is much lower, averaging 2%+3% and ranging from 0% to 20% (Fig Age (Ma) Figure 3. (a) The/5 80 values ofbenthic foraminifera C. wuellerstorfi from site 659 [Tiedemann et al., 1994], (b) the/5 80 values of C. wuellerstorfi from the Gardar drift site, (c) the/5 3C values of C. wuellerstorfi from the Gardar drift site, and (d) the percent coarse fraction from the Gardar drift site. The shaded bars indicate glacial intervals with numbers designating glacial marine isotopic stages [after Raymo et al., 1989].

5 _ MCINTYRE ET AL.' NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS ure 3d). Maximum percent coarse fraction at the Gardar drift is associated with interglacial cycles (Figure 3d). In the Feni drift record of % IRD and IRD >150!.tm/g and are spaced -10 kyr apart (Figure 4c). After 1.6 Ma, percent N. pachyderma sinistral decreases and varies with glacial-interglacial cycles (Figure 4c). sediment, peaks punctuate long periods of no IRD deposition, and the length of time between IRD peaks changes over the course of the interval from 2.0 to 1.4 Ma (Figure 4b). Prior to 4. Discussion 1.7 Ma, % IRD peaks are more frequent, occurring roughly every 10 kyr (Figure 4b). Between 1.7 and 1.55 Ma, there are only a few smaller % IRD peaks (Figure 4b). Younger than 1.55 Ma, large peaks in the % IRD are associated with glacial terminations. The IRD >150!.tm/g sediment varies in tandem with the % IRD, and only the amplitude of the peaks differs between these two proxies (Figure 4b). N. pachyderma sinistral first appears at 1.8 Ma, the biostratigraphic datum when this species became a major component of the North Atlantic foraminiferal assemblage [Hooper and Weaver, 1987] (Figure 4c). In the record from Feni drift site 981, percent N. pachyderma sinistral increases As we will describe below, interglacial circulation in the North Atlantic during the interval from 2.0 to 1.4 Ma was similar to that of the Holocene, while during glacial intervals there was no water mass analogous to GNAIW at the Gardar drift site prior to 1.7 Ma. Instead, this site was influenced by a low-buc water mass relative to other sites in the North Atlantic, which we interpreto reflect the continued influence of overflow waters during glacial periods between 2.0 and 1.7 Ma. After 1.7 Ma, benthic foraminiferal bl3c values for this site converge toward those for the Feni drift, reflecting the relative mixture of NADW and Southern Ocean water and, from 0 to 40% during the first glacial interval (stage 64) after during some glacial intervals, the presence of GNAIW. Fiits first appearance (-1.8 Ma; Figure 4c). Its abundance then decreases to <15% of the assemblage in the subsequent interglacial stage (Figure 4c). The percent N. pachyderma sinisnally, a comparison of benthic foraminiferal b lao and b l3c values from sites throughouthe North Atlantic during representative glacial and interglacial stages demonstrates how the tral increases again in glacial stage 62 (-1.76 Ma) and re- Gardar drift site evolved over the course of the interval from mains high until interglacial stage 53 (-1.55 Ma) (Figure 4c). Between 1.8 and 1.6 Ma the peaks in percent N. pachyderma sinistral occur at the same time as peaks in the IRD records 2.0 to 1.4 Ma and how the pattern of intermediate water circulation during the late Pliocene to early Pleistocene was different from that of the Holocene and last glacial maximum e e+3-3e e+3 - e+3 _ o ' I ' Age (Ma) Figure 4. C. wuellerstorfi b 80 data, ice-raftedebris (IRD) concentration, and percent Neogloboquadrina pachyderma sinistral versus age from the Feni drift site 981: (a) C. wuellerstorfi b 80 values with numbers indicatin glacial marine isotopic stages, (b) % IRD (dark lines and symbols) and the IRD/gram bulk sediment (dark lines), and (c) the percent N. pachyderma sinistral. The shaded bars indicate intervals of IRD input.

6 MCINTYRE ET AL.' NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS Intermediate Waters at the Feni Drift The benthie foraminiferal isotope records for the Feni Drift site are similar to those for western Atlantic site 607 [Raymo et al., 1989; Ruddiman et al., 1989] throughout most of the interval from 1.9 to 1.4 Ma (Figure 5), suggesting that the two sites were bathed by the same water mass. Presence of the same water mass at the Feni drift site, which is in the eastern basin of the North Atlantic and 1250 m shallower than westem Atlantic site 607 (Table 1), suggests that for the interval from 1.9 to 1.4 Ma the prevailing circulation was similar to that of the late Pleistocene [Ruddiman et al., 1989; Schmitz and McCartney, 1993]. In interglacial periods from 1.9 to 1.4 Ma, NADW filled the western basin of the North Atlantic and passed over site 607 as it flowed southward; a large component was then recirculated northward in the eastern basin and passed over eastern Atlantic site 659 (Table 1) [Tiedemann et al., 1994; Bickert et al., 1997] and the Feni drift site. While the ;5 3C of dissolved inorganicarbon in this water decreased as it aged and mixed with Southern Ocean water masses along the way, the relatively small difference in ;5 3C values between western Atlantic site 607 and the Feni drift site indi- cates that this recirculation occurred rapidly with little to no mixing (Figure 5b). Lower ;5 3C values during glacial periods for both sites indicate an increase in the amount of Southern Ocean water relative to NADW, but ;5 3C values in the Ma interval are never as low as late Pleistocene glacial values [Ruddiman et al., 1989], indicating a larger glacial component of NADW. The only major difference between the two sites is that ;5 3C values at the Feni drift during glacial stages 48 and 50 are higher than ;5 3C values at site 607, suggesting that there may have been some GNAIW affecting this site during these two glacial intervals Intermediate Depth Waters at the Gardar Drift High benthic foraminiferal ;5 3C values for the Gardar drift site during interglacial periods throughouthe interval from 2.0 to 1.4 Ma indicate that interglacial deep water circulation was similar to Holocene circulation (Figure 3c). The presence of high ;5 3C values at sites throughout the North Atlantic, including the Feni and Gardar drift sites, from 2.0 to 1.4 Ma indicates that interglacial NADW production was strong, filling the North Atlantic over a large range of depths (Figures 5b, 6b, and 7c,f). In addition, the interglacial benthie foraminiferal ;5 so values for the Gardar drift site are always higher than those for the Feni drift site (Figure 8a). This is true in the modem ocean because of the high density of overflow waters at the Gardar drift site [Bainbridge, 1981; Oppo and Lehman, 1993]; it follows that there was a strong overflow component of deep waters at this site during interglacial periods from 2.0 to 1.4 Ma. During glacial periods from 2.0 to 1.7 Ma the deep water mass occupying the Gardar drift site was not comparable to GNAIW, which occupied this site during late Pleistocene glacial stages 2 and 6 [Oppo et al., 1997]. Glacial Gardar drift site ;5 3C values prior to 1.7 Ma are as much as 1%0 lower than those from the Feni drift site and western Atlantic site 607 and as low or lower than those at deep Ceara Rise site 929 (Figures 7 and 8b). After 1.7 Ma, glacial Gardar drift site i i i Age (Ma) Figure 5. Feni drift site 981 and western Atlantic site 607 benthic foraminiferal stable isotopes versus age. (a) C. wuellerstorfi /5 ao data from the Feni drift site 981 (solid circles), from western North Atlantic site 607 (dark line), and from deep Ceara Rise site 929 (dashed lines). Numbers indicate marine isotopic stages discussed in Figure 7. (b) C. wuellerstorfi 6 3C data from the Feni drift site 981 (solid circles), from western Atlantic site 607 (dark line), and from deep Ceara Rise site 929 (dashed lines). The site 607 benthic foraminiferal isotope data are from Rayrno et al. [1989], and the Ceara Rise data are from Bickert et al. [1997].

7 , 330 MCINTYRE ET AL.' NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS I i Gardardrift.--o Caribbean Sea I I I I I Age (Ma) Figure 6. Gardar drift site 983 and Caribbean site 502 benthie foraminiferal stable isotopes versus age. (a) C. wuellerstorfi 6180 data from the Gardar drift site 983 (solid circles) and from Caribbean site 502 (open circles). (b) C. wuellerstorfi 613C data from the Gardar drift site (solid circles) and from Caribbean site 502 (open circles). The site 502 benthie foraminiferal stable isotope data are from Oppo et al. [1995]. b 3C values generally converge toward Feni drift (Figure 8b) values, indicating the presence of a more nutrient-depleted water mass, and glacial Gardar drift 6280 values are higher (Figures 3b and 8b), indicating that intermediate waters at the Gardar drift became colder or more saline. As at the Feni drift site, the Gardar drift b23c values during glacial stages 48 and 50 are higher than values at deeper sites 607 and 929, but these two glacial intervals are poorly represented in this record (Figure 8b). Because benthie foraminiferal 1523C values for the Gardar drift site are primarily as low or lower than values for other sites within the North Atlantic during glacial periods (Figure 70, there must not have been GNAIW at the Gardar drift site throughouthe interval from 2.0 to 1.7 Ma. However, benthie foraminiferal data from Caribbean site 502 (Figure 6b) [Oppo et al., 1995] indicate the presence of GNAIW at shallower depths in this interval. Oxygen isotopic values for the Feni drift site and for the Gardar drift site are always higher than those for site 502 because, as is the case in the modem ocean, these drifts sampled colder waters of a higher density than the intermediate water sampled by site 502 (Figure 6a). In combination with the data at site 502 our data indicate that from 2.0 to 1.7 Ma, GNAIW was probably not as deep in the North Atlantic and not as dense relative to deep waters, compared to the late Pleistocene. After 1.7 Ma, this water mass may be present at the Feni and Gardar drift sites, but only during some glacial intervals The Origin of Low Benthie Foraminiferal Isotopic Values at the Gardar Drift Site The benthie foraminiferal b 80 and 6 3C records for the Gardar drift site are often unique in the North Atlantic from 2.0 to 1.4 Ma. From 2.0 to 1.7 Ma, average glacial 6 80 values are the highest for the Gardar drift site relative to all other sites in the North Atlantic (Figures 7d, 7f, and 8a), which we interpret as indicating continued overflow from the Nordic seas. From glacial stage 58 to 68 (Figure 3b) these high benthic foraminiferal $280 values are offset by extremely low 6 80 values, masking the -0.6%0 ice volume change, which are likely to reflect the influence of waters formed by brine rejection on the Gardar drift site. During glacial stages the Gardar drift site has the lowest glacial $ 3C values of any site in the North Atlantic, and as discussed later, the origin of this signal is probably the Nordic seas and not the Southern Ocean. During glacial stages ( Ma), benthic foraminiferal $280 values for the Gardar drift are often very low (Figures 3b and 3c). Recent studies have found low $280 values at other sites in this region of the North Atlantic [Vidal et al., 1998; Raytoo et a/.,1998] and suggested that these low $ 80 values reflect the influence of dense waters formed from brines [Vidal et al., 1998]. As sea ice forms, it excludesalt, increasing the density of the remaining liquid water without fractionating oxygen isotopes, and thus creating dense waters with a low $280 value characteristic of surface waters [Craig and Gordon, 1965]. Vidal et al. [1998] found low benthic foraminiferal $280 values at a 2100 m water depth site in the Rockall trough during intervals of rapid surface water cooling and iceberg melt over the past 40 kyr. They also found that this signal is strongest at a site in the southern Norwegian sea, indicating that it must originate close to this site or elsewhere in the Norwegian sea. Raymo et al. [1998] found similarly low benthic foraminiferal/5280 values in conjunction with the deposition of ice-rafted debris at Gardar drift site 983 during an interval between -1.2 and 1.4 Ma. In both studies the low benthic foraminiferal $ 80 values are related to rapid, shortlived intervals of extensive sea ice melt at the surface, which could contribute to the low surface water $280 values, and to low benthic foraminiferal b23c values. Given this growing

8 MCINTYRE ET AL.: NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS 331 Interglacial Glacial O 552 ß Gardar o (5) Feni [] o Gardar 552 o Feni [] Holocene i LGM O o O ß Stage 51 Stage o o o [] Stage Stage s0 (per mil) 6 s0 (per mil) Figure 7. North Atlantic benthie foraminiferal 6 3C versus 6 80 values for three interglacial and three glacial stages: (a) the Holocene (stage 1), (b) the last glacial maximum (stage 2), (c) early Pleistocene interglacial stage 51, (d) early Pleistocene glacial stage 52, (e) late Pliocene interglacial stage 69, and (f) late Pliocene glacial stage 70. The symbols are intermediate waters (open hexagon), deep North Atlantic waters (open diamonds), Gardar drift waters (solid circle), and deepest North Atlantic waters (open square). Site depths and locations are presented in Table 1. Mean values and error (Is) are presented in Table 2. body of evidence for the influence of brine formation during glacial periods at sites within the Iceland basin, it is probable that dense waters formed via this process were responsible for low benthie foraminiferal 6'aO values at the Gardar drift site during the interval from 2.0 to 1.4 Ma. While the origin of the low 6 80 values from brine formation is fairly straightforward, the origin of low '3C values is less clear. Raymo et al. [1998] suggested that the low b 3C values at the Gardar drift site around 1.5 Ma can be explained by the mixing of a small amount of fresh melt water with a larger amount of low b 3C Southern Ocean water. However, it is hard to call upon Southern Ocean water as the only source of low 6 3C values at the Gardar drift site during the interval from 2.0 to 1.7 Ma because we do not see very low values for the Feni drift site or at any site within the North Atlantic at this time, except in some cases deep Ceara Rise site 929 [Bickert et al., 1997] (Figures 7, 8b, and 8e). In addition, despite spikes of low 15 ao values representing brine water the average benthie foraminiferal 15 O values at the Gardar drift site are often the highest in the North Atlantic during the interval from 2.0 to 1.4 Ma (e.g., glacial stage 70; Figure 3c), suggesting tha this site was continually bathed by overflow waters, even during glacial intervals. Sedimentary evidence supports the possibility of continued overflow from the Nordic seas to the Gardar drift. Throughout glacialinterglacial cycles in the interval from 2.0 to 1.4 Ma, the per-

9 332 MCINTYRE ET AL.' NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS, i ß 981 Feni drift 983 Gardar drift O - 3O - 10 loo ' Iceland basin 607 western north Atlantic 22 i Age (Ma) Figure 8. Benthic foraminiferal 5 80 and 5 3C values for the Feni and Gardar drifts sites 981 and 983, percent N. pachyderma sinistral and % IRD from the Feni drift site 981, the benthic foraminiferal 5 3C values from deep North Atlantic site 607 and Iceland basin site 552, and the Earth's obliquity, from 2.0 to 1.0 Ma. (a) The 5 s of C. wuellerstorfi from the Feni drift site 981 (solid circles) and from the Gardar drift site (open circles), (b) the 5 3C of C. wuellerstorfi from the Feni drift site 981 (solid circles) and from the Gardar drift site 983 (open circles), (c) the percent N. pachyderma sinistral from the Feni drift site 981, (d) the % IRD from the Feni drift site 981, (e) the 5 3C of C. wuellerstorfi from deep North Atlantic site 607 (dark line) and Iceland basin site 552 (dark line with solid circles), and (f) the tilt of the Earth's axis (obliquity). The site 607 C. wuellerstorfi data are from Rayrno et al. [1989], and the site 552 data are from Shackleton and Hall [1984]. The obliquity calculations are fromlaskar et al. [1993]. cent coarse fraction at Gardar drift is low and sedimentation rates are very high, indicating a continuou supply of drift material. These drift sediments require a persistent strong bottom current in frictional contact with a slower water mass [Robinson and McCave, 1994]. Since high bottom current velocities in this region are related to the compression of southward flowing overflow waters against the adjacent ridges [Robinson and McCave, 1994], there must have been some overflow water entering the North Atlantic basin throughout glacial and interglacial cycles at this time. If the deep waters at the Gardar drift continued to be influenced by overflow water during glacial periods from 2.0 to 1.4 Ma, the very low glacial 15 3C values can be explained by a mixture of brine waters with poorly ventilate deep waters of the Nordic seas. The work that documented the presence of brine-derivedeep waters in this region during younger intervals also found that these waters had a very low 15 3C signature [Vidal et al., 1998; Raymo et al., 1998]. Brines may carry with them the 15 3C values of the surface water they are formed from, which could be quite low if the surface waters have high dissolved nutrient concentrations. If these waters are formed on shelves within the Nordic seas or in the Arctic Ocean, they could mix with poorly ventilated, nutrientenriched intermediate or deep waters in these basins before flowing over the gateways into the Atlantic. There is some evidence for poorly ventilated intermediate and deep waters in the Nordic seas during this time interval. A number of studies have demonstrated that calcium carbonate preservation in the Nordic seas was extremely low until the latest Pliocene and then increased sporadically [,Jansen et al., 1989,

10 MCINTYRE ET AL.: NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS ; Heinrich and Baumann, 1994]. In particular, Heinrich and Baumann [ 1994] found increasing carbonate preservation for ODP site 644A on the V6ring plateau from 2 to 1 Ma. While low percent calcium carbonate in the Nordic seas can be attributed to low productivity in surface waters, it may also indicate poorly ventilated, nutrient-enriched low-bl3c deep they are representative of glacial and interglacial conditions in all three time intervals. Comparin glacial stages 52 and 70 to the last glacial maximum, two features of glaciations from 2.0 to 1.4 Ma become clear. First, the glacial benthic foraminiferal b lso values for the Gardar drift indicate continued overflow from the Nordic seas (Figure 7). Second, the deep waters during this interval. waters seen at western North Atlantic site 607 and at the Feni There are other possible sources of the low benthic foraminiferal ( 13C and {5180 values at the Gardar drift, but we consider them less likely given the strong evidence for brine formation, for continued overflow, and for overflow from the Nordic seas that we have presented. Intermediate waters drift (site 981) during glacial stage 70 were influenced by a high b lso, low b l3c water mass, similar to that recorded by the Gardar drift. In both glacial stages 52 and 70 the average benthic foraminiferal blso values for the Gardar drift site are higher than formed in the Mediterranean and the Antarctic could have in- at any other site in the North Atlantic, including deep Ceara fluenced this site, but the preformed isotopic values of both these water masses would have had to have been very different from their modem values in order to explain the isotopic values for the Gardar drift site. Alternately, C. wuellerstorfi Rise site 929 (Figures 7d and 7f). This is also true in the modem ocean (Figure 7a) because of the relatively salty overflow waters from the Nordic seas. High b lso values for the Gardar drift site during glacial stages 52 and 70 indicate that ( 13C values could reflecthe biology or the microenvironment dense overflow waters continued to bathe this site during glaof this species instead of deep water values. Mackensen et al. cial periods from 2.0 to 1.4 Ma. The combination of high [1996] found that C. wuellerstorfi calcite could be low in {513C b lso and low b l3c values at the Gardar drift site during stage relative to seawater in regions where there are large accumu- 70 supports our interpretation that the Gardar drift site was lations of organic matter at depth. This seems like an unlikely not bathed in GNAIW at this time. Benthic foraminiferal valmechanism to explain low {513C excursions at the Gardar drift as organic matter concentrations this site are very low and ues from Iceland basin site 552 also support the idea that GNAIW was not present in this part of the Atlantic during the lower in this interval than in the late Pleistocene [Jansen et older interval. While late Pleistocene benthic foraminiferal al., 1996]. isotopic values from site 552 reflect NADW during interglacial intervals and GNAIW during glacial intervals (Figures 7a 4.4. Time Slice Reconstructions and 7b) [de Menocal et al., 1992; Oppo et al., 1995], the available data from this site between 2.0 and 1.4 Ma are We consider benthic foraminiferal ( 180 versus {513C values from sites throughouthe North Atlantic for selected interglacial and glacial intervals to examine trends in North Atlantic intermediate and deep water mass structure and circulation in the interval from 2.0 to 1.4 Ma. We identified intervals of minimum and maximum {51 O values: Holocene and last glacial maximum, early Pleistocene stages 51 and 52 (-1.55 Ma), and late Pliocene stages 69 and 70 (-1.90 Ma) (Figure 7 and Table 2). We chose these stages because they are distinctive and easily identified in all the records and because closer to values from deep Atlantic site 607 than they are to those at site 502 (Figure 8e). While the shift at western Atlantic site 607 and at the Feni drift site (Figure 7f) toward lower b l3c values during glacial stage 70 can be explained by an increased amount of nutrient rich Southern Ocean water, this change cannot also explain the high b lso values. Generating the benthic foraminiferal b lso and b 3C values for western Atlantic site 607 and the Feni drift site requires a mixture of deep Ceara Rise Southern Ocean waters with a high b 3C and blso water mass. Such a Table 2. Mean Benthic Foraminiferal b 80 and b 3C Values for North Atlantic Sites for Selected Isotopic Stages Location Holocene LGM Stage 51 Stage 52 Stage 69 Stage 70 Gardar drift a (12) b (17) (6) (6) (16) (4) (14) b (9) Feni drift d (4) (14) (8) (2) (7) (4) (16) Deep western (3) (2) (4) (3) (16) (6) North Atlantic c Caribbean Sea (3) (4) (3) (4) (3) 2.43(1) Deep Ceara (4) (3) (2) (2) Rise ½ Glacial and interglacial periods are defined as intervals of maximum and minimum values. a Holocene and LGM values are from site V23-81 on the Feni drift and V28-73 in the Iceland basin (Table 1) [Oppo and Lehman, 1993]. V28-73 is at a similar depth to Gardar drift site 983. The values in the first row for each location are b 80 values, and b 3C values are in the second row. Means are given +ls (where s is the standard deviation), with n given in parentheses. c Values from site 607 (Table 1) [Ruddiman et al., 1989; Raytoo et al., 1989]. d Values from site 502 [de Menocal et al., 1992; Oppo et al., 1995]. ½ Bickert et al. [1997].

11 334 MCINTYRE ET AL.: NORTH ATLANTIC INTERMEDIATE AND DEEP WATERS water mass might form in regions for which there are no con- Pleistocene, composed of upwelled deep waters recirculated tinuous data available, such as high b 80 waters entering the fiom the south in the eastern basin of the Atlantic. While in- North Atlantic from the Greenland sea via the Denmark Strait terglacial circulation at the Gardar drift was similar to modem or high b 80 waters formed in the Labrador Sea with a b 3C signature similar to that for Caribbean site 502. Alternately, this water mass could be created by a mixture of low b 3C, circulation during this interval, our data indicate that during glacial periods prior to 1.7 Ma, intermediate waters at the Gardar drift had very low 3C values of dissolved inorganic high bl o waters like those at the Gardar drift with high 3C, carbon relative to other sites in the North Atlantic. The conlow b l80 waters like those at Caribbean site 502. This evi- tinuously high sedimentation rates and low percent coarse dence for a high b l O and 3C NADW end-member suggests fraction combined with the high density of waters at the that NADW production did continue during glacial periods of the late Pliocene and early Pleistocene, possibly to a greater extent than was previously thought. Gardar drift site relative to deeper sites in the North Atlantic indicate that there was always some component of overflow from the Nordic seas at this site. Thus low 3C values at this site probably have their origin in poorly ventilated deep wa Synthesis ters of the Nordic seas. In some cases these low b 3C values Long-term changes in the character of glacial deep water accompany rapid inputs of low b 80 waters formed from sea seen at the Gardar drift site were accompanied by changes in' ice formation. Overall, our Gardar drift data, combined with the formation of NADW. Increases in the glacial benthic fodata from nearby site 552, indicate that there was no GNAIW raminiferal b13c and bl80 values for the Gardar drift site occupying the Iceland basin from 2.0 to 1.7 Ma. After 1.7 around 1.65 Ma (stage 58) occur at the same time as other Ma the 3C values at the Gardar drift increase, suggesting authors have identified the break down of NADW production that this site was bathed in GNAIW during at least some gladuring glacial intervals [Raymo et al., 1989; Ruddiman et al., cial intervals. Our reconstructions of specific glacial and in- 1989; Bickert et al., 1997]. Surface water signals from the terglacial periods in the 2.0 to 1.4 Ma interval demonstrate Feni drift site indicate an interval of cooler sea surface tem- that the high-b O waters at the Gardar drift may have signifiperatures and frequent inputs of ice rafted debris that ended at cantly contributed to the mix of the intermediate and deep the same time that the changes in deep and intermediate water waters that fill the North Atlantic basin. Overall, the interval formation occurred (Figures 8c and 8d). All these changes from 2.0 to 1.4 Ma is one in which the intermediate and deep occurred at the same time as an increase in the amplitude of water circulation of the North Atlantic changed in concert the 41 kyr obliquity cycle (Figure 8f) [Laskar et al., 1993], with changes in sea surface temperature and salinity, possibly and this increase in amplitude may have triggered the cross- in response to changes in the amplitude of the Earth's obliqing of a threshold within the overall cooling of the late Cenouity cycle. zoic. Once this threshold was crossed, NADW formation was significantly reduced during glacial periods, and glacial overflow waters had higher 3C values, reflecting increased ventilation of the Nordic Sea basins. Future work at other inter- Acknowledgments. K.McI. thanks the shipboard scientific party on Ocean Drilling Program leg 162 for a great cruise. Ocean Drillmediate depth sites in the north Atlantic, closer to the Den- ing Program samples were supplied through the assistance of the National Science Foundation. At U.C. Santa Cruz this research was mark straits and to the Labrador sea, should resolve where improved through discussions with K. Billups, L. Anderson, M. GNAIW was created during the Ma interval and re- Wara, and K. Faul. We thank R. Hoag, H. Lao, D. Neenan, and C. solve when and where GNAIW evolved after this time. Wilson for sample preparation and G. Koehler for maintaining the UCSC light stable isotope facility. We thank Maureen Raytoo and 5. Conclusions Joe Ortiz for assistance with time scales and J. Wright, B. Flower, and K. Miller for reviews and suggestions. 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