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ABSTRACT: Monthly d18O and Sr/Ca records generated from modern and fossil corals from Southwestern Pacific Ocean sites in the Republic of Vanuatu are used to assess the differences in mean climate state, seasonality, and interannual variability between a glacial and interglacial period. The modern coral contains a well-defined annual signal in d18O and Sr/Ca. The top 40 cm of the coral used in this study has a mean d18O value of -4.99+/-0.13%VPDB (2s) and a mean Sr/Ca value of 8.691+/-0.015mmol/mol (2s). El Nino-Southern Oscillation (ENSO) events are characterized by positive d18O and Sr/Ca anomalies, consistent with cooler temperatures and reduced rainfall that typifies ENSO at Vanuatu. The ~12cm long fossil coral is dated to 346 ka + 25, - 9, based on uranium-series analysis and stratigraphic forward modeling, indicating that the fossil coral grew during MIS10 - a glacial period. X-ray diffraction, petrographic inspection, SEM analysis, and geochemical considerations indicate excellent preservation. The mean d18O value is enriched by 0.74%, and the mean Sr/Ca value is equivalent, compared to the modern coral. Mathematical modeling of Pleistocene mean SST and SSS results in temperature estimates up to ~2?C warmer and salinity up to ~2 psu saltier than present-day conditions, if seawater Sr/Ca were 1-2% higher in MIS10. Our fossil coral data and modeling results preclude colder SST and lower SSS at Vanuatu during MIS10. Accurate estimates of past values of seawater Sr/Ca remain the largest obstacle to accurately reconstructing past tropical SST using pristine fossil corals. The fossil coral Sr/Ca annual range is similar to the modern range, indicating that seasonal SST ranges were similar, whereas the d18O annual range is about half that of the modern coral, indicating weaker past seasonal salinity variations. The reduced seasonal SSS variations and increased SSTs near Vanuatu are interpreted as evidence that the SPCZ was displaced from its present location while the fossil coral lived. The geochemical response to El Nino events in the modern coral is observed twice in the fossil coral record, indicating that ENSO-like processes are not unique to interglacial time periods, but characterize the tropical Pacific at least back to MIS 10.
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ABSTRACT: Monthly d18O and Sr/Ca records generated from modern and fossil corals from Southwestern Pacific Ocean sites in the Republic of Vanuatu are used to assess the differences in mean climate state, seasonality, and interannual variability between a glacial and interglacial period. The modern coral contains a well-defined annual signal in d18O and Sr/Ca. The top 40 cm of the coral used in this study has a mean d18O value of -4.99+/-0.13%VPDB (2s) and a mean Sr/Ca value of 8.691+/-0.015mmol/mol (2s). El Nino-Southern Oscillation (ENSO) events are characterized by positive d18O and Sr/Ca anomalies, consistent with cooler temperatures and reduced rainfall that typifies ENSO at Vanuatu. The ~12cm long fossil coral is dated to 346 ka + 25, 9, based on uranium-series analysis and stratigraphic forward modeling, indicating that the fossil coral grew during MIS10 a glacial period. X-ray diffraction, petrographic inspection, SEM analysis, and geochemical considerations indicate excellent preservation. The mean d18O value is enriched by 0.74%, and the mean Sr/Ca value is equivalent, compared to the modern coral. Mathematical modeling of Pleistocene mean SST and SSS results in temperature estimates up to ~2?C warmer and salinity up to ~2 psu saltier than present-day conditions, if seawater Sr/Ca were 1-2% higher in MIS10. Our fossil coral data and modeling results preclude colder SST and lower SSS at Vanuatu during MIS10. Accurate estimates of past values of seawater Sr/Ca remain the largest obstacle to accurately reconstructing past tropical SST using pristine fossil corals. The fossil coral Sr/Ca annual range is similar to the modern range, indicating that seasonal SST ranges were similar, whereas the d18O annual range is about half that of the modern coral, indicating weaker past seasonal salinity variations. The reduced seasonal SSS variations and increased SSTs near Vanuatu are interpreted as evidence that the SPCZ was displaced from its present location while the fossil coral lived. The geochemical response to El Nino events in the modern coral is observed twice in the fossil coral record, indicating that ENSO-like processes are not unique to interglacial time periods, but characterize the tropical Pacific at least back to MIS 10.
ACKNOWLEDGMENTS Several people in the College of Marine Science deserve special thanks for their unending support. I would like to recognize first and foremost my advisor Terry Quinn, whose constant advice and advocacy were vital to my success. My committee members Ben Flower, David Hollander, and Gary Mitchum also were extremely helpful and always available for questions. Gary Mitchum deserves special recognition for teaching me much about quantitatively assessing my data. Many others have contributed to my success in more indirect ways. Richard Long has kept our research lab going by continuously and miraculously leaping over tall bureaucratic barriers, thereby enabling us to all focus on science. My fellow lab-mates, Amy Wright, Christie Stephans, Heather Hill, and Jennifer Smith, helped in many different ways, but all provided camaraderie and acted as sounding boards for ideas. This research was funded mainly by a grant from the National Science Foundation. The Von Rosenstiel Fellowship for new students, and the Paul Getting Fellowship made additional valuable contributions.
vii SSS variations and increased SSTs near Vanuatu are interpreted as evidence that the SPCZ was displaced from its present location while the fossil coral lived. The geochemical response to El Nio events in the modern coral is observed twice in the fossil coral record, indicating that ENSOÂ…like processes are not unique to interglacial time periods, but characterize the tropical Pacific at least back to MIS 10.
3 from the Galapagos Islands that extended back to 1587 to illustrate that spectral power within the ENSO frequency band shifted on decadal time scales and postulated that solar variability may modulate interannual and decadal climate variability in the tropics. In another study, two Holocene coral records (8920 yr BP and 7375 yr BP) indicated mean surface temperatures in Papua New Guinea were cooler than they are today (McCulloch et al., 1996). Higher Sr/Ca variance in the same fossil corals was interpreted as representing increased frequency of high amplitude ENSO events (McCulloch et al., 1996). Oxygen isotopic variations in a last interglacial coral (~124 ka) from Indonesia exhibit behavior much like that observed in modern corals from the same area, indicating that ENSO activity at that time was similar to today (Hughen et al., 1999). Modeling results suggest that tropical climate variability is orbitally controlled by seasonal insolation changes arising from the precession of the equinoxes (Clement et al., 1999; 2000; Kukla et al., 2002). Tudhope et al. (2001) used the record of geochemical variations in a suite of fossil corals from New Guinea, ranging in age from modern to 130 ka, to demonstrate that ENSO activity is a feature of both the interglacial and glacial ocean-atmosphere system. The corals from glacial times have weaker variance at the ENSO frequency bands in accordance with the model, however more paleoclimate data are needed to assess the predictions of the model with confidence. Climatological interpretation of geochemical data from fossil coral samples must also account for any changes in seawater chemistry that occur on glacial-interglacial time scales. The stable oxygen isotopic composition of seawater is known to change in response to sea-level changes such that during glacial times the seawater 18O values increase as 16O is preferentially sequestered in ice sheets. The current best estimate for the change in seawater 18O during the last glacial maximum (18ka) is 1.00.1Â‹ (Schrag et al., 2002). Seawater Sr/Ca ratios, long thought invariant on glacial-interglacial time scales, likely vary in response to changes in sea level (Stoll and Schrag, 1998). These authors used a geochemical box model to demonstrate that the diagenetic alteration of Srrich shelf carbonates during sea-level low stands results in a flux of Sr to the global oceans that raises the seawater Sr/Ca ratio. Hence, glacial intervals of lower sea level have a higher seawater Sr/Ca ratio than do interglacial intervals.
4 Here we present 8O and Sr/Ca data from an exceptionally preserved fossil coral drilled on Bougainville Guyot (1600.56ÂS, 16640.34ÂE) near Espiritu Santo Island in the Republic of Vanuatu (Fig. 1). The fossil coral is ~350 ka old, an age that places coral growth during deep-sea isotope stage 10 a glacial interval. In this study, we compare geochemical variations in the fossil coral with those from a modern coral from near Espiritu Santo Island (15.7S, 167.2E; Fig. 1) to assess the differences and similarities in mean climate state, seasonality, and interannual variability between a glacial and interglacial period as recorded in coral 18O and Sr/Ca.
5 2. OCEAN-ATMOSPHERIC INTERACTIONS AT VANUATU Ocean-atmosphere interactions in the Vanuatu region of the South Pacific are dominated by spatial and temporal variability in the linkages between warm SST associated with the Western Pacific Warm Pool (WPWP) and atmospheric convection associated with the South Pacific Convergence Zone (SPCZ). This study will address this variability on intra-annual and interannual time scales in the modern and glacial ocean. 2.1. South Pacific Convergence Zone The SPCZ is a zone of atmospheric convergence of the trade winds, and can be thought of as an extension of the ITCZ that stretches roughly from Papua New Guinea east to 120W, 30S (Fig.2, Vincent, 1994). The SPCZ can be defined by several variables: sea-level pressure, convective activity, outgoing longwave radiation, or surface wind-field convergence. There are four leading hypotheses that explain the existence of the SPCZ (Vincent, 1994). One hypothesis states that SST gradients around the WPWP (Fig. 2) cause surface pressure gradients that drive low-level wind, resulting in moisture convergence. A second hypothesis states that SPCZ strength is determined by the heating and circulation patterns over Australia. Model results testing the second hypothesis indicate that removing Australia from the model weakened the strength of the SPCZ, although the SPCZ remained in the same location (Kiladis et al. 1989). The third hypothesis suggests that the strength of the SPCZ is dependent on forcing from the monsoon systems of the Southern Hemisphere, including the Australian Monsoon and Indonesian Monsoon (Davidson and Hendon, 1989). According to Vincent (1994), the most viable hypothesis is that both tropical and extra-tropical forcing affects the strength and location of the SPCZ in a variety of different ways. Surface temperatures, midlatitude wave activity and tropical convection are all likely to be involved in SPCZ activity and variability.
10 precipitation data from Luganville, Vanuatu, located on the coastal plain just beside Malo Channel. The climatological range in precipitation is large, from ~ 100 mm per month to ~ 350 mm per month (Fig. 4). As explained above, heavy rainfall in the winter to early spring months is due to strong atmospheric convergence, and is correlated with low salinity in the region. Important regional patterns are also clear in the annual rainfall totals over the past half-century. El Nio and La Nia years can be differentiated as especially dry and wet years respectively, and the climate regime shift of the mid-1970s is readily discernable. SSS and SST anomalies (SSSA and SSTA, respectively) in the Vanuatu region are strongly affected by ENSO activity. During warm phase (cool phase) years, the SPCZ weakens (strengthens) and Vanuatu is dry (wet). Salinity increases sharply during warm phases due to advection of salty water from south of the SPCZ and due to the local E-P balance as described above. SSTÂs decrease (increase) during warm phase (cool phase) years, though the deviation is not as large as with SSS. As with the seasonal cycle, interannual variations of SSS and SST are inversely related and cause an additive response in the coral 18O signal. The maximum salinity in warm phase years is 0.28 psu saltier than the average salinity maximum for non-warm phase years for the period 19801993 (calculated with data from Gouriou and Delcroix, in press). This salinity value of 0.28 psu is 78% of the entire range of the annual SSS cycle. SST data (GISST2.36; Parker et al., 1995) can be used to demonstrate that winter minima in SST during warm phase years are 0.76C cooler than the minima in non-warm phase years for the period 1980-1993. The difference between normal years and warm phase years is 30% of the average annual range over the same period. SST and SSS values quoted above give the correct sense of the response; however it is important to note that not all ENSO events manifest in the exact same manner at Vanuatu (Fig. 5). There are some warm phase years where a small SSTA is associated with a strong SSSA (1983), while other years have strong SSTA and weak SSSA (1987). Still other ÂeventsÂŽ last for multiple years, such as the mild El Nio of the early 1990Âs (Trenberth and Hoar, 1996; 1997). The key to recognizing ENSO from temperature and salinity records in the Vanuatu region is determining the sense and duration of the SSSA and SSTA.
11 For the purpose of this study ENSO events must be defined in terms recognizable in a proxy record, faithful to the local modern instrument record, and consistent with conventional definitions. In a fashion similar to Trenberth (1997), the 5-month running mean of surface anomalies are used to define an ENSO event. A simultaneous SSTA decrease (increase) and SSSA increase (decrease), of a magnitude greater than one standard deviation of the running mean for more than 6 months of either SSSA or SSTA ( 18O or Sr/Ca anomalies in proxy situations), indicates a warm (cool) event (Fig. 6). The definition above differs from the definition of Trenberth (1997) in two ways. Surface salinity anomalies are included in the definition in addition to SSTA because the local response to El Nio at Vanuatu can be either strong salinity anomalies or strong temperature anomalies, and is often both. Another difference is the use of a fractional standard deviation threshold as opposed to a specific temperature (0.4C) as in Trenberth (1997). The fractional standard deviation threshold captures anomalies that are proportionally large compared to the variance in the specific record. When considering records of past SSTA and SSSA that may have a smaller or larger amount of variance, the standard deviation threshold is a more robust indicator of significant change relative to the mean.
14 estimated to be 0.15 % (0.013 mmol/mol), based on 86 determinations of the coral standard solution. Instrumental parameters of the ICP-OES are important for obtaining accurate results. The samples were introduced to the plasma at a rate of 1.0 ml/min using a Perkin Elmer AS93 plus auto sampler with a Meinhard TR-50 nebulizer and a cyclonic spray chamber. The sample probe was washed for 30 seconds in 2% trace metal-grade nitric acid between samples. Concentrations of strontium and calcium for the Sr/Ca ratios were measured on the ICP-OES by the intensity of two spectral lines: Sr (421.552 nm) and Ca (422.673 nm). The Perkin Elmer Optima 4300DV has two different detectors, each with its own high frequency noise (time period of seconds). The spectral lines used in this study were chosen because they are both measured on the same detector, thus minimizing the effect of high frequency noise. Geochemical data were converted from the depth domain to the time domain by matching the maxima (minima) in Sr/Ca to minima (maxima) in SST data and linearly interpolating between, using the software package AnalySeries (Paillard et al., 1996). In the modern coral, SST data as extracted from a 1 by 1 grid box (GISST 2.36; Parker et al., 1995) was matched to the coral Sr/Ca, and the first tie point on the top of the core corresponds to the sampling date. For BG831, the monthly climatological average SST served as a substitute for monthly SST data. Sr/Ca was chosen over 18O to create the age model because coral 18O is affected by seawater temperature and 18O variations, whereas coral Sr/Ca ratios are only affected by seawater temperature variations on short time scales. Checking the age models visually against annual banding evident in the Xradiographs ensured accuracy of our depth to time conversions.
18 ocean setting, therefore is taken as evidence that BG831 grew when glacial ice was more extensive than today, i.e. during a glacial period. The annual 18O range averages 0.27 0.09Â‹ in BG831, compared to a value of 0.45 0.23 Â‹ in MCB (Fig. 13). Sr/Ca ratios are similar in both modern and fossil corals (Fig.11b). The mean Sr/Ca ratios for BG831 and MCB are 8.712 0.017 mmol/mol and 8.691 0.015 mmol/mol respectively (1 ). These mean values are the same within a single standard deviation. With the variance of these records, there would have to be 135 degrees of freedom in the data before the two means were significantly different at the 95% confidence level, many more degrees of freedom than are available from a ~12-year quasi-sinusoidal curve. The mean annual cycle in the two Sr/Ca records is also very similar (Fig. 13b). The 95% confidence limits overlap and differences between the curves are mostly within analytical error. The average annual Sr/Ca range is 0.13 0.04 mmol/mol and 0.12 0.03 mmol/mol (2 ) for BG831 and MCB, respectively.
19 5. DISCUSSION 5.1. Modern Proxies and Instrumental Data 5.1.1. Calibration of SST and Coral Sr/Ca Climate proxies must be calibrated and validated against the instrumental record of climate change before they can be applied with confidence to paleoclimate questions (e.g., Crowley et al., 1999). There is no in situ, instrumental SST record at Vanuatu, therefore a global SST product, which integrates shipboard and satellite SST data and produces SST estimates at 1 latitude by 1 longitude grid points, must be used. The GISST2.36 database (Parker et al., 1995) and the NCEP reanalysis database (Reynolds and Smith, 1994) have been used in this study. The relationship between coral Sr/Ca and SST at Vanuatu was calibrated using a reduced major axis (RMA) linear regression (Davis, 1986) on data from 1980-1990. The most recent few years of the core were not used to minimize any contamination from coral tissue at the top of the core (Fig. 15). RMA regression assumes equivalent error in both the dependent and independent variable, in contrast to ordinary least squares regression that assumes error only in the dependent variable (e.g., Davis, 1986). Calibration of MCB coral Sr/Ca measurements against the GISST and NCEP SST records produces the following equations (95% confidence on the slope and intercept):Sr/Ca9.962600.04620.0046T =()ÂŠ()Â€ .0066,r = Â…0.83 (GISST)(1)Sr/Ca9.89800.04460.0044T =()ÂŠ()Â€ 0 0059 .,r = Â…0.84 (NCEP).(2) The calibration slopes are not statistically different, and the minor difference in slopes can result in temperature errors < 0.5C at 25C. The difference in intercept values can be explained by the fact that the temperature data sets are nearly constantly offset from each other.
20 The slope of the Porites spp. Sr/Ca-SST relationship in equations 1 and 2 are less than the 0.0620.014 (2 ) slope value that Gagan et al. (2000) recently reported as the average slope value of 9 coral Sr/Ca-SST calibrations. Much debate has focused on the origin of the observed variability in the Porites Sr/Ca-SST relationships and many factors have been cited as possible causative agents including: differences in field and laboratory sampling procedures, inter-laboratory differences in Sr/Ca spike for TIMS analyses, inter-laboratory differences in standard determination, seawater Sr/Ca variability, calcification rate, symbiont activity, and lack of standardization of instrumental SSTs among calibration sites (de Villiers et al., 1995; Gagan et al., 2000; Cohen et al., 2002; Marshall and McCulloch 2002; Quinn and Sampson, 2002) A key for coral paleoclimate reconstructions is that there is a robust, quantifiable relationship between in situ SST variations and those recorded in the gridded SST database. If this is true, then the corals recording local SST changes will correlate well with regional SST, as is the case in this study (Figs. 14, 15). Regional SST variations are ultimately the goal in paleoclimate reconstruction, thus this method is justified. 5.1.2. Relationship between Coral 18O, SST and SSS The 18O composition of coralline aragonite is a function of seawater temperature and the 18O of seawater at the time of skeletal precipitation (e.g., Goreau, 1977; Weber and Woodhead, 1972; McConnaughey, 1989). In oceanic regions characterized by strong atmospheric convection, like the tropical western Pacific, seawater 18O values are highly correlated with salinity (Fairbanks et al., 1997). Coral 18O variations at Vanuatu also reflect the combined influences of SST and SSS-driven changes in seawater 18O (Quinn et al., 1993; this study). Separate regressions for both 18O-SST and 18O-SSS are justified because the monthly resolved instrumental records of SST and SSS that are used in the regression analyses are not significantly correlated (r=-0.0086). This is not to say that SSS and SST at Vanuatu have no relationship, they are quite correlated on interannual time scales, and therefore future work on this core should include a simultaneously solved regression for both SST and SSS.
22 SST (GISST) data have a correlation coefficient that is not significantly different from zero (r= Â…0.0086). 5.1.3. Skeletal 18O and Sr/Ca Variations and ENSO at Vanuatu The response of the climate proxies (coral 18O and Sr/Ca) to seasonal SST and SSS forcing has been defined, but the relationship of these climate proxies to interannual forcing, especially ENSO forcing, must also be demonstrated in the modern record before interannual relationships in the fossil record can be considered. Interannual climate variability in the tropical Pacific Ocean is dominated by ENSO variations. Warm (cold) events are manifest in this region as cool (warm) and dry (wet) anomalies (Delcroix, 1998). Corals respond to lower (higher) SST and higher (lower) SSS values attendant with a warm (cool) phase by precipitating skeletal material that has higher (lower) 18O and Sr/Ca values. The 18O and Sr/Ca anomaly plots from MCB are very similar to the SSSA and SSTA plots from Vanuatu (Fig. 16, A and B). Coral 18O and Sr/Ca both increase during the 1983, 1987, and 1991 El Nio events, and they decrease in the 1989 La Nia event. Additionally, modern coral geochemistry provides details about the characteristics of each event that we know to be true from the instrumental record. The 1983 warm phase event was characterized in the region by a large salinity anomaly and a small temperature anomaly. Salinity in the Western Pacific lagged behind the eastern Pacific warming (represented by gray bars in Fig.15, A and B) during this event. The timing and relative magnitude of the temperature versus salinity anomalies in the proxy data are perfectly in line with the instrumental record (Fig. 16, A and B). The 1987 ENSO event manifest in this region as large temperature anomalies and much smaller salinity anomalies. Again, the proxy record shows this detail (Fig. 16, B). By using the definition of ENSO events described in section 2.2, one can easily identify modern ENSO events in the geochemical proxy records, and even further describe details about the temperature and salinity anomalies associated with these events.
30 secondary calcite can be applied to negate the above scenario. Recrystallization can also be patchy and the coral was sampled at greater than monthly resolution. A particular sample that was pristine would show quite a different Sr/Ca result from a sample that had calcite recrystallization. The record would have a sample-to-sample variance almost as large as the seasonal cycle even if the patches were only 1% calcite. The Sr/Ca data do not contain large deviations between near-by samples and recrystallization is not judged to be a problem in BG831. Secondary aragonite is an inorganically precipitated marine cement. BG831 has been exposed to marine conditions since its origin so we must entertain the possibility of secondary aragonite contamination in BG831. The distribution coefficient for Sr in inorganic aragonite is higher (KSr~ 1.14 at 25C; Kinsman and Holland, 1969; KSr = 1.210.03: Enmar et al., 2000) than the distribution coefficient for Sr in coral aragonite (e.g., for Porites near Japan KSr A~1.0560.003; Livingston and Thompson, 1971; Shen et al., 1996; Marshall and McCulloch 2002). This implies that inorganic aragonite will have a higher Sr/Ca ratio than coral aragonite precipitated from the same solution. Secondary aragonite has been shown to affect SST reconstructions from corals resulting in calculated temperatures that are lower than measured temperatures (Enmar et al., 2000; Mller et al., 2001). A quick calculation using values from Enmar et al. (2000) can confirm that minor amounts (~4%) of inorganically precipitated aragonite can have a significant affect on climate reconstruction. The bulk coral Sr/Ca ratio is equal to a weighted average of the coral Sr/Ca and the secondary aragonite Sr/Ca. Using 0.0229 (10.4751 mmol/mol) for secondary aragonite and 0.0203 (9.2858 mmol/mol) for primary aragonite, the Sr/Ca ratio of the bulk, contaminated coral would be 0.2040 (9.3333 mmol/mol). The effect translates to 0.8C using the Sr/Ca-SST relationship of Beck et al. (1997) and 1.0C using the Sr/Ca-SST relationship from this study. In reality, small percentages (1-4%) of secondary aragonite are not likely to be missed if the coral samples are examined for their presence because secondary aragonite is discernable using standard techniques. Although it is theoretically possible to underestimate SST by 0.4 0.5C given the undiagnosed presence of 2% secondary aragonite, it is not likely that the calculated temperatures from fossil coral BG831 are too low since they are close to the upper bound of physically possible SSTs.
33 climate solution consistent with the fossil coral data needs to be determined. According to the foraminiferal Mg/Ca record of Lea et al. (2000), the Western Pacific (specifically the Ontong Java Plateau; OJP (15922ÂE, 019ÂN) warmed early during MIS 10 so that by ~340 ka, surface temperatures were close to their present value. The age estimate of BG831 ranges between 337 and 371, overlapping the early warming event so one cannot discount that BG831 grew during that early warming to explain why seawater during a glacial period was similar or warmer than present day conditions. Another simple explanation is that meridional thermal gradients in the Pacific are compressed and shifted southwards during glacial times because of sea ice in the northern Hemisphere. Vanuatu is on the edge of the WPWP, and a southward shift in the warm pool could maintain or actually increase local temperatures, even during modest cooling of the warm pool. Although the western equatorial Pacific seems to have cooled by a couple of degrees Celsius during the MIS 10 glacial maximum as is evidenced by planktonic foraminiferal Mg/Ca data (Lea et al., 2000), there is also evidence that it warmed back up to almost 28 C around 350 ka. Vanuatu would have experienced similar to warmer than present SST if both Vanuatu and the OJP were bathed by the warmest equatorial waters at the time (i.e. both in the warm pool). A southward shift in the northern edge of the WPWP is documented by other data (Trent-Staid and Prell, 2002), but a southward shift in the southern border of the WPWP has not yet been documented by other studies. Another scenario that has been suggested is that salinity in the western Pacific decreases during glacial periods. Lea et al. (2000) suggested this when they found a smaller range of seawater 18O in the western equatorial Pacific (WEP) than the eastern equatorial Pacific (EEP) over glacial/interglacial cycles. Since seawater 18O in the EEP changed by the same amount as the global average, salinity changes in the WEP were invoked. Model results (Fig. 18) illustrate that salinity was not likely to be less than today in the waters where BG831 grew. To have even a slight salinity decrease, one must assume negligible changes to seawater Sr/Ca over glacial/interglacial cycles. While this is possible, it is not probable, since at least some increase in seawater Sr/Ca is expected during a glacial period (Stoll and Schrag 1998).
35 Kitoh and Murakami, 2002) and not all of them agree. Recent modeling studies (Bush and Philander, 1998; 1999; Hewitt et al., 2001) illustrate an alternative hypothesis, which is consistent with the data at Vanuatu and Indonesia (Stott et al., 2002). Bush and Philander (1998, 1999) concluded that the center of atmospheric convection moves west into the Indian Ocean and the tropical Pacific is characterized by a deeper thermocline, increased trade wind activity, increased salinity, and decreased convection between about 120E and 180E during glacial periods. The model predicts a simultaneous decrease in trade wind strength and decrease in atmospheric pressure in the Indian Ocean between 60E and 120E (Bush and Philander, 1998; 1999). These physical phenomena are consistent with a variety of paleo-data including: eolian deposits, planktonic foraminifera, pore waters, pollen records, and ice cores (Bush and Philander, 1998; 1999; and references therein). However they are inconsistent with a freshening of the WPWP (Lea et al., 2000). SST data from foraminiferal assemblage work similar to the CLIMAP project (CLIMAP project members 1976, 1981) also support a center of convergence in the Indian Ocean rather than over the Indo-Pacific region (Trend-Staid and Prell, 2002). Faunal evidence suggest decreased average temperatures around Indonesia, a smaller WPWP, warmer temperatures in the equatorial Indian Ocean, and an increased zonal thermal gradient across the Pacific Ocean (Trend-Staid and Prell, 2002). All of these features are consistent with increased trade winds in the WEP and increased convection in the equatorial Indian Ocean. Admittedly, the coupled model results in SSTÂs that are cooler than the foraminiferal data indicate, but the general patterns are similar. In summary, the data from this study are consistent with two different models of glacial periods that both predict a displaced center of convergence during glacial periods. Additional ÂpaleoÂŽ data are needed to constrain the east-west thermal and salinity gradients across both the Pacific and Indian Oceans during glacial periods, and to help resolve the discrepancies in current model predictions.
37 of seawater Sr/Ca remain the largest obstacle to the accurate reconstruction of tropical SST in the past using pristine fossil corals. 2) During this same time window, seasonal SST ranges were very similar to modern seasonal SST ranges, while seasonal hydrologic variations were reduced in amplitude compared to modern. The reduced seasonal SSS variations and increased SSTs near Vanuatu are interpreted as evidence that the SPCZ was displaced from its present location during at least part of MIS 10. 3) ENSO or ENSO-like interannual variations are not unique to interglacial time periods, nor just the past 130 kyrs, but characterize the tropical Pacific at least back to MIS 10, between 347-371 ka.
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54 -2 0 2 1980198219841986198819901992199419961998Nino3.4 Anomaly A -1.0 -0.5 0.0 0.5 1.0SSTA (C) B -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1980198219841986198819901992199419961998SSSA CMulti-year Multi-year Strong SST Weak SSS Weak SST Strong SSSFigure 6. Five month running average of monthly SSSA and SSTA at Vanuatu. Data source is as listed in Fig. 4. Anomalies are calculated as deviations from the average annual cycle between 1980-1992 (Fig. 2). Shaded regions are ENSO warm phase years ( Trenberth, 1997), and the labels on the lower graphs indicate the local response to ENSO. Black horizontal lines are 1.5 standard deviations of the anomaly data and represent the threshold over which anomalies at Vanuatu are functionally defined as ENSO events.
67 Figure 17. Sr/Ca and 18O anomalies from fossil coral BG831 illustrating fossil ENSO warm phase events (gray bars). The horizontal lines represent 1.5 standard deviations of the anomaly data (i.e., the criteria defined through the analysis of the modern relationship between coral proxy records and instrumental records). -0.20 0.00 0.20 012345678910111218O Anomaly (Â‹)Fossil Coral "Years" -0.10 -0.05 0.00 0.05 0.10 0123 456789 101112Sr/Ca Anomaly (mmol/mol)Fossil Coral "Years"
68 0 1 2 3 4 5 6 -101234 Sea Surface Temperature Sea Surface Salinity Mean SST= 30oCIce Volume=0m Ice Vol.= 120m 120m 120m 120m Sr/Ca=0% Sr/Ca=1% Sr/Ca=2% Sr/Ca=3% 98m 93m 50m 98m 93m 93m 93m 98m 98m 50m 50m 50mFigure 18. Four model solution sets of Pleistocene SST changes relative to modern ( T, C) and SSS changes relative to modern ( S) representing seawater Sr/Ca changes relative to modern of 0%, 1%, 2%, and 3% as labeled. Plots are limited to those SST and SSS values that correspond with ice volume changes no greater that 120m. The blue dots represent the maximum and minimum age/ice-volume boundaries from the forward stratigraphic modeling (see Fig. 8). The red open crosses denote the central date and ice volume obtained by forward stratigraphic modeling (see Fig. 8). A thin black horizontal line denotes a mean annual SST of 30C, a value assumed to be the upper bound of possible mean annual SST in this region. The gray shaded area illustrates the ÂmostlikelyÂŽ solutions of SST and SSS given model inputs for changes in ice volume/sea level and physical SST limits (see text for complete discussion).
69 Table 1. List of values and sources used to solve equations 6 and 7 for temperature, ice volume and salinity. Symbol Values Used Source O1 T -0.18 Â‹/oC Gagan et al. (1998) O2 I 0.00833Â‹/mSchrag et al. (2002) O3 S 0.273Â‹/psuFairbanks et al. (1997) Sr4 T -0.0462 mmol/mol oC-1This study Sr5 I 0-3% per 120m IceStoll and Schrag (1998) 1 O/ T; change in coral 18O (Â‹) per 1C change in SST.2 O/ I; change in coral 18O (Â‹) per 1m change in ice volume.3 O/ S; change in coral 18O (Â‹) per 1psu change in salinity.4 Sr/ T; change in coral Sr/Ca (mmol/mol) per 1C change in SST.5 Sr/ I; change in coral Sr/Ca (mmol/mol) per 1m change in ice volume.

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