A centennial-scale variability of tropical North Atlantic surface hydrography during the late Holocene

Palaeogeography, Palaeoclimatology, Palaeoecology 183 (2002) 25^41 www.elsevier.com/locate/palaeo

A centennial-scale variability of tropical North Atlantic surface hydrography during the late Holocene Johan Nyberg a;
, Bjo«rn A. Malmgren a , Antoon Kuijpers b , Amos Winter c a

c

Department of Earth Sciences-Marine Geology, Go«teborg University, P.O. Box 460, SE-405 30 Go«teborg, Sweden b Geological Survey of Denmark and Greenland (GEUS), Thoravej 8, DK-2400 Copenhagen NV, Denmark Department of Marine Sciences, University of Puerto Rico, P.O. Box 9013, PR 00681-9013 Mayaguez, Puerto Rico Received 13 September 2000; accepted 18 October 2001

Abstract Sea-surface temperature (SST) and sea-surface salinity (SSS) fluctuations in the northeastern Caribbean have been reconstructed through the last 2000 yr using an artificial neural network and N18 O analyses of planktonic foraminifera. A warmer period prevailed in the NE Caribbean from AD V700^950, which may reflect the occurrence of stronger and/or more frequent El Nin‹o events. A V2‡C cooling of winter SSTs, from AD V1400 to 1550, coincides with the occurrence of reduced solar output, the Spo«rer event. Episodes of lower SSSs with marked minima at the onsets of the Dark Ages in Europe (AD V500^600) and Little Ice Age (AD V1400) are cyclically recurrent at intervals of 200^400 yr, and coincide with drier periods in Mexico. This may indicate that the tropical Atlantic evaporation^precipitation budget and SSSs are affected by a centennial-scale modulation involving the freshwater export (import) from (into) the Atlantic Ocean. Coeval changes recorded in the deep North Atlantic circulation indicate that low-latitude SSS anomalies may be advected polewards by the North Atlantic current system, thus affecting deep-ocean convection and strength of the thermohaline circulation. @ 2002 Elsevier Science B.V. All rights reserved. Keywords: northeastern Caribbean Sea; North Atlantic; late Holocene; climate variability; temperature; salinity

1. Introduction The northeastern Caribbean Sea is well positioned to monitor variability on centennial-to-mil-

* Corresponding author. Tel.: +46-31-7732839; Fax: +46-31-7734903. Present address: Department of Geology, Royal Holloway University of London, Egham, Surrey, UK. E-mail addresses: [email protected] (J. Nyberg), [email protected] (B.A. Malmgren), [email protected] (A. Kuijpers), [email protected] (A. Winter).

lennial scales of tropical Atlantic climate as well as large-scale circulation of the Atlantic Ocean. Today, the precipitation system over the northeastern Caribbean undergoes seasonal as well as annual £uctuations, which are in£uenced by changes in the position of the Inter-Tropical Convergence Zone (ITCZ). Zonal trade winds maintained by high-pressure systems in the North Atlantic undergo latitudinal shifts with seasonal displacement of the ITCZ. This phenomenon is responsible for the seasonal shifts between wet and dry seasons in the northeastern Caribbean and also for annual variations (Etter et al.,

0031-0182 / 02 / $ ^ see front matter @ 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 4 4 6 - 1

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1987; Malmgren et al., 1998; George and Saunders, 2001). In the dry season (February to May), the ITCZ is close to the Equator and in the rainy season (August to October) the ITCZ is at its most northerly position between 6 and 10‡N (Etter et al., 1987). The timing of oceanic variability in the northeastern Caribbean Sea is coherent with variability in much of the North Atlantic and is most likely due to water-mass changes brought about by interannual variability of the North Atlantic circulation. The most probable mechanism for communication between the Caribbean and North Atlantic are water masses which pass through shallow passages in the eastern Caribbean and ultimately feed the Gulf Stream (Gordon, 1967; Morrison and Nowlin, 1982). During winter, when the thermal equator and ITCZ are furthest south, the water masses of the tropical Atlantic Ocean £ow with increased strength westward into the Caribbean Sea. In summer, when the thermal equator shifts north and the ITCZ lies between 6 and 10‡N, the surface waters in the Caribbean Sea are in£uenced by increasing precipitation (Etter et al., 1987). Here, we present the results of a study of the variability in surface-water hydrography (warm and cold sea-surface temperatures (SSTs) and mean annual sea-surface salinities (SSSs)) through the last 2000 years as recorded in a sediment core from the northeastern Caribbean Sea. The time interval is characterized by the occurrence of two climatically anomalous periods, the Little Ice Age (LIA) and the Medieval Warm Period. Existing observations indicate that the timings of climate change associated with these episodes were not simultaneous around the globe (Lamb, 1982; Ro«thlisberger, 1986). In contrast, several other observations point to a general interhemispheric synchronity of climate change as, for instance, recorded by changes in precipitation and wind patterns (Lamb, 1982; Ro«thlisberger, 1986). The SST and SSS £uctuations have been reconstructed using an arti¢cial neural network (ANN) trained on a modern data base of planktonic foraminifer census data, and oxygen isotope ratios of the planktonic foraminifer Globigerinoides ruber (white variety). The results are compared with sediment core data from the southern Caribbean

Sea, ice-core records from Peru and Greenland, a lake record from Mexico, a deep-sea record from the Iceland Basin, and solar activity.

2. Materials and methods 2.1. Study site The climate record is obtained from a piston core (PRP 12) and a box core (PRB 12) with lengths of 228 and 24 cm, respectively, retrieved south of Puerto Rico (17‡53.27PN, 66‡36.02PW; 349 m water depth) (Fig. 1). The cores are relatively ¢ne-grained and consist mainly of silty calcareous clay/ooze. No burrows were detected in the cores, which indicates that intense bioturbation has not occurred. The surface current system in the northeastern Caribbean Sea, south of Puerto Rico, is characterized by a predominantly westward drift with velocities of 10^40 cm/s in which occasionally eddies and meanders are formed (Wu«st, 1964; Metcalf, 1976; Morrison and Nowlin, 1982). The current system is modi¢ed by the local winds as well as the strength of the North Equatorial Current (Wood et al., 1975). The two upper water masses in the region are the Caribbean Surface Water and the Subtropical Underwater (Wu«st, 1964). The Caribbean Surface Water originates from the North Equatorial and Guiana currents with large input in October^November of low-salinity waters from the Amazon and Orinoco rivers (Froelich et al., 1978; Morrison and Nowlin, 1982). The Subtropical Underwater, extending down to 200 m, originates in the surface waters of the North Atlantic Central Gyre, and moves into the eastern Caribbean through the Anegada^ Jungfern Passage. The maximum SSS (W36.5x) and minimum SST (W26‡C) occur during winter, while minimum salinity (W35.1x) and maximum temperature (W29.5‡C) occur during summer (Yoshioka et al., 1985; Levitus et al., 1994). 2.2. Chronology The chronology of core PRP 12 is based on 14

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Fig. 1. Location of piston core (PRP 12) together with schematic surface circulation patterns in the North Atlantic and the two main deep-water convection sites in the Greenland and Labrador seas.

accelerator mass spectrometry (AMS) radiocarbon analyses (Table 1; Fig. 2) from samples of benthic gastropods and mussels in the s 150-Wm fraction (see also Nyberg et al., 2001). The dating was performed at the The Svedberg Laboratory, Uppsala University, Sweden, and University of Aarhus, Denmark. Calibrated ages in calendar years were obtained from Stuiver et al. (1998) by means of the Seattle calibration program CALIB version 4.0 (Stuiver and Reimer, 1993).

The top of the core yields a negative age after reservoir correction. With the exception of one date (at 24 cm), the corrected ages produce a down-core trend. Ages of samples that were not AMS 14 C dated in PRP 12 were determined through linear interpolation between the AMS 14 C dates. The upper 8 cm of the core represents the period since AD V1624 (301 reservoir corrected 14 C yr) and shows the lowest sedimentation rates of V0.24 mm/yr, while the interval between

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Table 1 Results of AMS Depth

14

C datings for the piston core PRP 12 retrieved from the southern insular shelf o¡ Puerto Rico

Lab No.

(cm) 0 8 20 24 28 40 80 100 120 130 160 170 200 216

AAR-4632 AAR-4555 AAR-4633 Ua-13675 AAR-4556 AAR-4634 AAR-4557 AAR-4635 AAR-4558 Ua-12082 AAR-4559 AAR-4636 AAR-4637 AAR-2768

14

C age

(BP)

Reservoir-corrected 14 C age (BP)

Calibrated age (cal yr AD)

(AD)

385 Q 45 710 Q 45 760 Q 50 925 Q 75a 825 Q 40a 1025 Q 50 1380 Q 55 1550 Q 40 1600 Q 55 1680 Q 65 1910 Q 55 1955 Q 55 2155 Q 45 2265 Q 55

315 Q 45 301 Q 45 351 Q 50 516 Q 75 425 Q 40 616 Q 50 971 Q 55 1150 Q 40 1200 Q 55 1271 Q 65 1510 Q 55 1555 Q 55 1746 Q 45 1856 Q 55

after 1960 1624 1520 1429 1483 1320 1000 925 870 810 590 540 217 82

1533^1656 1492^1625 1384^1467 1457^1512 1307^1403 987^1059 880^970 780^890 664^850 540^630 430^600 143^259 24^138

Cal yr range Q 1N

Sedimentation rate (mm/yr) 0.24 1.15 1.15b 0.95 1.25 2.6 3.6 1.7 1.4 2.0 0.93 1.2

The calibrated ages in calendar (cal) years with ranges Q 1c are obtained from Stuiver et al. (1998) by means of the Seattle calibration program CALIB version 4.0 (Stuiver and Reimer, 1993). a These two raw 14 C ages were averaged using the CALIB program, which yielded a 14 C age of 848 Q 36 (BP) and a calibrated age of AD 1448 (1468) 1492 ( Q 1N). b Sedimentation rate calculated from the average of the two 14 C ages at 24 and 28 cm depth, respectively.

120 and 80 cm (from 1200 to 971 reservoir corrected 14 C yr BP,AD V900^1000) has the highest sedimentation rates of V2.6^3.6 mm/yr. The period around AD 900 is marked by an abrupt increase in magnetic susceptibility and a decrease in CaCO3 , which may indicate an increased out wash of terrigenous material from Puerto Rico. This could be caused by a change towards a more humid climate (Hodell et al., 1991; Nyberg et al., 2001). The depth from 40 to 24 cm in PRP 12, where the age reversal appears, is characterized by a higher organic carbon content compared to the other parts of the core (Nyberg et al., 2001). This may have favored enhanced benthic productivity and bioturbation, resulting in increased sediment reworking. An average of the raw 14 C results from 24 and 28 cm depth, respectively, using the CALIB program, yielded a 14 C age of 848 Q 36 (BP) and a calibrated calendar age of AD 1448 (1468) 1492 ( Q 1c). 210 Pb/137 Cs dating for box core PRB 12 was carried out at the RisR National Laboratory, Denmark. The age model for the 24-cm-long box core is based on 16 measurements of 210 Pb and 137 Cs activity at 0^1, 1^2, 2^3, 3^4, 4^5, 5^6, 6^7,

7^8, 8^9, 9^10, 11^12, 12^13, 13^14, 15^16, 17^ 18, and 22^23 cm depths. Use of the ‘constant rate of supply modeling’ method (Kunzendorf et al., 1998), indicates that the top 15 cm of the core spans an interval from AD 1994 to about AD 1945 (Fig. 3). This age is also supported by Cs-137 activity measurements from the same interval. The error of the age model due to measurement uncertainty is smaller than 10%. Extrapolating the dating results to greater depths places a depth of 22 cm to the middle of the 19th century, which suggests that the sedimentation rates found before the 1940s in the box core (V1 mm/yr) are similar to the rates determined in the piston core using 14 C dating results (Table 1). 2.3. SST estimates Twenty-six 1-cm-thick samples were taken every 4th cm down to 30 cm depth and every 10th cm down to 216 cm depth in the piston core (PRP 12). In the box core (PRB 12) 13 1-cm-thick samples were taken every 2 cm. All samples were oven dried at 50‡C, weighed and disaggrated by soaking in water with continuous

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Fig. 2. Age^depth relationship based on AMS used to establish chronology in piston core PRP 12. Compare Table 1.

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shaking on a rotating table and wet sieved through a 63-Wm sieve. The dried residues were weighed and sieved through a 150-Wm sieve. In each sample planktonic foraminifer census data were generated by counting approximately 300 or more specimens of 26 species of planktonic foraminifera in the s 150 Wm fraction following the taxonomy established by Parker (1962), Be¤ (1967) and P£aumann et al. (1996). No signs of calcite dissolution were observed, which is con¢rmed by low percentages of planktonic foraminifer fragments in all samples. We reconstructed past SSTs using a back propagation ANN, which is a branch of arti¢cial intelligence (Malmgren and Nordlund, 1996; Malmgren et al., 2001). The network was trained on the relationship between the relative abundances of 26 species of planktonic foraminifera and measured SSTs (0^75 m depth) in 80% of a total of 740 surface-sediment samples from the Atlantic Ocean (P£aumann et al., 1996; Malmgren et al., 2001) and tested on the remaining 20% of the samples. The dataset consisted of 738 surfacesediment samples from the Atlantic database compiled by P£aumann et al. (1996) and two additional samples from the Caribbean Sea (Malm-

Fig. 3. (A) Cs-137 activity values against depth in box core PRB 12. (B) Age^depth model for box core PRB 12 using the ‘constant rate of supply modeling’ method on 210 Pb activity measurements (Kunzendorf et al., 1998).

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gren et al., 2001). Separate training sessions of the network on summer (August^October) and winter (February^April) SSTs permit reconstructions of climate change on a seasonal basis. An assessment of the precision of SST predictions resulted from 10 such independent runs per season using di¡er-

ent training and test-set con¢gurations. A comparison of the ANN approach, both with (ANND) and without (ANN) use of geographical information regarding the location of the surfacesediment samples, with other procedures employed for paleotemperature predictions (Imbrie-

Fig. 4. Comparison between measured and estimated warm and cold SSTs over the time interval for which reliable instrumental observations were available. (A) Instrumentally measured warm (August^October) and annual mean SSTs together with estimated SSTs. (B) Instrumentally measured cold (February^April) together with estimated SSTs. Error bars for estimated SSTs represent 95% con¢dence intervals based on 10 separate runs of the ANN using di¡erent training- and test-set con¢gurations. The age model with error bars for PRB 12 is based on the 210 Pb and 137 Cs dating. A ¢ve-year running mean is applied to the measured SSTs to account for the di¡erences in resolution. Each reconstructed SST value represents about 3^6 yr of measured SSTs, and is based on a sample thickness of 1 cm.

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Kipp transfer functions [Imbrie and Kipp, 1971; Kipp, 1976]), the modern analog technique (Hutson, 1980), SIMMAX (P£aumann et al., 1996), and revised analog method (RAM ; Waelbroeck et al., 1998) indicates that the ANND, together with SIMMAX, generate the lowest root-meansquare errors (RMSE) of predictions on modern estimates (Malmgren et al., 2001). A comparison of the various techniques for the data obtained in PRB 12 with the instrumental records over the last V45 yr suggests that ANN and ANND may provide better paleo SST estimates than both RAM and SIMMAX. To determine the reliability of the ANN estimates, we compared estimated and instrumentally measured SSTs on the basis of eight 1-cm-thick samples taken every 2 cm from the top 15 cm of the 210 Pb- and 137 Cs-dated box core PRB 12 (Fig. 4). Each 1-cm-thick interval should approximately represent a time interval of 3^6 yr. We obtained the reliable instrumentally measured summer (August^October), winter (February^April), and mean annual SSTs from 1949 to 1994 from the Comprehensive Ocean-Atmosphere Data Set (COADS ; Woodru¡ et al., 1987) and da Silva et al. (1994) in a 2U2‡ box centered at 67‡W and 17‡N. SST measurements for the entire water depth of 0^75 m, on which the neural network was trained, are available only for the year 1994 (da Silva et al., 1994). Therefore, estimates of SSTs obtained from the neural network are expected to be some 0.5‡C lower than the instrumental record. The estimated warm SSTs are slightly lower than the actual summer SSTs. This is most likely due to the facts that the surface-sediment database includes very few samples deposited under extreme warm conditions and that the instrumentally recorded summer SSTs are based on an average of the 3 months marked by considerably higher SSTs than the other months of the year. Instead, the estimated warm SST record is synchronous with the measured annual mean series. The RMSE for the estimate of the annual average SST, excluding the e¡ect of the expected approximately 0.5‡C di¡erence discussed above, is only 0.13‡C, while it is 0.92‡C for summer SST (August^October). Predicted cold SSTs correlate ex-

31

ceptionally well with the measured winter SSTs (RMSE = 0.06‡C). The warm temperature peak around 1960 coincides with the strong El Nin‹o years of 1957^1958 and is recorded by the ANN predictions for both warm and cold SSTs (Fig. 4). The reconstructed SST minimum around 1980, which coincides with a temperature maximum in the instrumental record, is within the error bar on dating. 2.4. SSS estimates The reconstructed SSS £uctuations are derived from oxygen isotope ratios of the surface-dwelling planktonic foraminifer G. ruber (white variety) and the estimated warm SSTs using the basic relationship between water temperature and the N18 O signal of carbonate (O’Neil et al., 1969; Shackleton, 1974; Duplessy et al., 1991): T ¼ 16:934:38ðN 18 Ocarbonate 3N 18 Owater Þþ 0:1ðN 18 Ocarbonate 3 N 18 Owater Þ2

ð1Þ

Approximately 100 Wg of G. ruber (white) was analyzed for oxygen isotopes using a VG Prism Series II mass spectrometer Preparation system on line at the Earth Sciences Center, Go«teborg University. The analytical precision is better than 0.03x. Wang et al. (1995) demonstrated that the N18 O values of G. ruber (white) in the low-latitude Atlantic Ocean record the annual mean temperature and salinity of the water at 0^50 m depth, although to a slightly higher degree for the summer (August^October) SST and SSS. In the Sargasso Sea, the N18 O signal of G. ruber (white) tests has been shown to record the annual mean SST and SSS (Deuser, 1987). Thus, if we use the estimated warm (August^October) SSTs at our site, which in our calibration study also are a measure of mean annual temperatures (Fig. 4), the N18 O values of G. ruber can be used to decipher the mean annual and to a certain degree the summer N18 O of the paleowater masses at 0^50 m depth. When we predict SSS from (Eq. 1) we need to know the relationship between salinity and

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Atlantic (Wang et al., 1995): T iso ¼ 3:147 þ 0:963T m

Fig. 5. Salinity and N18 Owater values derived from seawater samples taken from 1991 to 1995 outside La Parguera, SW Puerto Rico.

N18 Owater and the habitat e¡ect of G. ruber, which translates into the following relationship between the measured summer SST at 0^50 m depth (Tm ) and predictive isotopic temperature (Tiso ) of G. ruber tests calculated from N18 O values of G. ruber and measured SSS in the low-latitude

ð2Þ

The relationship between salinity and N18 Owater is derived from seawater samples taken during all seasons from 1991 to 1995 outside La Parguera, SW Puerto Rico, located 50 km west of the PRP 12 site. SSS and N18 Owater are positively correlated (r2 = 0.91), and this relationship holds throughout the year (Fig. 5). SSS can be determined from N18 Owater using the following linear regression function: SSS ¼ 33:670 þ 4:155 N 18 Owater

ðin PDBÞ:

ð3Þ

The conversion from standard mean ocean water to Pee Dee belemnite (PDB) is assessed by an adjustment of 30.27 (Hut, 1987). The standard errors for salinity and N18 Owater are 0.28 and 0.06x, respectively. The reconstructed SSS £uctuations are then derived by inserting the habitat e¡ect (Eq. 2),

Fig. 6. (A) Estimated mean annual SSSs and N18 O values of the planktonic foraminifera species Globigerinoides ruber (white variety) in PRB 12. (B) Comparison of estimated mean annual SSSs in PRB 12 (17‡53 N, 66‡36) with observed mean annual SSSs at Bermuda (32‡N, 64‡W, 0^50 m depth). A ¢ve-year running mean is applied to the observed SSSs to account for the di¡erences in resolution.

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the relationship between SSS and N18 Owater (Eq. 3), oxygen isotope ratios of G. ruber (white variety), and the reconstructed warm SST into Eq. 1. The SSS can then be assessed using the equation : 18

SSS ¼ 33:670 þ 4:155 ðN Ocarbonate 321:9þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 310:6 þ 10Wð0:963WSST þ 3:147ÞÞ:

ð4Þ

The standard error of the salinity estimates can be calculated by adding the errors comprised in Eq. 4 (see Wang et al., 1995; also Schmidt, 1999), which involves the basic relationship between water temperature and the N18 O signal of carbonate, the habitat e¡ect outlined in Eq. 2, the relationship between SSS and N18 Owater (Eq. 3), the reconstructed warm SST, and the analytical uncertainties in the oxygen isotope measurements. The calculated additive standard error is V Q 1.0x (Press et al., 1990). Unfortunately, no time series of measured SSSs exist in the Caribbean region. In order to determine the reliability of the SSSs estimates, we therefore compare estimated SSSs in PRB 12 with Levitus et al. (1994) and the longest available SSS observations in the low-latitude Atlantic taken at Bermuda (32‡N, 64‡W) at water depths of 0^50 m spanning the time interval from 1954 to 1994. The direct comparison between estimated mean annual SSS (36.1x; Fig. 6) in the top sample, which should span an interval between December 1994 and the fall of 1991, and observed mean annual SSSs during 1994 (35.7x) at latitude 17.5‡N and longitude 66.5‡W at water depths between 0 and 50 m (Levitus et al., 1994), reveals a di¡erence of 0.4x. The estimated SSS £uctuations in PRB 12 correlate well with those observed at Bermuda (Fig. 6B). High SSS values concur in the mid 1960s, late 1980s, and 1990s. Low SSS values coincide around 1970 and the early 1980s. Since changes in SSSs at Bermuda are linked to freshwater £uxes between the tropical Paci¢c and Atlantic oceans (Latif, 2001), the estimated SSSs in PRB 12 may record the general trend of SSS £uctuations in the tropical Atlantic as well as anomalous freshwater £uxes between the two ocean basins.

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3. Results 3.1. SSTs The estimated SSTs derived from PRP 12 show that a warmer period prevailed from AD V700^ 950 with cold (winter) and warm (summer) SSTs close to or slightly above the 20th century temperatures (Fig. 7). Another signi¢cant feature is the higher summer SSTs around AD 1400 and the following V2‡C cooling of winter SSTs from AD V1400 to 1550, which coincides with an occurrence of reduced solar output, the Spo«rer event. Both warmer winter and summer SSTs occurred around AD 1550^1600. The seasonal SST range (warm minus cold season SST) is high between AD 850 and 950 and during the beginning of the LIA, from AD V1400 to 1550. 3.2. SSSs In PRP 12 lower reconstructed SSS values coincide with lighter N18 O values of G. ruber at AD V200, 600, 850, 1000, 1400 and 1600 (Fig. 7). If the major part of the £uctuations in the N18 O composition of G. ruber were controlled by temperature changes, assuming a constant vital e¡ect, the mean annual temperatures could have changed up to 3‡C during these periods (O’Neil et al., 1969). However, since the independent ANN estimates of SSTs do not show a complete correlation with N18 O, and since it is not likely that mean annual temperature variations of 3‡C could have occurred several times during the last 2000 yr in tropical^subtropical areas, we assume that a freshening of the surface-water mass has been involved in producing the peaks of more negative N18 O values.

4. Discussion 4.1. AD 700^950 The warming recorded in the northeastern Caribbean Sea around AD 700^950 coincides (within the dating uncertainties) with an increase in the Cl3 content in the GISP2 ice core from central

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Fig. 7. Comparison of estimated cold-season SSTs (February^April), warm-season SSTs (August^October), mean annual SSSs, seasonal SST range, and measured N18 O of Globigerinoides ruber (white variety) in piston core PRP 12 and box core PRB 12, retrieved in the northeastern Caribbean Sea, south of Puerto Rico at 17‡53.27PN, 66‡36.02PW (349 m water depth), with the abundance of the planktonic foraminifer Globigerina bulloides in a sediment core retrieved in the Cariaco Basin in the southern Caribbean (Black et al., 1999), Cl3 in GISP2 ice core (Greenland), N18 O values of the ostracod Cytheridella ilosvayi in a sediment core from Lake Punta Laguna, Mexico (Curtis et al., 1996), particle concentrations greater than 0.63 Wm in the Quelccaya ice core, Peru (Thompson et al., 1985; 1986; 1988), sortable silt size mean from a piston core obtained from 2848 m depth in the south Iceland Basin (Bianchi and McCave, 1999), and solar activity (N14 C residual series) (Stuiver et al., 1998). Higher abundance of G. bulloides indicates stronger northeasterly trade winds, higher concentrations of Cl3 imply an increase in the intensity of tropospheric aerosol transport, higher ostracod N18 O values indicate higher evaporation and drier climate, increases in microparticle concentration are interpreted as indicating arid events in the Andean altiplano, and a higher sortable silt mean size indicates an increase in ISOW £ow and vice versa. Dashed lines for SSTs represent 95% con¢dence intervals. The thick lines represent ¢vepoint smoothing averages, which correspond to time steps of V5^40 yr, except for the Cl3 record, which is smoothed using 20point means, corresponding to a time step of V30^40 yr. This scale is shown on the right-hand side of the graph.

Greenland, higher ostracod N18 O values in Lake Punta Laguna, Mexico, higher concentrations of microparticles in the Quelccaya ice core (Fig. 7) as well as with a cooling in the North Atlantic (Bond et al., 1997), a SST warming o¡ Cap Blanc, West Africa (deMenocal et al., 2000) and a signi¢cantly enhanced dust deposition indicating drier conditions in northeastern Nigeria (Street-Perrott et al., 2000). The Cl3 content indicates enhanced deposition of sea salts due to intensi¢ed meridional air£ow (O’Brien et al., 1995; Yang et al., 1996; Kreutz et al., 1997), higher ostracod N18 O values indicate higher evaporation and drier climate (Curtis et al., 1996), and a high concentration of microparticles in the Quelccaya ice core indicates arid events in the Andean altiplano (Thompson et al., 1985; 1986; 1988). From instrumental observations during the 20th century, warmer periods in the northeastern Caribbean and waters outside West Africa are associated with wetter tropical climate, reduced trade wind strength, and weakened meridional pressure gradient (Hastenrath, 1976, 1984; Hastenrath and Kaczmarczyk, 1991; George and Saunders, 2001). This may indicate that the warming in northeastern Caribbean and the waters outside West Africa is at odds with the other records during this interval when compared to the 20th century. An explanation for these contrasts may be increased impact of Paci¢c climate variability on tropical Atlantic climate. Warmer SSTs outside Puerto Rico are found to correlate with El Nin‹o years during the 20th century (Malmgren et al., 1998). During El Nin‹o years there is a transfer of warm temperatures from

the eastern equatorial Paci¢c to the western equatorial Atlantic, which is very distinct up to 10‡N, but also a¡ects the latitude of Puerto Rico (18‡N) (Kawamura, 1994). The period from AD V750 to 950 is characterized by exceptionally severe or frequent El Nin‹o events, as inferred from low Nile £ood levels (Quinn, 1992) and from paleo£ood chronologies over southwestern North America (Ely et al., 1993). Thus, the warming in northeastern Caribbean may be due to stronger in£uence of Paci¢c climate. 4.2. AD 1400^1550 The V2‡C cooling of winter SST and the larger seasonal SST range from AD V1400 to 1550 concur with a higher abundance of the planktonic foraminifer Globigerina bulloides in a sediment core retrieved in the Cariaco Basin, southern Caribbean Sea, indicating stronger northeasterly trade winds (Black et al., 1999) (Fig. 7). These conditions are coeval with an increase in the Cl3 content in the GISP2 ice core from central Greenland (Fig. 7). These records, together with an annually dated ice core from the Siple dome, West Antarctica (Kreutz et al., 1997), suggest enhanced meridional vs. zonal atmospheric circulation. Such a circulation pattern, including shifts in the latitudinal belt of stronger westerlies and southwestward shifts in storm tracks, may explain the transitions towards an apparently more arid climate in Africa (Street-Perrott and Perrott, 1990), increases in precipitation in California and Patagonia (Stine, 1994), and longer and

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more intense upwelling at lower latitudes, especially at the beginning of LIA, AD V1400^1500 (Kreutz et al., 1997; Nyberg et al., 2001). The larger seasonal SST range in the northeastern Caribbean, which according to the ANN is caused by cooler winter SSTs, may have resulted from troughs of cold air penetrating further south than present into the northern Caribbean, particularly during winters. Enhanced coastal upwelling in relation with increased (winter) trade wind strength may also have played a role. For the 20th century it has also been observed that years of a cooler northern Atlantic Ocean, including the northeastern Caribbean Sea, are associated with a southward shift of the ITCZ, which leads to an intensi¢cation of the northeasterly trade winds and a weakening of the southeasterly trade winds (Hastenrath and Greischar, 1993; Black et al., 1999). These years are also characterized by a warmer South Atlantic relative to North Atlantic and a drier climate in the northern tropical regions. Cooler tropical North Atlantic temperatures relative to the South Atlantic reduce the cross-equatorial SST gradient, increase the surface pressure over the North Atlantic and shift the ITCZ southwards. The stronger northeasterly trade winds cause lower moisture content in the atmosphere and subsequently lower precipitation due to enhanced latent heat £ux (Hastenrath, 1976, 1984; Hastenrath and Kaczmarczyk, 1991; Malmgren et al., 1998; George and Saunders, 2001). An additional e¡ect of the southward shift of the ITCZ is that the meridional pressure gradient is forced equatorward of the subtropical high. Reverse scenarios occur when the North Atlantic is warmer than the South Atlantic. A decadal variability in the patterns of this cross-equatorial SST gradient has been observed (Carton et al., 1996; Kushnir, 1994). The mechanisms that have been proposed to explain this variability range from air^sea interactions in the tropics (Chang et al., 1997), teleconnections between El Nin‹o^Southern Oscillation (ENSO) events in the Paci¢c (En¢eld and Mayer, 1997), and variability in the Atlantic meridional overturning circulation (MOC) (Black et al., 1999; Yang, 1999). An involvement of MOC, which is driven by

deep-water formation in the Labrador and Nordic seas, may be evident at the beginning of LIA, AD V1450^1600, from a lower sortable silt size mean in a piston core obtained from 2848 m depth in the south Iceland Basin (Bianchi and McCave, 1999) and a cooler northern tropical Atlantic (Fig. 7). Hence, the observed enhanced meridional circulation in the northern hemisphere from AD V1450 to 1600, which is marked by a higher Cl3 content in GISP2 and higher abundance of G. bulloides in the Cariaco Basin may be associated with a reduced deep-water production in the North Atlantic. A lower sortable silt size mean indicates a slower £ow of Iceland^Scotland over£ow water (ISOW). The deep £ow of ISOW partly maintains and counterbalances the warm and saline waters of the poleward North Atlantic Current, and a slower £ow of the ISOW has been linked to cooler surface water in the Sargasso Sea for the last 3500 yr (Bianchi and McCave, 1999). The slower £ow of ISOW from AD V1450 to 1600 may indicate a weakened deep-water formation and southward transport of cold deep waters by the deep Western Boundary Current, which is balanced by a reduced cross-equatorial heat £ux in the upper ocean layer (Yang, 1999). This may imply that a reduced northward £ow of equatorial warm water into the North Atlantic occurred, which resulted in a cooler North Atlantic relative to the South Atlantic. Apparently, a single factor is not the cause for the intensi¢cation of the meridional atmospheric circulation in both hemispheres at the beginning of the LIA around AD V1400^1450 (Kreutz et al., 1997). Noteworthy is, however the reduced solar output during the Spo«rer event at AD V1450^1550, which is coeval with the reduced deep-water production discussed above, and which can be correlated with an enhanced meridional atmospheric circulation in the northern hemisphere (Fig. 7). This is indicated by the higher abundance of Globigerina bulloides in the southern Caribbean and higher Cl3 content in GISP2 (Fig. 7). However, there are indications that a long-term variability of the oceans (V1500 Q 500 yr) (Bond et al., 1997) may have played a key role in climate variations associated with the LIA (Broecker et al., 1999). A 3^4‡C

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cooling of SSTs in the waters o¡ Cap Blanc, West Africa, around AD 1300 has been explained as a result of a southward advection of cooler temperate or subpolar waters related to abrupt millenialscale cooling events linked to this long-term variability of the oceans (deMenocal et al., 2000). The interruption of the £ow of ISOW around AD 1350 may have been caused by a southward advection of less dense subpolar waters and support the occurrence of a cooling event during this time. This may imply that these temperate or subpolar waters also entered the northeastern Caribbean through the North Equatorial Current and caused the lowering of SSSs around AD 1400 (Fig. 7). Lowered SSSs with associated density changes in Caribbean and tropical Atlantic water may, in turn, have caused a further reduced deepwater formation in the North Atlantic due to weakened northward £ow of the equatorial warm-water pool into the North Atlantic. A weakened northward £ow may reduce the volume £ux of the North Atlantic Current, which is thought to be a large component of the ISOW (Mauritzen, 1996). The reconstructed lower SSSs in the northeastern Caribbean correlate well with the initiation of a slower ISOW £ow around AD 1400, indicating reduced deep-water formation in the North Atlantic. Similar observations were made during the 1970s when intensi¢cation of northeasterly trade winds coincided with large negative salinity anomalies in the North Atlantic, which at a later stage were associated with reduced deep-water convection and unusually cold SSTs in the Greenland Sea (Schlosser et al., 1991). Incidental weakening of the £ow of the Gulf Stream was coeval with the reduced deep-water convection and the increased trade-wind strength. On the other hand, during the 1950s when the trade-wind strength was weaker an increase in the £ow of the Gulf Stream was observed (Greatbatch et al., 1991). 4.3. Cyclical variations in SSSs Comparable changes of magnitudes in the North may have occurred due to advective processes at the

di¡erent scales and Atlantic circulation low-latitude oceanic onsets of the Dark

37

Ages in Europe (AD V550^600; Lamb, 1982), and LIA (V1400) (within the dating errors of Q 100 yr) when lower SSSs coincide with reductions of ISOW £ow. The available record of trade-wind strength in the southern Caribbean as recorded by the abundance of G. bulloides, suggests that weakening of the northeasterly trade winds (i.e. a northward shift of the ITCZ) leads the lower recorded SSSs (Fig. 7). This may indicate that changes in the evaporation^precipitation budget at lower latitudes are involved in inducing the SSS anomalies, possibly in combination with more temperate and subpolar waters entering the Caribbean Sea by the North Equatorial Current. The cyclical behavior of the reconstructed SSSs as well as the coinciding lower SSSs, drier periods in Mexico, and shifts in the £ow patterns of ISOW at AD V200, 1000, and 1600 (Fig. 7) may indicate an involvement of freshwater £uxes between the Atlantic and Paci¢c oceans. Weakened water vapor export from the tropical Atlantic, as well as enhanced water vapor import into the tropical Atlantic, occur during cold ENSO phases (Schmittner et al., 2000). These conditions with associated higher rainfall together with a decreased latent heat £ux due to lower wind speed, may depress SSSs in the tropical Atlantic if they persist su⁄ciently long. Furthermore, an associated northward shift of the ITCZ weakens the drift of the tropical Atlantic water masses westward into the Caribbean Sea (Etter et al., 1987). SSS anomalies induced in this way may be transferred poleward by the North Atlantic Current system, which a¡ect surface water density in the deep-ocean convection regions of the North Atlantic and thus deep-water production. A stabilized thermohaline circulation may have prevailed around AD 850, which could explain why the £ow of ISOW did not markedly change at the AD 850 event of lower SSSs in the northeastern Caribbean. This period is characterized by exceptionally severe or frequent ENSO events (Quinn, 1992; Ely et al., 1993). During warm ENSO phases there is an enhanced export of water vapor from the North Atlantic (Schmittner et al., 2000). This increase in freshwater export may have provided a stabilizing e¡ect on Atlantic SSSs and thus the thermohaline circulation. A

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short-term disturbance in the freshwater £ux may be inadequate to induce a reduction or disturbance in the deep-water production in the North Atlantic. Moreover, a stabilized thermohaline circulation may have favored the northward £ow of equatorial warm water into the Caribbean and additionally explain the warming recorded in the northeastern Caribbean around AD 700^950 (Fig. 7).

5. Summary We have reconstructed SST and SSS £uctuations in the northeastern Caribbean Sea through the last 2000 yr. The past SSTs were reconstructed using an ANN trained on the relative abundances of 26 species of planktonic foraminifera. The reconstructed SSS £uctuations were derived from oxygen isotope ratios of the planktonic foraminifer G. ruber (white variety) and the reconstructed warm SSTs. (1) A pronounced warming is recorded from AD V700 to 950. Climate records from North Africa and southwestern America indicate that this interval was marked by exceptionally severe or frequent El Nin‹o events. Since a direct relationship exists between El Nin‹o events and enhanced SSTs in the northeastern Caribbean, this warming event may be related to the occurrence of stronger and/or more frequent El Nin‹o events. (2) A V2‡C cooling of winter SSTs and a larger seasonal SST range occurred at the beginning of the LIA from AD V1400 to 1550. The cooler winter SSTs may have resulted from troughs of cold air penetrating further south than at present into the northern Caribbean, particularly during winters. Increased trade-wind strength and associated enhanced coastal upwelling may also have played a role. An additional explanation for cooler SSTs during this time is reduced deep-water formation in the northern North Atlantic. Weakened deep-water formation can be associated with reduced northward advection of equatorial warm water into the North Atlantic. A reduced transport of warm water into the tropical North Atlantic may have resulted in a cooler northeastern Caribbean Sea. A cooler North Atlantic relative

to the South Atlantic may also have increased surface pressure over the North Atlantic and resulted in a southward shift of ITCZ as well as an enhanced meridional circulation. (3) Periods of low SSSs in the northeastern Caribbean concur with reductions in North Atlantic deep water (ISOW) circulation at AD V500^600 and 1400. SSS anomalies transferred northwards by the North Atlantic Current system may have a¡ected surface water density in the deep-ocean convection regions of the North Atlantic and thus deep-water production. In addition, at AD V200, 1000, and 1600, changes in the evaporation^precipitation budget at lower latitudes and/ or temperate and subpolar waters entering the Caribbean Sea by the North Equatorial Current may be invoked in lowering SSS and density in the tropical Atlantic. The cyclical behavior of the reconstructed SSSs and the coinciding lower SSSs, drier periods in Mexico, and shifts in the £ow patterns of ISOW may indicate an involvement of freshwater £uxes between the Atlantic and Paci¢c oceans.

Acknowledgements This work was supported by Grants G-AA/GU 04076-322, 332, and 334 from the Swedish Natural Science Research Council (NFR) to B.A.M. The Geological Survey of Denmark and Greenland (GEUS) and Go«teborgs Marina Forskningscentrum (GMF) are thanked for their support as well. J.N. also thanks the American-Scandinavian Foundation, the Swedish Institute, Professor Sven Lindquist’s Foundation, the Lars-Hierta Foundation, and Adlerbertska Foundation for supplying the grants for his visits to Puerto Rico, and the Royal Swedish Academy of Sciences (HiertaRetzius’ Foundation) for supplying equipment. We are also grateful to Claire Waelbroeck and an anonymous reviewer for valuable comments, which improved the paper. The SST and SSS reconstructions in PRB 12 and PRP 12 are available in digital form at the National Oceanic and Atmospheric Association-National Geophysical Data Center paleoclimate database http :// www.ngdc.noaa.gov/paleo/paleo.html.

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