Oceanographic variability in the southern Gulf of California over the past 400 years: Evidence from faunal and isotopic records from planktic foraminifera

Palaeogeography, Palaeoclimatology, Palaeoecology 284 (2009) 337–354

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Palaeogeography, Palaeoclimatology, Palaeoecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p a l a e o

Oceanographic variability in the southern Gulf of California over the past 400 years: Evidence from faunal and isotopic records from planktic foraminifera Francisca Staines-Urías ⁎, Robert G. Douglas, Donn S. Gorsline Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, USA

a r t i c l e

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Article history: Received 22 April 2009 Received in revised form 12 October 2009 Accepted 18 October 2009 Available online 24 October 2009 Keywords: Planktonic foraminifera Stable isotopes Eastern subtropical pacific Sea surface temperature PDO

a b s t r a c t Oceanographic conditions during the past four centuries–sea surface temperature (SST) centennial and decadal variability, and changes in the water column structure–were reconstructed in high resolution from foraminiferal assemblages and stable isotope analyses completed on laminated sediments from two multicores (MC15, 415 m depth and MC26, 600 m depth) from the southern Gulf of California (eastern subtropical Pacific). Total abundance counts and oxygen isotopic analyses were performed on two planktonic species, Globigerina bulloides (a proxy for winter SST) and Pulleniatina obliquiloculata (a proxy for late fall subsurface water temperature). Multidecadal oscillations in δ18O values are considered to result primarily from extratropical forcing–sea level pressure anomalies in the north Pacific–via northwesterly winds weakening/strengthening in association with changes in the average position and intensity of the Aleutian Low. The downcore evolution of both biological proxies revealed an increase in SST since the 1600s, reflecting a gradual decline in upwelling activity during this period. This temperature trend implies a longterm change that cannot be explained by the same mechanisms associated with the multidecadal oceanographic variability. We proposed that the progressive warming of the surface waters is linked to a general northward shift of the ITZC resulted from increased solar irradiation, which accounts for larger net surface shortwave fluxes, affecting regional heating patterns and consequently the sensitive regional climate. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The Gulf of California is a unique example of a marginal sea where the global atmospheric signal is amplified and modified by regional effects. The Gulf is the only evaporitic basin in the Pacific and modifies Pacific intermediate circulation on a regional scale by exporting and importing heat (Roden, 1964, Castro et al., 1994; Castro et al., 2000b; Beron-Vera and Ripa, 2000; Mascarenhas et al., 2004). Due to a welldeveloped oxygen minimum zone (OMZ), the Gulf is also one of the classic locations for annually accumulating laminated (varved) sediments. A distinctive feature of the Gulf is its climatically driven high primary productivity (Thunell et al., 1996; Thunell, 1998). Windgenerated upwelling produces Ekman transport that advects nutrientrich deep waters to the surface, supporting high primary production. The high primary productivity, in association with moderate rates of deep thermohaline ventilation, creates an OMZ between −300 and −800 m (Fernández-Barajas et al., 1994). Laminated sediments are observed in areas where bottom oxygen concentration is lower than <9 µM (0.2 mL L− 1), roughly where the OMZ impinges on the slope (Calvert, 1966). ⁎ Corresponding author. Present address: GEUS-Geological Survey of Denmark and Greenland, Øster Volgade 10, DK 1350, Copenhagen K, Denmark. Tel.: +45 381 425 63. E-mail address: [email protected] (F. Staines-Urías). 0031-0182/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2009.10.016

Dry and cool winter/spring conditions persist in the Gulf as the East Pacific High (EPH) and the Inter-Tropical Convergence Zone (ITCZ) retreat towards the Equator. A stable pressure gradient exists and northwesterly surface winds are funneled along the Gulf axis (ParesSierra et al., 2003; Bordoni et al., 2004; Jiménez et al., 2005). Northwesterly winds generate coastal upwelling along the eastern side of the Gulf, supporting high primary productivity along this margin. In contrast, during summer/fall, when the EPH and the ITCZ move northward and the wind field changes, northwesterly winds cease, and winds crossing the Gulf alternate with intermittent surges that transport moisture northward along the Gulf (Bordoni et al., 2004). Upwelling ceases and primary productivity declines. Consequently, the Gulf presents a strong seasonal signal in temperature and primary productivity (Soto-Mardones et al., 1999; Beron-Vera and Ripa, 2000). Maximum temperatures occur during summer and minimum during winter. Minimum SSTs vary among regions with maximum geographic variation occurring during winter (Soto-Mardones et al., 1999). The amplitude of the seasonal variation is larger along the east side than along the west side (Castro et al., 2000b; Lavín et al., 2003). The observed atmospheric circulation is regarded as monsoonal, based on seasonal reversal of pressure and wind patterns, energy and mass transfers, and typical regimes of rainfall and temperature (Adams and Comrie, 1997), and is commonly referred as the North American Monsoon (NAM). Although less impressive than its Asian counterpart, this regional scale circulation pattern is an important

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feature of the atmospheric circulation that develops over the eastern tropical and subtropical Pacific, including the Gulf of California and large areas of northwestern Mexico and the southwestern United States (Bordoni et al., 2004). Observational studies suggest that the southerly summer wind blowing over the Gulf of California and the intermittent gulf surges (relatively cool transient events) that bring moist maritime air from the Tropical Pacific into the southwestern desert region are the primary source of moisture to the western flanks of the Sierra Madre Occidental (Mexico) and to the arid southwest deserts (Douglas, 1995). Along the Gulf, the biological response to climate forcing is best expressed in the southern region where mid-latitude and tropical influences are most important. Because the southern Gulf is deep, local processes, such as tidal mixing and turbulence due to coastaltrapped waves, are less influential, and seasonal variability in sea surface temperature (SST) and primary production are clearly associated with climate forcing (Santamaría-del-Ángel et al., 1994). In this region, a SST lateral pattern is present. Alongside the western margin SSTs are higher and less variable (24.75 ± 2.5 °C) than in the eastern side (21.20 ± 5.5 °C) (Soto-Mardones et al., 1999). This spatial pattern is related with the occurrence of winter wind-driven upwelling along the eastern side (Castro et al., 2000b; Beron-Vera and Ripa, 2002; Lavín et al., 2003).

Previous research has revealed an east–west asymmetry in the sedimentary record of the southern Gulf associated with the asymmetry in oceanographic conditions (Van Andel, 1964; Baba et al., 1991; Douglas et al., 2002; Barron et al., 2005; Douglas et al., 2007). Along each margin, microfabrics of the laminated sediments are not uniform, but differ in appearance, thickness and composition (Douglas et al., 2002; González-Yajimovich et al., 2005). The stations for this investigation were selected to evaluate these differences (Fig. 1). The primary goal of this research was to reconstruct and investigate the oceanographic variability during the past four centuries in the southern Gulf of California (SST variation, periodicity of the SST variability, and changes in the water column structure). Furthermore we were interested in understanding how the oceanographic changes in the Gulf are influenced by the oceanographic variability of the Northern Pacific. This study took advantage of the unique sedimentary characteristics of anoxic basins located on opposite sides of the southern region of the Gulf. The oceanographic setting and particular characteristics of these basins allowed an ultra high resolution reconstruction of past variability in the water column structure derived from the stable isotope composition of two planktic foraminifera, Globigerina bulloides (d'Orbigny, 1826) and Pulleniatina obliquiloculata (Parker and Jones, 1865).

Fig. 1. Map of the Gulf of California (a) showing simplified bathymetry (meters), locations of the investigated cores (diamonds) and detailed bathymetry of Alfonso (b) and Pescadero (c) Basins.

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One of the investigation sites is located in Alfonso Basin, on the western side of the Gulf (station 15, Fig. 1b). This basin is a relatively small closed depression, with a maximum depth of −400 m and an effective sill depth of about − 320 m (Nava-Sánchez et al., 2001). Here primary production is lower and mostly sustained by upwelled water transported from the eastern side by across-Gulf eddies (Pegau et al., 2002; Douglas et al., 2007). Low-oxygen intermediate water enters the basin and below 200 m, waters become suboxic to anoxic (<0.5 µM) (Berelson et al., 2005). A second site is located on the eastern side, in the Pescadero Basin slope (station 26, Fig. 1c). This basin is located at the entrance of the Gulf, beneath major upwelling plumes. Its position and depth (a maximum of − 3000 m) allow free exchange with the waters of the Pacific (Castro et al., 2000b). In this area, inputs of fresh organic material are enhanced from November through April, during the upwelling season. The accumulation and decomposition of organic matter reduces oxygen levels (<0.2 µM) and limits oxygen penetration in the sediment (Berelson et al., 2005).

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attesting the Northern Pacific influence over the Gulf's climate (Bernal et al., 2001). In this study, additional relations were also observed between the PDO index, SST and precipitation, however due to the sparsity of the data, the statistically significance of such correlations was questioned (Bernal et al., 2001; González-Yajimovich, 2004). Thus, even though the PDO and the ENSO were in positive phases during this period and climatic trends are still unclear after the last regimen shift of the PDO (Lluch-Cota et al., 2003), this mode of variability clearly has a strong influence on the climate and oceanography of the region. Therefore, verifying the existence of periodicities in the 50–70 and bidecadal year bands (Minobe, 1999; Minobe, 2000; Zhu and Yang, 2003; McDonald and Case, 2005) in the δ18O records from foraminifera carbonate is of much significance in understanding the influence of the extratropical North Pacific over the Gulf region as well as an important step towards predicting future climatic and oceanographic variations in the area. 3. Ecological differences between Globigerina bulloides and Pulleniatina obliquiloculata

2. Pacific Ocean influence over the Gulf of California The El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) are modes of variability of basin-scale spatial structure that include the entire Pacific Ocean (Mantua et al., 1997). These global climate phenomena have a significant influence on both ocean and atmospheric circulation, and drastically affect wind-driven upwelling. Both modes exhibit a similar spatial structure. However, the climatic fingerprints of the ENSO are most prominent in the tropics with only secondary signatures in the North Pacific/North American sector, and the opposite is true for the PDO (Hare et al., 1999; LluchCota et al., 2001; Lluch-Cota et al., 2003). Furthermore, while the two climate oscillations have similar spatial fingerprints, they have very different behavior in time: PDO events persist for 20–30 y, while typical ENSO events persisted for 6–18 months. The analysis of historic records in the Gulf of California indicates that ENSO is a major source of interannual variability, having a strong influence on the climate and oceanography, and ultimately on the distribution and abundance of marine species in the region (LluchCota et al., 2001; Lavín et al, 2003). As available oceanographic time series data for the Gulf are relatively short and sparse, it is difficult to objectively summarize the research about multidecadal variability in the region in a way that reflects scientific consensus. There is evidence, however, suggesting synchrony with phases of the PDO and oceanographic and climatic changes in the Gulf of California and contiguous continental areas (Higgins and Shi, 2000; Lluch-Cota et al., 2001; Castro et al., 2001). The PDO is often considered as the leading mode of multidecadal SST variability in the extratropical North Pacific (Zhang et al. 1997; Mantua and Hare, 2002). Extremes in the PDO pattern are marked by widespread variations in the Pacific Basin and North American climate. In parallel with the ENSO, the extreme phases of the PDO have been classified as being either warm (positive) or cool (negative), as defined by SST anomalies in the eastern North Pacific Ocean, off the California coast. Although the origin of these multidecadal SST variations remain largely unexplained (Ware, 1995, Newman et al., 2003), even though they were recognized many years ago (Hubbs, 1948), there is evidence showing a strong connection between North Pacific SST variability and changes in the strength and the average position of the Aleutian Low (Castro et al., 2000a; Schneider and Cornuelle, 2005). Recent climate data indicated that in the Gulf of California the amplitude of seasonal SST variation and summer precipitation increased during the last positive phase of the PDO, from the mid1970s to the late 1990s (Castro et al., 2000a; Lluch-Belda et al., 2003). Furthermore, the analysis of 40 y of climate and oceanographic data from La Paz Bay (southern Gulf of California), revealed an association between the PDO index and wind variability, especially during winter,

Many previous studies have used oxygen isotopic composition of planktic foraminifera to reconstruct past water temperature variability (e.g. Prell and Curry, 1981; Ravelo and Fairbanks, 1992; Bemis et al., 1998) and environmental conditions in regions such as the Arabian Sea (Curry et al., 1992; Naidu and Niitsuma, 2003), the Panama Basin (Thunell and Reynolds, 1984), San Pedro Basin (Thunell and Sautter, 1992), the Peru Margin (Mohtadi et al., 2005), the Southern Ocean (Grant and Dickens, 2002), the Northeast Pacific (Ortiz et al., 1996) and the North Atlantic (Ganssen and Kroon, 2000). Like the Gulf of California, seasonal upwelling and high productivity characterize most of these regions. On a global scale, the foraminifera Globigerina bulloides is associated with upwelling and a homogenized or well-mixed water column (Thunell and Reynolds 1984; Thunell and Sautter, 1992). Sediment trap data from the North, Eastern and South Pacific suggest that G. bulloides thrives in the surface mixed layer above the thermocline, preferably in upwelling environments where phytoplankton density and prey abundance are high (Fairbanks et al., 1982; Sautter and Thunell, 1991; Bemis et al., 1998; Marchant et al., 2004; Mohtadi et al., 2005). As high abundances of G. bulloides are associated with conditions of abundant food supply resulting from a well-mixed water column and/or upwelling, changes in abundance of G. bulloides have been used to infer changes in oceanographic conditions (Curry et al., 1992; Thiede and Junger, 1992; Black et al., 2001; Gupta et al., 2003; Black et al., 2004). Particularly, the use of G. bulloides as an upwelling indicator has been calibrated using modern sea-floor samples and sediment traps in different locations around the world and tested over a range of timescales (Fairbanks et al., 1982; Thunell and Reynolds, 1984; Black et al., 2001, 2004; Marchant et al., 2004; Mohtadi et al., 2005). The advantages of G. bulloides as a proxy in the Gulf of California are (1) the unique association of the species with the winter winds, as it is absent in tropical waters except during wind-driven upwelling; (2) the correlation between abundance of the species and surface cooling due to upwelling, which is apparently unbiased by other influences; and (3) the sensitivity of the species to wind speed and, consequently, to the atmospheric pressure gradient. In the Gulf of California, this species is abundant throughout the winter/spring period of high primary productivity, and is greatly reduced during summer and fall (Brinton et al., 1986). The seasonal distribution of this foraminifer is similar to that of long-term average pigment concentrations, which reflect nutrient availability and primary productivity (Pride, 1997). The large seasonal range and low average oxygen isotopic values of Globigerina bulloides suggest that this species provides a record of seasonal heating and cooling of the surface mixed layer (Pride, 1997; Herguera et al., 2003). Similar depth distributions have been inferred for the Pacific Ocean off

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California (Thunell and Sautter, 1992), the Panama Basin (Thunell and Reynolds, 1984), the northwestern Arabian Sea (Naidu and Niitsuma, 2003), and off the coast of Chile (Marchant et al., 2004). Thus, we considered that the oxygen isotopic values for G. bulloides are an indicator of the winter/spring surface mixed layer temperatures. In contrast, Pulleniatina obliquiloculata is a tropical to subtropical species (Coulbourn et al., 1980; Li et al., 1997) that has been identified as a thermocline dweller (Fairbanks et al., 1982; Ravelo and Fairbanks, 1992). The highest abundance of this species in surface sediments occurs in a narrow belt between about 10°N and 10°S (Belyaeva, 1976). This species has a limited seasonal occurrence due to its sensitivity to low temperatures (Jian et al., 1996; Li et al., 1997). At times, it is strongly dominant in oligotrophic areas, for example, comprising up to 90% of all foraminifera in samples from the Sargasso Sea (Cifelli and Smith, 1974). In the Gulf of California because the abundance of P. obliquiloculata is sensitive to relatively cool winter temperatures, the annual range of δ18O in P. obliquiloculata is less than that of surface-dwelling species suggesting a deeper habitat that undergoes little seasonal temperature change (Pride, 1997; Bernal, 2001). Moderate abundances occur following the end of the summer, with maximum numbers at the onset of strong northwesterly winds and the breakdown of the thermocline. Such conditions expand the habitat of P. obliquiloculata, allowing it to flourish when primary productivity is high, and food is readily available (Mohtadi et al., 2005). Numbers drop rapidly at the establishment of the winter conditions (Pride, 1997). In the central Gulf of California (Guaymas Basin), Pulleniatina obliquiloculata exhibits maximum fluxes during October–November as surface waters begin to cool down (Pride, 1997). In the southern Gulf, this species appears most frequently from September to November (Brinton et al., 1986). Therefore, the isotopic values of P. obliquiloculata are used as an indicator of fall conditions at the bottom of the mixed layer. Together, the sediment abundances and δ18O signals of Globigerina bulloides and P. obliquiloculata provide information about upwelling intensity (Black et al., 2001; Naidu and Niitsuma, 2003). 4. Methodology 4.1. Recovering, sampling and processing Laminated sediments were obtained with an 8-tube multicorer during November 2001 (NH01-CALMEX cruise). Two cores were recovered, one from station 15 (MC15) and one from station 26 (MC26). Core location, water depth, core length, maximum sediment sampling depth, and oceanographic variables at the moment of sampling are presented in Table 1. Immediately after core retrieval, with the assistance of a mechanical high-precision core extruder, sediment was carefully extracted from the multicore tube and sampled every 2 mm (7.30 cm3 per sample). This core extruder allowed the vertical extrusion of the sediment in very precise intervals, corresponding to the selected sampling depth. As a result, extrusion and sampling developed into an integrated process, as

Table 1 Location, water depth, total length, maximum sampling depth, and oceanographic variables at core recovery in Alfonso (MC15) and Pescadero (MC26) Basins. Parameter Station ID Latitude (N) Longitude (W) Multicore total length Water depth Sediment sampling depth Temperature Salinity

Unit

Alfonso Basin

Pescadero Basin Slope

mm m mm °C psu

MC15 24° 16′ 70″ 110° 36′ 06″ 390 415 0–162 9.1 34.62

MC26 24° 16′ 70″ 108° 11′ 67″ 490 600 0–306 6.7 34.54

sediment was extruded from the tube in 2 mm intervals at a time, it was drawn toward a funnel and rinsed into a sampling container. All samples were wet preserved (Sperling et al., 2002). In the laboratory, Globigerina bulloides and Pulleniatina obliquiloculata specimens were counted in the sediment size fraction >63 µm (sand fraction). For each station, a second multicore was used to calculate dry bulk densities in order to transform volumetric-based abundances into abundance per gram of dried sediment. Oxygen stable isotope analyses (δ18O) were conducted on samples weighing between 40–70 µg, about 20 specimens of Globigerina bulloides and 5 to 8 specimens of Pulleniatina obliquiloculata per sample. The specimens used for isotopic measurements were selected within a narrower size range (125–180 µm) than the range used for faunal analysis (>63 µm). To improve the repeatability and accuracy of the measurements, after selection, shells were cleaned and prepared by following a procedure first developed by Boyle (1995) and modified to work with smaller size samples (Staines-Urías and Douglas, 2009). Isotope analyses were carried out using a VG Prism stable isotope ratio mass spectrometer (IRMS), equipped with an automated common acid carbonate system. Samples were reacted in 100% phosphoric acid at 90 °C. Resulting CO2 gas was then analyzed on the IRMS. Oxygen isotopic data are reported in delta notation relative to PDB standard, Vienna. The long-term standard reproducibility for δ18O is ±0.1‰, based on replicate measurements of a reference standard. During the transition from living to fossil faunas, oxidation of organic matter results in increasing dissolution of foraminifera tests and other carbonate particles. Thin-shelled specimens, more delicate species, and more delicate parts of shells are preferentially dissolved. Due to preferential dissolution, the remnant of the original foraminifera assemblage, and isotopic composition of the remnant shells may not accurately represent the environmental conditions at the time the assemblage/shell was formed. In the present study, therefore, the degree of foraminifera preservation was carefully evaluated. Each sample was ranked according to a preservation index developed for use at the Sedimentary Laboratory of the University of Southern California (R. Douglas unpublished data, Fig. 2). The Gulf's Foraminifera Preservation Ranking (GFPR) ranks planktonic taxa by their susceptibility to dissolution, and uses the ratio of more susceptible to more resistant forms to document the effects of dissolution (Berger, 1970; Thompson and Saito, 1974; Thunell and Honjo, 1981). Other, similar indices have been developed (Berger, 1968; 1970; Le and Shackleton, 1994), but the GFPR has the advantage of being developed specifically for species and assemblages commonly found in the Gulf of California. For all samples, GFPR related counts were performed on a fraction of the total wet sample. The used fraction varied from 1/64 to 1/16 of the original volume. Total planktonic foraminifera abundances (specimens g− 1 of dry sediment) were complete on a lower resolution than that use for all other proxies (one 2 mm-depth sample every 10 mm, instead of every 2 mm). All planktic foraminifera in the fraction > 63 µm were considered. These data was employed to calculate past variations in the relative abundances (%) of Globigerina bulloides and Pulleniatina obliquiloculata. 4.2. Age model Age models for MC15 and MC26 were constructed by a combination of excess 210Pb profiles, AMS radiocarbon dates, and varve counts (derived from grey-scale analyses). The Geochemistry laboratory of the University of Southern California (USC) performed all 210Pb analyses. The AMS analyses were performed at the Livermore National Laboratory (California), and the USGS National Center (Virginia). AMS 14C dates were converted to calendar years using the CALIB 4.4.1 age-calibration software of Stuvier and Reimer (1993), with a delta R of 249 ± 18 y (all values are based on a 95% confidence interval).

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Fig. 2. Gulf of California Foraminifera Preservation Ranking (GFPR): sample preservation index used to determine the condition of the analyzed samples.

For the ocean reservoir correction, a date of 547 ± 18 y was used, obtained from the regional mean value for the rhodolith Lithothannium crassiusculum (Frantz et al., 2000). For each location, a multicore replica was frozen, and then split and cut horizontally into 1.5 cm thick slabs. All the slabs were X-rayed using a Penetrex industrial X-ray apparatus at the Sedimentary Laboratory at USC. The X-radiographs were then scanned at 1200 dpi resolution in a Mikroteck ScanMaker 1000XL scanner (Fig. 3). To obtain a grey-scale digital transformation, scanned images were processed using Xplor-NIH 1.06 (a National Institutes of Health software), to measure changes in optical density. The digital files were then imported to Excel and the grey-scale values were plotted as a function of the distance downcore. Varve counts were obtained for every 5 cm segment of record and were transformed to sedimentation rates, expressed as millimeters per year. Using this method on core BAP96CP (Alfonso Basin), Douglas et al. (2002) establish and identical accumulation rate to that based on excess 210Pb profiles, concluding that couplets represent annual accumulations or varves. Rates obtained this way were incorporated into the age models. Core chronostratigraphy for MC15 (station15, Alfonso Basin) was based on an integration of the 210Pb excess profiles, varve counting, and corrected AMS dates (one planktic foraminifera sample from the 38 cm depth horizon), all in good agreement, indicating an average sedimentation rate of 0.37 mm y− 1 for the top 40 cm. This rate conforms to sedimentation rates calculated by radiocarbon dating in this same basin at similar depth intervals (Barron and Bukry, 2007). Considering the selected sampling interval and sedimentation rate, sampling resolution in Alfonso Basin corresponds to 5.4 y per sample. At station 26 (Pescadero Basin), excess 210Pb profiles for core MC26 yielded a sedimentation rate of 3.35 mm y− 1. However, there was considerable scatter in the data, and this rate far exceeds rates estimated for similar sedimentary settings in the Gulf (Barron and Bukry, 2007; Douglas et al., 2007). Therefore, to estimate sedimentation rates in MC26, the age model was constructed based on varve counts and two planktic foraminifera AMS radiocarbon dates. These

AMS dates were obtained from two depth horizons–350 and 408 cm– of a gravity core (GC26) collected at the same time in the same location as MC26. Both cores were correlated matching downcore similarities in sediment texture and total inorganic carbonate content. Sediment samples were obtained from MC26 and GC26 at 1 cm intervals. The sediment was then dried at 80 °C for a period of 24 h and manually pulverized in a mortar. A c. 20 mg subsample was weighed and analyzed for total inorganic carbon (TIC, dry weight % of CaCO3) in a Carbon Dioxide Coulometer (UIC Inc. model CM5012). Corresponding depths between cores were established based on signature sequences (downcore variability) observed in each of the TIC series. In both TIC series, a sharp decreased in CaCO3 content (c. 4%) was observed–160 to 180 mm depth in MC26, and 40 to 60 mm depth in GC26–followed by a slight increase in CaCO3 and a 40 mm section of low but steady carbonate values. Based on this sequence, clearly recognizable in both cores, we determined that 120 mm of sediment were lost at the top of GC26, and the two cores were accurately correlated (Fig. 3). Lamina thickness, coloring and succession (greyscale variability) were also studied to ensure the validity of the proposed correlation. Furthermore, Dry Bulk Density (DBD) measurements estimated by the GRAPE system (Herbert and Mayer, 1991), revealed a comparable linear increase in DBD (c. 0.16 g cm− 3) for the first 380 mm in GC26, and along the entire length of MC26 (490 mm). Below 395 mm, GC26 DBD values showed a drastic change in porosity, from 5.9 to 3.8 g cm− 3. This change is not registered in MC26 indicating that the multicore did not penetrate at this depth. In MC26 and GC26 top section (0–380 mm), varve count sedimentation rates ranged from 0.76 to 0.94 mm y− 1, averaging 0.83 ± 0.007 mm y− 1. No significant difference was observed between MC26 and GC26 in the overlap section (Fig. 3). In the GC26 lower section (−380 to −4010 mm), varve counts showed more variability, and sedimentation rates ranged from 0.26 to 2.94 mm y− 1. However, the average rate is remarkably similar to that of the top section 0.80 ± 0.011 mm y− 1. AMS dates (350 and 408 cm) were interpolated to the present and the average sedimentation rate calculated for eastern Pescadero Basin

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Fig. 3. Intercore depth-correlation based on the scanned X-radiographs of cores MC26 (right) and GC26 (left), Total Inorganic Carbon content (% CaCO3) and lamina sequence. Overlap section (dashed line) and corresponding depth intervals (shade boxes) are marked.

is 0.79 mm y− 1. Using both, varve count and AMS values, a best guess compromise was obtained and a curve was plotted and fitted with a linear regression. From this, the average sedimentation rate is estimated at 0.80 mm y− 1. As a result, sampling resolution in this basin corresponds to 2.5 y per sample. 4.3. Time series analyses To identify time dependent patterns in the faunal and isotopic records, moving averages (average of n surrounding elements) were calculated for each series. The average window (n) varied by location but it was always selected to encompass a period c. 20 y. In some cases, in order to verify the significance of the observed trends, regression analyses were performed. Time series periodicity analysis (exploration of cyclical patterns) was accomplished by performing single and cross-spectral analyses. Spectral analyses will decompose any time series into constituent sinusoidal components, however most oceanographic processes are a composite of aperiodic and periodic components and, most commonly, the periodic activity is not sinusoidal and is seldom stationary (Bengtsson, 2003). Therefore, the spectral decomposition will distribute variance at low frequencies for the trends and slow aperiodic processes and to high frequencies for the harmonics of the true periodic or pseudoperiodic

processes in the series (e.g. PDO, ENSO). Thus, the peak frequencies identified in the spectral analysis of these data may not correspond to periodic processes. To avoid the inclusion of the non-stationary components of the series, all frequencies lower than 0.005 y− 1 (periods >200 y) were filtered. This frequency threshold corresponds to periods larger than 0.5 of the total length of the series. Single spectral analyses were performed by the Blackman–Tuckey (B–T) method. For crossspectral analyses the Cross–Blackman–Tuckey (CBT) method was chosen. All methods are provided in the AnalySeries 2.0 2005 software package (Paillard et al., 1996). Gu and Philander (1995) demonstrated the utility of wavelet (frequency–time) analysis in objectively investigating interdecadal modulations of interannual variability in the tropical Pacific. This analysis is particularly useful for analyzing cycle periodicities in nonstationary data. Various climate research applications of wavelet analysis can be found in Weng and Lau (1994), Lau and Weng (1995), and Torrence and Compo (1998). The essential methodology is documented in the aforementioned references. Morlet Wavelet Analyses, commonly used to determine whether a climate time series is distinguishable from any well-defined process, including the output of a deterministic, chaotic system (Lau and Weng, 1995) were completed on the unfiltered series to assess how the dominant frequencies of variability varied over time (Torrence and Compo,

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1998). The wavelet spectra were tested for significance at a 95% confidence level against a white-noise process. 5. Results 5.1. Planktic foraminifera abundance Time series of total abundance (no. of specimens gr− 1) of Globigerina bulloides and Pulleniatina obliquiloculata were developed for Alfonso and Pescadero basins extending back to calendar year 1600, a

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total time span of about 400 y (Fig. 4). Lower resolution series of total planktonic foraminifera abundance, and G. bulloides and P. obliquiloculata species relative abundance (%) are included in Fig. 5. In both locations, Globigerina bulloides is more abundant than Pulleniatina obliquiloculata. However, the comparison of the total abundance of these two planktic foraminifera reveals a conspicuous interbasin difference. In the Pescadero Basin, both species are twice as abundant as in the Alfonso Basin, in accordance with the observed difference in primary productivity between the two basins (Santamaría-del-Ángel et al., 1994; Prokopenko et al. 2006).

Fig. 4. Time series of total abundance (specimens per gram of dry sediment) of Globigerina bulloides and Pulleniatina obliquiloculata from cores MC26-Pescadero Basin (b, c) and MC15-Alfonso Basin (d, e). The heavy dark lines denote calculated moving averages. Note that scales are different for each core. Downcore variation in the preservation of foraminifera shells is shown on the top and bottom panels (a, f). GFPR preservation values varied from 1.5 to 3 (excellent to good).

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Fig. 5. Time series of total planktonic foraminifera abundance (no. specimens per gram of dry sediment), and relative abundance (%) of Globigerina bulloides and Pulleniatina obliquiloculata from Pescadero and Alfonso Basins.

In Alfonso as in Pescadero, with the exception of one sample, downcore preservation ranking values varied from Excellent to Good (1.5 to 3 in the GFPR). Because all samples were ranked as well preserved, the observed interbasin difference does not appear to be the result of differential dissolution. Furthermore, those samples where Globigerina bulloides abundance–a species most susceptible to dissolution than Pulleniatina obliquiloculata–are the lowest do not correspond to those samples ranked as Good but rather as Very Good or Excellent preserved (Fig. 4a, f). The relative abundance of these two foraminifera also differs between the basins. In the Pescadero Basin, with the exception of the last 40y or so, Globigerina bulloides dominates the fossil assemblages, representing as much as 55% of the total planktic assemblage. In contrast, in the Alfonso Basin, G. bulloides is abundant but not dominant, showing similar relative abundances to Pulleniatina obliquiloculata (Fig. 5). Furthermore, correlation analyses indicate a weak inverse correlation between G. bulloides and P. obliquiloculata abundances in the Pescadero Basin (r2 = −0.33, p < 0.01), and a positive correlation in the Alfonso Basin (r2 = 0.36, p < 0.01). In the Pescadero Basin, the historical variations in the total abundances of Globigerina bulloides and Pulleniatina obliquiloculata are represented by two distinctive and opposite trends. The chronological variation of G. bulloides abundance is characterized by a continuous decrease in the number of specimens per gram of sediment for the whole period encompassed in the series (Fig. 4b). This decrease appears more prominent following the end of the 18th Century. Higher abundances (above 200 and as high as 250 specimens g− 1) occurred prior to 1800, dropping progressively towards total abundances as low as 50 specimens g− 1 between 1985 and 2000. By contrast,

P. obliquiloculata abundances show a positive trend. This species numbers seem to increase gradually from the 1600s to the late 1900s (Fig. 4c). No such historic trends were detected in the Globigerina bulloides abundance record from Alfonso Basin. In this basin, G. bulloides abundance appears steady through time, with the exception of three brief periods, 1675 to 1700, 1875 to 1900 and 1950–2000 (Fig. 4d). During these periods, G. bulloides abundance shows a drastic decrease and specimens of this species represent about 10% of the total planktic assemblage, a percentage significantly lower than the species average for this location (Fig. 5). As mentioned before, given the preservation state of the samples, the observed variability in the G. bulloides number cannot be explained by differences in preservation. A weak positive trend was detected for the Pulleniatina obliquiloculata abundance series, however the calculated slope of the corresponding linear trend is not significantly different from zero (Fig. 4e). 5.2. Oxygen isotopic record The oxygen isotopic composition of foraminiferal calcite is a function of both calcification temperature and the oxygen isotopic composition of seawater (δ18Ow), with the latter fluctuating on glacial–interglacial time scales because of changes in ice volume. In addition, local or regional physical factors (i.e., evaporation, precipitation, river runoff) also modify δ18Ow on shorter time scales (Fairbanks, 1989; Delaygue et al., 2001). As a result, noticeably different δ18Ow:salinity relationships exist in connection with different water masses and water source. Because of this, the δ18O composition of seawater has been used to identify different water

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masses, track circulation patterns, and assess the freshwater balance of a region (Craig and Gordon, 1965; Fairbanks, 1982; Bauch et al., 1995; Gat, 1996; Bigg and Rohling, 2000; Frew et al., 2000; Schlosser et al., 2002; Benway and Mix, 2004). Thus, both temperature and salinity must be considered when interpreting foraminiferal δ18O records. Both salinity and δ18Ow are affected by the local evaporation and precipitation balance as well as regional ocean processes (i.e., advection, sea ice calving). In surface waters, δ18Ow and salinity have been shown to covary linearly (Östlund et al., 1987; Fairbanks et al., 1992; Rostek et al., 1993; Watanabe et al., 2001; LeGrande and Schmidt, 2006). For instance, Wang et al. (1995) showed that in the low-latitude Atlantic (40°N–40°S) there is only a slight variation in the δ18Ow:salinity relationship. Furthermore, equatorial surface waters typically have high salinities and high δ18Ow values due to high evaporation (Östlund et al., 1987; Fairbanks et al., 1992; Watanabe et al., 2001). Ice volume and sea level changes during the late Holocene have been relatively small (Fairbanks, 1989) and should not have greatly influenced seawater δ18O in the Gulf of California during the past 400 y. Additionally, observations in the southern Gulf of California show almost constant sea surface salinity (SSS) values for the past century. Instrumental data from 1939–1990 indicate that SSS in the southern Gulf changes seasonally, but that values for a given season have remained steady through time (Beron-Vera and Ripa, 2002). However, there is considerable spatial variation. Because the Gulf is an evaporitic basin, saline water is formed in the north. This water mass, the Gulf of California Water, flows from the northern Gulf to the Pacific along the western side (Castro et al., 2000b). In contrast, along the western margin, California Current Water spreads across the mouth during spring, and the Costa Rica Coastal Current transports Tropical Surface Waters into the Gulf during fall (Roden, 1964; Wyrtki, 1967; Spearman, 1993). Due to this circulation pattern, salinity–and most likely the δ18Ow:salinity relation as well–exhibits a transverse gradient (Torres-Orozco, 1993; Beron-Vera and Ripa, 2002). Fresher waters occur next to mainland Mexico, with mean salinity from 0 to − 200 m depth <34.65, and saltier waters occur near the Baja California Peninsula, with mean values >34.85 (Castro et al., 2000b). Since changes in the water budget (precipitation/evaporation/ runoff) can significantly affect seawater composition in marginal basins and coastal regions (Karr and Showers, 2002), in the Gulf of California, interannual changes in the δ18Ow:salinity relation might be suspected, further complicating the interpretation of foraminifera δ18O data. However, regional climate factors such as annual precipitation and river runoff are not of considerable influence in the area. Because arid provinces confine the Gulf, river runoff is thought to have little influence on the Gulf's hydrography (BeronVera and Ripa, 2000; Castro et al., 2000b). In addition mean annual precipitation is considerably low (67.9 mm y− 1) (Álvarez et al., 2007). As a consequence, the seasonal cycle of precipitation has a minimal influence in the discharge of the local rivers and on the isotopic composition of surface water, further indicating that the majority of the δ18O foraminiferal calcite variability is temperature related. After considering the across-Gulf differences, the δ18O values of both planktic species can be use as references of past SST variability, with lower (more negative) δ18O values indicating warmer temperatures and higher (more positive) δ18O values as indication of colder temperatures. The δ18O time series of Globigerina bulloides and Pulleniatina obliquiloculata cover a time span of about 350 y (Fig. 6) and provide evidence of significant temporal variation during that time. At both locations, δ18O values for P. obliquiloculata are higher than values for G. bulloides, indicating that tests of G. bulloides are calcified at shallower depths, which is in conformity with their known depth– habitat differences.

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The range and variation of δ18O values is greater for Globigerina bulloides than for Pulleniatina. obliquiloculata. As expected, considering that temperature variation throughout the upwelling season (the residence interval of G. bulloides) is larger than the relatively narrow temperature change occurring in deeper waters from late summer to fall (the residence period of P. obliquiloculata). The difference in δ18O values between species is better expressed in Pescadero Basin. In this basin, as all along the east side of the southern Gulf of California, SST changes are particularly significant during the winter–spring transition (Soto-Mardones et al., 1999; Lavín et al., 2003). Commonly, SST varies by up to 6 °C during G. bulloides blooming season (Pride, 1997). The δ18O values of both species had lower variability (i.e. standard deviation) in samples from the Alfonso Basin than from the Pescadero Basin, which is consistent with the relatively low surface and subsurface temperature variability observed on the western side of the Gulf (Soto-Mardones et al., 1999; Lavín et al., 2003). In Alfonso as in Pescadero Basin, the Globigerina bulloides δ18O records show a trend towards more negative values (roughly − 0.5‰), indicating a progressive increase in SST. In the two basins the trend becomes more noticeable following the second half of the 19th Century. In contrast, in both basins, the Pulleniatina obliquiloculata δ18O series do not show any evident trend (Fig. 6). The spectral analyses of the δ18O Globigerina bulloides and Pulleniatina obliquiloculata records extracted multidecadal (50–55 y) periodicities that are comparable to those of the PDO (Fig. 7; Minobe, 1999; Mantua and Hare, 2002; Zhu and Yang, 2003). Indicating that this multidecadal pattern of SST variability has been a significant mode of variability over the past four centuries. Higher frequency signals (25–35 y) were revealed in both δ18O G. bulloides records. Such variability is compatible with changes in SST associated with the PDO phases (Lluch-Cota et al, 2003; Zhu and Yang, 2003). The results of wavelet analysis show similar patterns to those detected by the B–T spectral analysis. The frequency–time analysis of the δ18O Globigerina bulloides series from the Pescadero Basin indicates that overall there is significant power in the 30 to 60 y band (Fig. 8). These periodicities of interdecadal variability are consistent with those of the PDO shifts (Gedalof et al., 2002; McDonald and Case, 2005). Decadal variability (16 to 32 y band), consistent with higher frequency variability present in observed PDO behavior, was significant during two periods: 1800–1850 and 1950– 2000 (Fig. 8). Interannual variability (4 to 8 y band)–typically interpreted as ENSO influences (Newman et al., 2003; Sang-Wook and Kirtman, 2004)–was not significant at any time. In the Alfonso Basin, by contrast, wavelet analysis showed that the variability corresponding to the multidecadal frequencies was constrained to the period before 1900. Decadal variability was significant from 1800 to 1850 (Fig. 8). Both records exhibited significant variability at centennial time scales (>128 y band). This variability is most probably related to the non-stationary or aperiodic components in the series (Bengtsson, 2003), such as the observed trend towards lighter δ18O G. bulloides (Fig. 4). The absence of comparable frequencies in the B–T spectral analyses performed on the filtered series (Fig. 7) supports this interpretation. To better understand the influence of North Pacific oceanographic variability, and especially that associated with fluctuations in the position and strength of the Aleutian Low (An and Wang, 2005; Schneider and Cornuelle, 2005), over the winter oceanographic conditions in the Gulf of California, a PDO-Winter Index (PDOw), corresponding to the months of November, December, January, and February, was calculated from the original PDO Index data (http:// jisao.washington.edu/pdo/PDO.latest, Mantua and Hare, 2002). To emphasize their covariation, the PDOw Index was superimposed to the corresponding 100-year (1900–2000) time series from the Alfonso and Pescadero δ18O Globigerina bulloides records (Fig. 9a). Together, the remarkable similarity between the three series and the similarity in the periodicities of variability and high coherence

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Fig. 6. Oxygen isotopic composition (δ18O) profiles of G. bulloides and P. obliquiloculata (light grey line) over the past 350 y from Alfonso (MC15) and Pescadero (MC26) basins (note different scale ranges for each series), including calculated moving average (heavy black line).

observed in the cross-spectral analysis (Fig. 9b), suggest a strong association between SST variability in North Pacific and in the Gulf (Carleton et al., 1990; Castro et al., 2001). We observed, however, that the covariation between the PDOw and the δ18O G. bulloides record was not as clear for the Alfonso as for the Pescadero record. Two factors might be responsible for this difference (1) sedimentary rates are lower in Alfonso, leading to lower resolution in the record and the averaging of some, otherwise, prominent features, (2) the fact that North Pacific influence over the Gulf's winter climate is a direct consequence of the Northwesterly winds, whose intensity and duration varies depending on the strength and position of the Aleutian Low (Castro et al., 2000a; Higgins and Shi, 2000; Castro et al., 2001). Since SST variability along the eastern side of the Gulf (Pescadero Basin) is tightly coupled with these winds via upwelling, a

stronger PDO signal is expected in the Pescadero Basin compared to the Alfonso Basin. Therefore, a weakening of the PDO signal will be most noticeable in Alfonso. The lack of significant periodicities in the PDO range that was observed in the Alfonso Basin's wavelet analysis during the 20th Century can also be explained by the lower sensibility of this location to the Northwesterly winds. 6. Discussion 6.1. Faunal records The interbasin difference in the relative abundance of Globigerina bulloides and Pulleniatina obliquiloculata and the contrasting relationship in abundance variability between species, as indicated by the

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Fig. 7. Frequency analysis of δ18O variability in G. bulloides and P. obliquiloculata time series. At both locations, the G. bulloides frequency spectra (a, b) identify two prominent periods (light grey bands) of multidecadal (50–55 y) and bidecadal (35–25 y) variability. For P. obliquiloculata multidecadal periodicities are also significant. However, no significant variability was observed in the bidecadal band (c, d). The analysis of the linear filtered series was performed using a 4-year step, bandwidth of 0.00967, and 31 lags. Similar results were returned at both 80% and 95% confidence levels.

correlation analyses, can be explained by contrasting oceanographic conditions in the basins. In the Pescadero Basin, strong winter upwelling favors blooms of G. bulloides, and the drastic decline in temperature during late summer limits growth of P. obliquiloculata. On the contrary, in the Alfonso Basin, upwelling does not occur or is much reduced in intensity and duration, limiting G. bulloides populations and favoring longer residence periods for P. obliquiloculata, ultimately resulting in similar abundances of both species in the fossil record. The distinction between the basins corresponds to the upwelling asymmetry across the Gulf, and supports the hypothesis that primary productivity along the western margin is sustained, mostly, by upwelled water transported from the eastern side (Pegau et al., 2002). Despite similar shell preservation in the Pescadero and the Alfonso Basins (Fig. 4), both benthic (Douglas and Staines-Urías, 2007; StainesUrías and Douglas, 2009) and planktic foraminifera are more abundant in the sedimentary record in the former. Yet, calculated CaCO3 mass accumulation rates (MARs) for the last 500 y are higher in the Alfonso Basin (0.6–1.8 mg cm− 2 y− 1) than in the Pescadero Basin (0.4– 1.00 mg cm− 2 y− 1; González-Yajimovich et al., 2005). The high CaCO3 MAR in the Alfonso Basin is attributable to high coccolithophore abundance and the greater preservability of coccoliths (Young and Ziveri, 2000; Flores et al., 2003). Because in the Gulf coccolithophores are indicators of more oligotrophic conditions and stratified water column (Thunell et al., 1996, Thunell, 1998), all evidence indicates lower productivity rates in Alfonso compared to Pescadero Basin– today as for the past 400 y–and support the use of Globigerina bulloides as a reliable upwelling indicator for the Gulf of California. Further

verification of contrasting organic carbon accumulation rates between basins, consequence of the upwelling asymmetry existing across the Gulf, was provided by Prokopenko et al. (2006). By measuring the isotopic composition of nitrogen in pore water, these authors determined that pore water ammonium was enriched in 15N isotopes in both basins. Such enrichment indicates decomposition of organic matter. However, isotopic composition of pore water ammonium in the sediments at Pescadero was much lighter (closer to the average δ15N value of the organic matter raining to the sea floor) than in Alfonso, indicating higher organic carbon content and accumulation rates in Pescadero (Altabet et al., 1999; Pride et al., 1999). In Pescadero Basin, the observed trend and the amplified variability in Globigerina bulloides abundance, especially after the 1800s, appears to be the result of changes in wind-induced upwelling intensity. Assuming that the previously demonstrated correlation between wind stress and G. bulloides abundance holds true for older records, G. bulloides abundance in this basin suggests weaker winds and lower productivity at present compared to the 1600s. In this basin, the fossil record of Pulleniatina obliquiloculata provides further evidence of this change in the oceanographic conditions. From 1600 to the present, P. obliquiloculata progressively increased its abundance suggesting that fall-type conditions (warmer temperatures, well stratified water column) last longer during the year (Fig. 4). Furthermore, the inverse behavior of G. bulloides and P. obliquiloculata in recent history is also an indication of progressive stratification of the water column, a process that reduces the abundance of G. bulloides, which is upwelling-dependent, but favors P. obliquiloculata blooms.

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Fig. 8. Wavelet analysis of δ18O G. bulloides time series from Alfonso and Pescadero Basins. Significant periodicities (p > 0.05) are outlined in black on the wavelet power spectrum and extend to or beyond the dashed curve on the global wavelet profile. Crosshatching indicates the cone of influence where edge effects may be important.

6.2. Isotopic records 6.2.1. High-frequency oceanographic variability in the Gulf In the context of climate change and resources planning, the recognition of modes of variability such as the PDO is of key concern in predicting future climate variations and its consequences. In particular, the reconstruction of the PDO offers enormous potential in better understanding climate relations between the oceanographic variability in the North Pacific, Western North America, and the Gulf of California Region, including Northwestern Mexico. In a lower resolution SST reconstructed record from Alfonso Basin, Herguera et al. (2003) observed an association between isotopic derived SST and PDO variability. The comparison of Herguera et al. (2003) SST reconstructed record (also based on Globigerina bulloides δ18O values) with a tree-ring based PDO reconstruction (Biondi et al., 2001) indicated that SST in the southern Gulf varies contrary to the PDO phases. Thus, in opposite manner to SST off the coast of California (eastern North Pacific). This opposite relation, most notorious during the XIX century, indicated that positive phases of the PDO (warmer SST in the eastern North Pacific, colder in the central North Pacific) are associated with lower than normal SSTs in the southern Gulf of California. In contrast, negative PDO values (cooler eastern North Pacific) are associated with warmer or close to average winter SSTs in the Gulf. In good agreement with abovementioned study, our results also show that frequencies corresponding to the PDO are an important component of the oceanographic variability in the Gulf of California. Particularly, the covariation between the PDOw and the δ18O Globigerina bulloides records, revealing a comparable behavior (Fig. 9), links SST variability in the central North Pacific and in the southern Gulf of California. This association connects positive SST anomalies in the Gulf with positive SST anomalies in the central North Pacific. After extensive climate data analysis, including data from 1948 to 1956, Higgins and Shi (2000) proposed an analogous teleconnection. Other statistical models confirm that positive [negative] winter SST

anomalies in the Gulf region, are associated with positive [negative] SST anomalies in the central North Pacific, negative [positive] SST anomalies in the eastern North Pacific off the California coast (especially during summer), and drier [wetter] summers in the southwest of the United States (Higgins et al., 1998; Castro et al., 2001, Schneider and Cornuelle, 2005). All these features are linked to the weakening [intensifying] of the Aleutian Low that characterized the negative [positive] phase of the PDO. Particularly, in the central North Pacific, the deepening of the Aleutian Low produces advection of cool water and dry air from the north, due to intensified westerly winds, and SSTs that are colder than normal. In contrast, in the eastern North Pacific region, a robust Aleutian Low enhances poleward winds and leads to warm anomalies of surface temperature (Schneider and Cornuelle, 2005). The opposite is true for the negative phases of the PDO, when a weaker Aleutian Low results in weaker westerlies, positive SST anomalies in the central North Pacific, and negative SST in the eastern North Pacific, a pattern of changing sea level pressure that has attracted considerable attention given that its evolution correlates with anomalies of the Pacific ecosystem (Mantua et al., 1997). Considering that the bulk of the NAM moisture is advected at low levels from the eastern tropical Pacific Ocean and the Gulf of California (Douglas, 1995; Adams and Comrie, 1997) and that the analysis of climate and hydrologic variability from the Southwest United States indicated that PDO-like components are the largest contributors to cyclic hydrologic variability (Hanson et al, 2006), the interannual variability of the NAM precipitation patterns offers further evidence of the link between North Pacific oceanographic variability and climate changes over the Gulf of California. Correlations between NAM onset dates (onset of the summer in the Gulf of California and Southwest United States) and SST anomalies in the North Pacific from 1950 to 1995 show that the larger swings in the summer onset date correspond to the decades with the largest North Pacific SST anomalies (positive and negative), and that large negative SST anomalies in the central North Pacific, are associated with the early

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Fig. 9. (a) Comparison between the PDOw Index (light grey line) and calculated 4-year moving average (heavy grey line), and δ18O G. bulloides time series from Alfonso (black dashed line) and Pescadero (light black line) Basins. (b) Cross-spectral analysis between the PDOw index and Pescadero's δ18O G. bulloides records. The cross-spectral analysis was performed at 95% confidence level using a 2-year step, 23 lags, and a bandwidth of 0.02884. The grey bars marked the periods of significant variability. (c) Coherence (dark line) and cross-phase spectra (dashed grey line) are shown in the bottom panel. Non-zero coherence is higher than 0.6684 (horizontal dash line).

onset of the summer conditions and with colder than normal winter SST in the Gulf of California and off the coast of southern Mexico (Higgins and Shi, 2000). However, considering that correlation is not causality and statistical relations like this can only be disproved not proved it is necessary to present a coherent physical argument to explain the proposed teleconnections. If SST anomalies in the North Pacific influence the climate and oceanographic conditions in the Gulf of California region and over western North America, there must be a consistent set of atmospheric circulation features relating the two. Accordingly, it's been observed that during positive phases of the PDO anomalous cyclonic atmospheric circulation occurs over the Aleutians and anomalous anticyclonic circulation occurs over the subtropical Pacific strengthening the north–south SST gradient intensifying the Northwesterly winds over the Gulf, thus increasing Ekman transport and winter upwelling, producing colder than normal winter SST. Conversely, when anomalous anticyclonic circulation occurs over the Aleutians, anomalous cyclonic circulation is observed over the subtropical Pacific–typifying the negative state of the PDO–weakening the north–south SST thermal contrast, diminishing the intensity of the Northwesterly winds over the Gulf and producing winter positive SST anomalies. Therefore, as the westerlies weaken or strengthen, seasonally as interannualy, as a function of the evolving characteristics of the Aleutian Low and the corresponding seasonal and interannual developing pattern of North Pacific SST, SSTs in the Gulf of California fluctuate in response to the change in thermal contrast between the eastern North Pacific and the subtropical Pacific (Castro et al., 2001; Schneider and Cornuelle, 2005). Results from an atmospheric numerical model study–including stratospheric dynamics and ozone

chemistry simulations coupled to ocean, land and sea ice–further support the proposed physical mechanism (Meehl et al., 2009). This model revealed that relative small increases in solar energy, as those associated with the 11-y solar cycle, will produce greater UV absorption, resulting in a poleward shift of the subtropical jet in the troposphere, creating positive zonal wind anomalies near 40°N and 40°S and negative anomalies near 20°N and 20°S, indicating a transition to higher equatorial Pacific SSTs, and providing a mechanism to explain PDO-related variability as well as the link between SST in the Central North Pacific and the southern Gulf of California. 6.2.2. Centennial variability The Globigerina bulloides δ18O records from the Alfonso and the Pescadero Basins show a first period, from 1600s to mid-1800s, characterized by relatively stable SST. From the mid-1800s to 2000, these records indicated rapid warming, showing that SSTs in the second half of the 20th Century are the warmest of the series. Because of the connection that exists between salinity and the isotopic composition of the ocean water, the correct interpretation of the longterm (centennial) changes in the δ18O values of Globigerina bulloides and Pulleniatina obliquiloculata carbonate requires that we establish if centennial changes in the SSS have occurred in the southern Gulf of California. Reconstructed records of past precipitation indicate that since 4000 y BP, pluvial precipitation in the Gulf region has decreased progressively (González-Yajimovich et al., 2007; González-Yajimovich, 2004). In addition, since the 1800s fresh water input from the Colorado River, the main source of fresh water to the Gulf, has decreased rapidly due to extensive damming, basin transfer, and

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human utilization so that the river current discharge represents a very small fraction of the pre-damming volume (Carriquiry et al., 2001). Reference values indicate that c. 1800, water input from the Colorado was in the order of 550 m3 s− 1, declining to 40 m3 s− 1 at the beginning of the 20th Century, and that at present the river often goes complete dry before reaching the Gulf of California (Dahm et al., 2005). Thus, all evidence indicates that the Gulf of California might have become saltier not fresher, and that the observed negative trends in the δ18O Globigerina bulloides series most likely are the outcome of progressive warming of the water column, suggesting that winter SST are about a 1–1.5 °C warmer now than 400 y ago. Warming trends similar to the one found in the present study have been observed in other paleotemperature reconstructions for the Gulf of California. Herguera et al. (2003) lower resolution paleotemperature record also revealed progressive warming since the beginning of the 1800s. A high resolution, alkenone-based (UK'37) record of paleotemperature for the Guaymas Basin showed steady warming up to more than 2 °C in the surface waters of the central Gulf of California from early 1700s to 1950 (Goñi et al., 2001). Furthermore, in both our records, the period of faster warming–as indicated by the maximum change in the slope of the corresponding moving average–coincides with the end of the Little Ice Age (LIA; Thompson et al., 1986; Bradley and Jones, 1993; Mann, 2002). The LIA is a period of lower temperatures recognized in the Northern Hemisphere that was characterized by the expansion of Northwest European Glaciers (Grove, 2001). The generally accepted duration of the LIA is c. 1450–1850 (Thompson et al., 1986; Mann, 2002). Moreover, the difference between δ18O values for Globigerina bulloides and Pulleniatina obliquiloculata provides additional evidence of progressive thermal stratification of the water column since the mid-1800s. On the basis of the ecological differences between the two species, differences in δ18O should primarily reflect changes in tem-

perature at their calcification depth and, therefore, variability in stratification of the water column. In order to determine whether changes in the habitats of G. bulloides and P. obliquiloculata occurred gradually, the difference between the δ18O values of each species was calculated. This difference (Δδ18Oobli-bull) was calculated in order to show the difference between heavier (colder) and lighter (warmer) isotopic values (temperatures). The Δδ18Oobli-bull exhibited positive trends in both the Alfonso and Pescadero Basins (Fig. 10), indicating that the temperatures did diverge gradually over time. However, the correlation coefficient was statistically significant only for the Pescadero Basin (r2 = 0.578, p < 0.01). Trends in total abundance of the two species in the Pescadero Basin confirm this change. Globigerina bulloides started to decline in abundance around the mid-1800s, dropping from >260 to <140 g− 1. Pulleniatina obliquiloculata, on the other hand, showed moderated abundance increments between 1600 and 2000 (Fig. 4). The warming trend in the Gulf of California found in the present and previous studies is similar to trends seen in reconstructed SST and atmospheric temperature records from other parts of the world (e.g. Mann et al, 1998; Greene et al., 1999; Jones and Moberg, 2003; Malhi and Wright, 2004; Oerlemans, 2005), and coincides with the global temperature increase that followed the end of the LIA. The causes of progressive global warming over the last two centuries are attributed to changes in solar irradiance (Lean et al., 1995) and increase in greenhouse gases (Mann et al., 1998; Lean and Rind, 1998). However, the causes of detailed temperature fluctuations on a decadal to century scale, and of regional patterns of temperature change, are poorly understood. The causes of higher frequency fluctuations are also uncertain. However, historic and proxy reconstructions show strong links between solar variations and climate. Many investigators, for example, recognize reduced solar irradiance as the primary driving mechanism for the LIA event (e.g. Druffel 1982; Bradley and Jones,

Fig. 10. Total difference in δ18O values between G. bulloides and P. obliquiloculata (Δδ18Oobli-bull), as a proxy for stratification of the water column, in Pescadero (left) and Alfonso (right) basins.

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1993; Lean and Rind, 1998; Hong et al., 2000). A close correlation between reconstructed solar irradiance and surface temperature in the Northern Hemisphere suggests a dominant solar influence during the LIA (Lean et al., 1995), and reconstructed irradiance records for the past 300 y correlated closely with surface temperatures in the Northern Hemisphere (Hoyt and Schatten, 1993; Bradley and Jones, 1993). In the particular case of the Gulf of California, the observed warming can be explain by a decrease in winter upwelling linked to ocean-basin-wide change in the sea level pressure patterns. The increase in solar irradiance observed since the end of the LIA (Lean and Rind, 1998) may have caused a general northward shift of the ITZC and the EPH, resulting in weaker northwesterly winds, upwelling reduction, higher SSTs and, consequently, decreased δ18O Globigerina bulloides values. Detailed analysis of climate data from 1900 to 1994 in relation to the 11-year solar irradiation cycles reveal that during solar maxima the Aleutian Low moves westward and the EPH retains a more northern position (Christoforou and Hameed, 1997; van Loon et al., 2007; Meehl et al., 2009). This particular spatial configuration indicates that irradiation maxima reinforce the ITCZ and produce above normal pressure in the North Pacific over the eastern half of the Aleutian Low and North America, a sea level pressure pattern that results in negative SST anomalies off the coast of North America, positive SST anomalies in the central North Pacific and in the southern Gulf of California (van Loon et al., 2007; Meehl et al., 2008, 2009) and is associated with anomaly wet summers in the Southwest United States and Northwestern Mexico (Higgins et al., 2004). There is still much controversy about the link between climate change and solar irradiance variability, particularly during the past millennium. For the most part, links between irradiation variability and climate changes are often dismissed on the basis that the changes in solar energy between solar maxima and minima at the top of the atmosphere are too small to be significant (0.2 Wm−2, Lean et al., 2005). However it is necessary to consider that in tropical and subtropical areas where there are few clouds and the sun is more directly overhead, such as the Gulf of California region, there can be net surface shortwave fluxes an order of magnitude larger than the global average forcing (van Loon et al., 2007). Furthermore, considering the sensitivity of the regional climate to changes in heating patters (Meehl et al., 2008, 2009), and the strong dependency on sea level pressure variability of large atmospheric circulation patterns such as the NAM (Higgins and Shi, 2000, Castro et al., 2001), the inferred changes in winter SST in the Gulf of California appear physically consistent with the observed changes in solar irradiation (Lean and Rind, 1998, Lean et al., 2005). 7. Summary and conclusions The present study highlights the utility of the oxygen isotopic composition of Globigerina bulloides as a proxy for winter SSTs, and that of Pulleniatina obliquiloculata as a proxy for fall thermocline temperatures in the southern Gulf of California. Furthermore, the distinctive relation between species abundance and the differences in their relative abundance observed between locations, corresponding to the observed upwelling asymmetry across the Gulf, supports the use of both species as a proxy for upwelling-related variations in primary productivity. The covariation between the PDO-Winter Index and the δ18O Globigerina bulloides time series (as a proxy for winter SSTs in the Gulf) also illustrates the potential of the G. bulloides obtained from the sedimentary record in the southern Gulf of California for the reconstruction of past PDO variability in the region. Additionally, the observed correlation between δ18O G. bulloides variability and SST anomalies in the north Pacific (represented by the PDOw Index) indicates that at multidecadal scales, winter SSTs in the Gulf of California most likely fluctuate in response to changes in the thermal contrast

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between the eastern North Pacific and the tropical/subtropical Pacific. A thermal contrast that varies as a function of the multidecadal variation in the intensity of the westerlies associated with the developing pattern of North Pacific SST evolving in response to changes in the average position and strength of the Aleutian Low. Each phase of the PDO is characterized by a specific pattern of sea level pressure, a positive PDO is characterized by anomalous cyclonic atmospheric circulation over the Aleutians and anomalous anticyclonic circulation over the subtropical Pacific, a pattern that strengths the north–south SST gradient, while an opposite pattern of sea level pressure and a weaker thermal gradient is expected during negative phases of the PDO. The centennial changes in SST (inferred from the observed trends towards lighter δ18O Globigerina bulloides values) evidences a gradual warming of the water column, in addition to the inferred change in water column stratification calculated from the G. bulloides and Pulleniatina obliquiloculata δ18O records, are both in good agreement with previous SST reconstruction from the Gulf (Herguera et al., 2003; Goñi et al., 2001). Despite the controversy that exists about links between solar irradiation variability and climate changes due to the subtropical location of the Gulf, that account for larger net surface shortwave fluxes, the sensitivity of regional climate to changes in heating patters, and the strong dependency on sea level pressure variability of large atmospheric circulation patterns such as the NAM, the observed changes in solar irradiation appear physically consistent with the changes in SST inferred from the reconstructed record from the Gulf of California. Acknowledgments This research was supported by funding provided by the National Science Foundation International Programs (US–Mexico, INT 0304933 to Robert Douglas and Donn Gorsline), the Consejo Nacional de Ciencia y Tecnologia of Mexico (Paleoceanografia del Holoceno en el Golfo de California y el Borde Continental Californiano, no. 365), and the USC-Department of Earth Sciences Research Funding. Fellowship support for the first author was generously provided by CONACyT (no. 110221). The authors wish to acknowledge the captain and crew of the R/V New Horizon during the 2001 CALMEX Cruise to the Gulf of California for their assistance in collecting the sediment cores. We are also thankful to J. Yu, for his assistance in sorting out our many computer network related difficulties. References Adams, D.K., Comrie, A.C., 1997. The North American Monsoon. Bulletin of the American Meteorological Society 78, 2197–2213. Álvarez, M., Michel, R., Reyes-Coca, S., Troncoso-Gaytan, R., 2007. Pluvial precipitation in Baja California and the National Astronomical Observatory at San Pedro Martir Sierra. Revista Mexicana de Astronomia y Astrofisica (Serie de Conferencias) 31, 111–119. An, S.I., Wang, B., 2005. The forced and intrinsic low frequency modes of the North Pacific. Journal of Climate 18, 876–885. Altabet, M.A., Pilskaln, C., Thunell, R.C., Pride, C., Sigman, D., Chavez, F., Francois, R., 1999. The nitrogen isotope biogeochemistry of sinking particles from the margin of the Eastern Tropical Pacific. Deep-Sea Research I 46, 655–679. Baba, J., Peterson, C.D., Schrader, H.J., 1991. Modern fine-grained sediments in the Gulf of California. In: Dauphin, J.P., Simoneit, B.R.T. (Eds.), The Gulf and Peninsular Province of the Californias, AAPG Memoir, vol. 47, pp. 569–587. Barron, J.A., Bukry, D., 2007. Solar forcing of Gulf of California climate during the past 2000 yr suggested by diatoms and silicoflagellates. Marine Micropaleontology 62, 115–139. Barron, J.A., Bukry, D., Dean, W.E., 2005. Paleoceanographic history of the Guaymas Basin, Gulf of California, during the past 15,000 years based on diatoms, silicoflagellates, and biogenic sediments. Marine Micropaleontology 56, 81–102. Bauch, D., Schlosser, P., Fairbanks, R.G., 1995. Freshwater balance and the sources of deep and bottom waters in the Arctic Ocean inferred from the distribution of H18 2 O. Progress in Oceanography 35, 53–80. doi:10.1016/0079-6611(95)00005-2. Belyaeva, N.V., 1976. Quantitative distribution of planktonic foraminifera in sediments of the World Ocean. In: Takayanagi, Y., Saito, T. (Eds.), Progress in Micropaleontology, Special Publication: Selected papers in honor of Prof. Kiyoshi Asanao, pp. 10–16. Bemis, B.E., Spero, H.J., Bijma, J., Lea, D.W., 1998. Reevaluation of the oxygen isotopic composition of planktonic foraminifera: experimental results and revised paleotemperature equations. Paleoceanography 13, 150–160. Bengtsson, L., 2003. Periodic and non-periodic processes in the Earth's atmosphere and oceans, and their relevance for climate prediction. In: Sterken, C. (Ed.), Interplay of

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