Santa Barbara Basin diatom and silicoflagellate response to global climate anomalies during the past 2200 years

Quaternary International 215 (2010) 34–44

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Santa Barbara Basin diatom and silicoflagellate response to global climate anomalies during the past 2200 years John A. Barron a, *, David Bukry a, David Field b a b

U.S. Geological Survey, MS 910, Menlo Park, CA 94025, United States Monterey Bay Aquarium Research Institute, Moss Landing, CA 95039-9644, United States

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 25 September 2008

Santa Barbara Basin (SBB) diatom and silicoflagellate assemblages are quantified from a box core record spanning AD 1940–2001 and an Ocean Drilling Program Hole 893A record from w220 BC to AD 1880. The combined relative abundance of the diatoms Fragilariopsis doliolus and Nitzschia interrupteseriata from continuous two-year sampling intervals in the box core varies with sea surface temperature (SST), suggesting its utility in SST reconstruction. The assemblage data from the ODP 893A record indicate a broad interval of generally cooler SSTs between wAD 800 and 1350, which corresponds to the Medieval Climate Anomaly (MCA), a period of generally warmer temperatures across other regions of the northern hemisphere. The assemblages also indicate an interval of generally warmer SSTs between wAD 1400 and 1800, a period of otherwise global cooling referred to as the Little Ice Age (LIA). The changes in assemblages of diatoms and silicoflagellates support the hypothesis that the widespread droughts of the Medieval Climate Anomaly in the Western US were associated with cooler eastern North Pacific SST. The box core assemblages have higher percentages of tropical and subtropical compared to temperate and subpolar species than the ODP samples, reflecting a response of phytoplankton communities to an unusual 20th century warming. Pseudonitzschia australis, a diatom linked with domoic acid production, begins to become more common (>3% of the diatom assemblage) in the box core only after AD 1985, suggesting a link to anthropogenic activity. Ó 2008 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The Medieval Climate Anomaly (MCA) and Little Ice Age (LIA) are generally regarded as the largest broad-scale deviations of climate change in the last several thousand years, exceeded only by recent warming (Mann and Jones, 2003). An increase in the abundance of tropical and subtropical species coeval with a decrease in abundance of temperate and subpolar species during the 20th century indicates that some recent ecosystem changes in the California Current are atypical of prior variations (Field et al., 2006a,b). However, it is not clear if this conclusion can be applied to a diverse range of taxa. Although the MCA was anomalously warm across much of the planet, particularly in the North Atlantic and Europe, there is evidence for a cool eastern Pacific in association with dry conditions over western North America (Kennett and Kennett, 2000; Mann and Jones, 2003; Graham et al., 2007). Conversely, warmer sea surface temperatures (SST) in the eastern Pacific have been linked with increased precipitation in much of

* Corresponding author. Tel.: þ1 650 329 4971; fax: þ1 650 329 5203. E-mail address: [email protected] (J.A. Barron). 1040-6182/$ – see front matter Ó 2008 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2008.08.007

western North America during the LIA (Kennett and Kennett, 2000; Graham et al., 2007), a generally cooler period globally (particularly in the North Atlantic and Europe). We use diatom and silicoflagellate assemblage variations as an additional test of the hypothesis that eastern Pacific SST patterns are opposite in sign from the patterns of the North Atlantic in the MCA and LIA. We then compare the assemblage variations of the 20th century with prior variations to examine how the response of marine phytoplankton to recent warming compares with those prior variations. Evidence of a megadrought during the MCA and other periods ˜ a-like conditions in the eastern Pacific come from many of La Nin sources (Graham et al., 2007). Cook et al. (2004) used a network of 835 tree-ring chronologies in a 2.5-degree grid of 286 points covering most of North America to reconstruct drought histories for North America during the past 1200 years. They documented an epic drought that extended throughout much of the West between AD 900 and 1300, a period broadly consistent with the MCA. Both tree-ring records from California to Alberta compiled by MacDonald and Case (2005) and other numerous records of precipitation (summarized by Graham et al., 2007) support La ˜ a-like winter Northern Hemisphere circulation patterns during Nin the MCA from marine and terrestrial records (Fig. 1). Herweijer

J.A. Barron et al. / Quaternary International 215 (2010) 34–44

et al. (2006) linked severe Western droughts of the latter half of ˜ athe 19th century with anomalously cool tropical Pacific (La Nin like) SSTs. They applied an atmospheric general circulation model to the tree-ring database and tropical Pacific coral SST record of Cobb et al. (2003) to argue that the epic North American droughts ˜ a-like conditions in the tropical of the MCA were caused by La Nin Pacific. ˜ a-like The relationships between western drought and La Nin conditions largely stem from the association of anomalously low SSTs with anomalously high pressure throughout the eastern Pacific, extending from the tropical Pacific to the Gulf of Alaska (Herweijer et al., 2006). However, there are few records of SST anomalies in the eastern Pacific to support the patterns proposed by Kennett and Kennett (2000). There are even fewer records that permit the comparison of the response of marine organisms to 20th century warming in comparison with previous climate changes (e.g. Field et al., 2006a). The Santa Barbara Basin is situated centrally within the horseshoe-like pattern of SST anomalies associated ˜ o/La Nin ˜ a and the PDO. Thus changes at this location with El Nin reflect much of the large-scale changes in the eastern Pacific (Field et al., 2006b).

2. Regional setting Waters off the coast of California lie near the modern-day boundary between the subarctic and subtropical gyres of the North Pacific, where they are influenced by the strength and character of the California Current (Huyer, 1983). During much of the spring and summer, juxtaposition of the North Pacific High and the North American Low results in strong, persistent northwesterly winds, which induce coastal upwelling and lead to high biologic productivity (Hood et al., 1999). Winters are influenced by a southward migration of the North Pacific High from w40 N to w30 N, and the southward migration of the jet stream from w48 N to an average position of 38 N due to a strengthening of the Aleutian Low (Bograd et al., 2002). Winters are typically mild, wet, and stormy, with an alternation of southwesterly winds and upwelling-favorable ˜o northwest winds (Huyer, 1983; Harms and Winant, 1998). El Nin events and positive PDO phases are associated with an enhanced Aleutian Low and strengthened cyclonic circulation in the North Pacific that result in greater poleward transport, higher SSTs, and a depression of isotherms throughout the eastern Pacific. Santa Barbara Basin (SBB), just southeast of Point Conception, is located on the inshore side of the low salinity core of the California Current. The combination of the geostrophic flow of the California Current offshore and upwelling-favorable winds result in a shallow thermocline and high productivity year round (Fig 2). Strong northerly winds associated with intensification of the offshore California Current induce coastal upwelling year round, but these are most intense in spring and summer. During other seasons of the year, the combination of upwelling-favorable wind events and the Southern California Countercurrent result in cyclonic circulation over the basin (Harms and Winant, 1998). The cyclonic circulation results in a combination of warmer waters from the south mixing with recently upwelled waters. During the summer and early fall, warm-waters from the south are transported into the SBB by the Southern California Countercurrent. The northward-flowing Davidson Current is a coastal countercurrent extending northward to w40 N (Hendershott and Winnant, 1996; Di Lorenzo, 2003) that is active in the winter. Average SSTs in the SBB range from about 13  C in the spring to 18  C in the late summer, although higher (lower) SSTs can occur seasonally during ˜ o (La Nin ˜ a) events (California Cooperative Oceanic Fisheries El Nin Investigations database http://www.calcofi.org/newhome/data/ database/database.htm)

35

3. Previous paleoclimate studies Owing to the presence of annually varved sediments that accumulate at relatively high rates (>120 cm/kyr), SBB has been the focus of numerous high-resolution Holocene paleoclimate studies (Kennett et al., 1994, 2007; Kennett and Kennett, 2000; Friddell et al., 2003; Field et al., 2006a; Fisler and Hendy, 2008). A number of single proxy records have been developed in SBB sediments across various timescales, but each has potential problems. A multiple proxy approach is necessary for convincing results. The principal evidence for a cool (warm) California Current during the MCA (LIA) comes from several studies done in the SBB. Kennett and Kennett (2000) compiled a high-resolution oxygen isotope record of the near surface-dwelling planktonic foraminifer Globigerina bulloides. If interpreted as SST, their d18O record of G. bulloides implies that a large portion of the MCA between wAD 900 and 1300 was characterized by anomalously low SSTs. Although changes in the d18O values of G. bulloides, have been shown to reflect seasonal and interannual variability in SST over the SBB (Thunell et al., 1999), several studies have shown that G. bulloides frequently reflects subsurface temperatures as stratification increases and a particular isotherm deepens (Field, 2004; Pak et al., 2004). Pak et al. (2004) found that both Mg/Ca and d18O values of G. bulloides from SBB sediment trap samples indicate that G. bulloides calcification temperatures often reflect SST but frequently correspond to observed temperatures at 30 m depth and below. Based on a study of modern plankton two samples from the SBB and surrounding waters, Field (2004) frequently found most individuals of G. bulloides within the mixed-layer but G. bulloides was preferentially found within the thermocline in conditions of greater stratification. In a comparison with instrumental SST records for the recent periods of AD 1950 through 1998, Field (2004) found that the d18O records of G. bulloides and Neogloboquadrina pachyderma do not track the decadal-scale warming since 1977 that has been clearly observed in SST measurements. Rather, the d18O records are affected by a downward shift in the habitat depths of both taxa (Field, 2004) and/or changes in seasons of greatest flux (Black et al., 2001). Thus d18O records will not necessarily be a reliable indicator of SST at high-resolution timescales, particularly during periods of enhanced stratification. A high-resolution alkenone (UK’37) SST record for the period AD 1440–1940 also provides some support for positive SST anomalies in the SBB during the LIA, Zhao et al. (2002) found that positive SST anomalies were generally between 0.0 and þ0.5  C from AD 1650 to 1850 with an exception of a cooler period (0 to 0.5  C) recorded between AD 1750 and 1770. However, Zhao et al. observed that individual increases in alkenone SST did not correlate well with historical ENSO records, which questions the reliability of alkenones as indicators of SST. In a study of sediment trap samples, Hardee and Thunell (2006) found that the alkenone temperature index reflects temperatures within the chlorophyll maximum, as one might expect. The chlorophyll maximum is deeper over the SBB when stratification and SST increase. Mollenhauer and Eglinton (2007) highlighted other fundamental problems with SST-indicating biomarkers in SBB. They concluded that some of the biomarkers are pre-aged (resuspended from elsewhere) and mixed with new biomarkers, therefore resulting in questionable SST records. Huguet et al. (2007) investigated the TEX86 temperature proxy based on marine crenarchaeotal membrane lipids in a two-yearlong sediment trap study and applied the technique to a highresolution sediment record from AD 1850 to 1987. After comparison with instrumental SST records, they concluded that TEX86 records reflect subsurface temperatures in the SBB but do not reflect interannual variability in temperature due to changes in habitat production of the crenarchaeotal.

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Long-Term Changes in Drought Area in the West

PDO Index (MacDonald & Case, 2005)

% Drought Area (Cook et al., 2004)

100 80

936

1034

1150

Drier 1253

60

Wetter

40 20

1.5 1 0.5 0 -0.5 -1 -1.5 -2

800

1000

1200

1400

1600

1800

2000

Year (AD) Fig. 1. Comparison of the smoothed versions of the PDO reconstructions of MacDonald and Case (2005) with the western drought record of Cook et al. (2004). Dashed lines around drought curve reflect two-tailed 95% bootstrap confidence intervals.

Assemblage data also have complications. Pisias (1978) studied radiolarians of the last 8000 years in the SBB. He calculated February SST using a transfer function and argued that SST exceeded modern values during the intervals of 0.8–1.8, 3.6–3.8, and 5.4–8.0 ka. Large variations in SST of over 5  C were inferred by estimated transfer functions, which question the application of transfer functions for directly estimating SST. However, Weinheimer et al. (1999) found that changes in the percentages of different species of radiolarians did respond to 20th century temperature variations, including the warming trend, but the changes in percent abundance of radiolarians have not been shown further back in time. Pospelova et al. (2006) studied dinoflagellates over the past 40 kyr. Their coverage of the Holocene was rather limited, but they did point out that principal component 1 of their assemblages suggests warming of SBB surface waters during the past 5000 years. Fisler and Hendy (2008) compiled a high-resolution record of planktonic foraminiferal abundances in ODP Site 893 during the past 5000 years. They showed that an increased ratio of sinistrally coiled N. pachyderma to dextrally coiled N. pachyderma (¼ N. incompta) during part of the MCA (wAD 800–1200) reflected cooling of SBB surface waters, in support of the d18O isotopic data of Kennett and Kennett (2000). However, during part of the LIA (wAD 1400–1650) where Kennett and Kennett’s (2000) isotopic data suggested warming of SBB surface waters, Fisler and Hendy’s (2008) N. pachyderma data indicate anomalously cool conditions. Similarly, Fisler and Hendy (2008) do not record a noteworthy increase in the relative abundance of warmwater species during the LIA. Field et al. (2006a,b) showed that tropical and subtropical species increased in abundance during the 20th century to levels that exceed prior decadal- and centennial-scale variability during the last 1400 years. They showed that the first principal component of the assemblage of planktonic foraminifera from two-year sampling intervals reflected SST variability during the last 100 years. Additionally, a decrease in abundance of temperate and subpolar species indicated that warming after the mid-1970s was unlike prior warm periods during the last 1400 years.

Thus although foraminiferal d18O and abundance data both indicate a cooling (warming) of the California Current during the MCA (LIA), only the foraminifera assemblage data reflect the late 20th century warming; the d18O records of G. bulloides do not track the observed warming. Other proxy records do not cover the timescales of the MCA, LIA, and recent 20th century. Thus additional proxies are needed to have reliable records of the MCA, LIA and 20th century warming and their relative effects on regional ocean temperatures and marine ecosystems. 4. Diatom studies in the Santa Barbara Basin Hemphill-Haley and Fourtanier (1995) tabulated the diatom assemblages in ODP Site 893 in a low-resolution study of the past 114,000 years. They reported that diatom preservation ranged from excellent in finely laminated sediments of the interglacials to poor in the bioturbated sediments that typified the glacials. They noted that the diatom assemblages were dominated by Chaetoceros spp. resting spores and Thalassionema nitzschioides and the Holocene was characterized by the appearance of the subtropical taxon, Fragilariopsis doliolus. Hemphill-Haley and Fourtanier (1995) observed that during the past 10,000 years, the subtropical diatoms F. doliolus, Azpeitia neocrenulatus, and Hemidiscus cuneiformis displayed a bimodal distribution, being more common between w9 and 6 ka and after w3 ka, than they were in the middle Holocene interval between w6 and 3 ka. This agrees with the results of Barron and Bukry (2007) off central and northern California, where they observed that the middle part of the Holocene was cool compared to the later and earlier parts. Other published reports on the climate-controlled aspects of diatoms in the SBB are from sediment trap and water column studies. Lange et al. (2000) reported on the fluxes of siliceous microorganisms (diatoms, silicoflagellates, and radiolarians), organic carbon, calcium carbonate, biogenic silica, and lithogenic particles from 1996 to early 1998 in a sediment trap set at 540 m ˜ o of 1997 to early depth in the SBB. They noted that during the El Nin 1998, fluxes of diatoms, silicoflagellates, and radiolarians were substantially lower than those seen by them between 1993 and

J.A. Barron et al. / Quaternary International 215 (2010) 34–44

A Sa

Pt. Conception

a nt r la C a e iv R

Santa Barbara X

r

0

60

ODP893

200

SBCC2902

400

200

40

0

60

400

0

Summer-Fall

Spring

50°N

Winter WWD

37

transport, and dissolution, alter diatom assemblages during the time spent settling through the water column to the sediment. Nonetheless, Venrick et al. (2006) found that changes in local circulation are well correlated with large-scale changes in circulation. Thus a positive PDO phase would result in enhanced transport from the south and the presence of more tropical to subtropical species. Thus some of the changes in diatoms assemblages are expected to reflect large-scale climate changes. To address the response of the California Current to climate changes on various timescales, we compare high-resolution diatom and silicoflagellate assemblage data in a SBB box core record of the years AD 1940–2001 with instrumental SST in order to determine which taxa best track modern SST change. These results will be applied to the ODP 893 record for the past 2000þ years in an effort to examine ocean temperature variations in the SBB during the MCA and LIA. We also examine whether the 20th century warming resulted in diatom and silicoflagellate assemblages that are typical of past variations or are indicative of conditions that are warmer than previous decadal and centennial variations of the last two millenia. 5. Materials and methods 5.1. Samples and chronology

WWD 40°

WWD CC

SBB

PtC CC

CC

DC

SCC

30° CC

B 20°

130°

120°

110°W

Fig. 2. A - Location map of the Santa Barbara Basin showing cores studied, bathymetry in meters, and general surface circulation. B - Seasonal circulation of the California Current (CC) modified from P.T. Strub, written comm., 2007; SBB ¼ Santa Barbara Basin; WWD ¼ West Wind Drift; SCC ¼ Southern California Countercurrent; PtC ¼ Pt. Conception; DC ¼ Davidson Current.

1996, whereas the flux of lithogenic material and calcium carbonate was higher. Like Thunell et al. (1995) and Thunell (1998), they found that the maximum diatom flux occurred in the spring (March–May). They reported that upwelling-diatoms and deep-living radiolarians were reduced in relative numbers during the August 1997–April ˜ o event compared to the previous January 1996–July 1998 El Nin 1997 period, whereas warm-water diatoms and radiolarians and non-planktonic diatoms displayed an opposite trend in relative abundance. They considered Asterolampra marylandica, Bacteriastrum comosum, and Hemiaulus hauckii as warm-water diatoms, and Chaetoceros radians as an upwelling-indicator. Venrick et al. (2003) compared the diatom and silicoflagellate assemblages collected biweekly in SBB sediment traps between August 1993 and April 2000 with mixed-layer phytoplankton samples from quarterly cruises. Both data sets revealed the presence of regular, strong spring blooms dominated by Chaetoceros spp. They noted that the silicoflagellates Distephanus speculum and Octactis pulchra were most common during the fall in their trap samples. A group (cluster) consisting of the diatoms Coscinodiscus centralis, C. radiatus, and C. wailesii was most common during the summer, whereas a group dominated by the diatoms Coscinodiscus perforatus, C. granii, Thalassionema frauenfeldii, and Thalassiothrix spp. and the silicoflagellate Dictyocha fibula was most common during the winter. Diatom and dinoflagellates assemblages are also affected by many physical properties that are often associated with SST, such as mixing, thermocline structure, and transport. Venrick et al. (2003) found that many processes, including high-frequency variability,

Box core SBBC2902 was collected at 3413.3’N, 120 01.7’W in the deepest portion of the SBB in October 2001 with a Soutar box corer (Field et al., 2006a,b; Fig. 2). The presence of bacterial mats at the surface indicated that the sediment surface was recovered for each box core used in this study. The site for this core was chosen based on the excellent laminae structure observed in core SBBC9210-1001 (BC1001). SBBC2902 was subsampled with a 14  14 cm acrylic liner that was inserted into the core on board ship. The core bottom was frozen with dry ice to prevent sediment slumping upon opening. The core was then opened and the frozen sediments surrounding the liner washed away until the acrylic liner could be sealed at the bottom. Cores were stored at w5  C and allowed to drain. Vertical sections of each type of core were sliced into slabs along the vertical axis of the core. A single trimmed slab was first sliced from the widest point at the shrunken top to the point of equivalent width at the bottom of the section. Each vertical section, now of equal width, was then sliced into six vertical slabs down the core, each of which revealed the laminated sequences. All slabs were x-radiographed from at least two different perspectives by centering the x-ray beam vertically over the top and bottom of the slab to obtain the best focus of the laminae structure. A detailed chronology was developed based on visual identification of individual varves from the X -radiographs of the different slabs from six different cores. Since sedimentary sequences vary across the basin, examination of slabs from several cores reveals different perspectives of the laminae structure. Gray layers were considered as instantaneous deposits (Hu¨lsemann and Emery, 1961) for the chronology developed. This chronology is anchored by the well-established varve count from several box cores extending to 1880 that has been verified by 210Pb dating (Soutar and Crill, 1977). ODP 893 is located at 3417.25´N, 120 2.2´W and at 576.5 m water depth (Fig. 2). A detailed varve chronology of Schimmelmann et al. (2006) is complemented by numerous 14C-AMS dates and detailed comparison with other varved cores in the SBB (Schimmelmann, written comm., 2006). Kennett et al. (1994) identified the Macoma layer (AD 1841) at a depth of 22 cm in ODP 893A. For the present study, samples were taken every 5 cm and averaged 1 cm in thickness. Varve chronology suggests that the sample intervals typically range between 8 and 32 years. The diatom assemblages in these 1 cm thick samples are assumed to represent a composite of between w2 and 8 years of deposition.

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An independent AMS chronology exists for ODP 893A (Fisler and Hendy, 2008), which results in ages that are as much as 200 years older than the varve chronology (these likely increase downcore). We assume that the varve chronology is more robust, as differences with the AMS chronology likely result from changing reservoir ages of the dissolved inorganic carbon in the SBB (Schimmelmann et al., 2006). 5.2. Processing Samples from ODP 893A were disaggregated in distilled water and then processed by boiling them in 30% hydrogen peroxide and 37% hydrochloric acid. The acid was then removed through centrifuging. The processed samples were stored in a vial containing at least 7–10 times as much distilled water as sample. To prepare slides, the vial was shaken and a drop of the suspension was taken after 5–10 s of settling from near the top of the vial, transferred to a 22  30 mm cover slip and allowed to dry on a warming tray overnight. Slides were then mounted in Naphrax (index of diffraction ¼ 1.74). Samples from SBBC2902 were not processed in hydrogen peroxide and hydrochloric acid. Rather, they were placed in a glass vial and covered with 7–10 times as much distilled water as sample. A disposable wooden stick was then used to disaggregate the samples in the vials by stirring the suspension. To prepare slides, the vial was shaken and a drop of the suspension was taken after 5– 10 s of settling from near the top of the vial, transferred to a 30  22 mm cover slip and allowed to dry on a warming tray overnight. Slides were then mounted in Naphrax (index of diffraction ¼ 1.74). 5.3. Diatoms At least 300 individual diatoms were counted per sample by making random traverses of the slide under the light microscope at 1250X using the counting techniques of Schrader and Gersonde (1978). Following Sancetta (1992) and Barron et al. (2003), Chaetoceros resting spores, which dominate in nearshore coastal upwelling environments (Lopes et al., 2006), were not counted in order to better resolve differences in offshore oceanic conditions. Chaetoceros spores are resistant to dissolution, so that their relative numbers may be enhanced in recycled sediments. Relative diatom abundances were estimated by recording the number of diatom valves encountered while making vertical traverses of the slide (length of traverse ¼ 22 mm) at 1250X (total area covered per traverse ¼ 4.114 mm2). Random traverses were made until >300 diatom valves were counted. The taxonomy of Barron et al. (2003) was followed (see Appendix A). 5.4. Silicoflagellates One to three slides were systematically tracked to obtain a representative count of 50–100 silicoflagellate specimens per sample. Counts typically were made at 250X magnification, with 500X used for checking questionable specimens. Taxonomy follows that used by Barron and Bukry (2007) and Barron et al. (2004, 2005) (see Appendix A). Intraspecific variants of silicoflagellate taxa were tabulated in an effort to determine environmental preferences. 6. Results 6.1. Diatoms The relative abundance of selected diatom taxa in box core SBBC2902 and ODP 893A is presented in Fig. 3. Census data are available in Tables 1 and 2 of the Supplementary data.

The diatom taxa are arranged on Fig. 3 with warm-water taxa Azpeitia nodulifera, F. doliolus, Nitzschia interrupteseriata, Azpeitia species (Azpeitia africana and Azpeitia tabularis), H. cuneiformis, and Roperia tesselata on the bottom. Transported diatoms on the top include freshwater and reworked taxa, benthic diatoms, and tychopelagic, shelf-associated taxa Actinoptychus spp., Paralia sulcata, Stephanopyxis spp. Taxa indicative of cooler waters and/or upwelling, such as Thalassiosira pacifica, T. spp. (indistinguishable small Thalassiosira, that likely include Thalassiosira decipiens, T. lineata, and T. pacifica), Actinocyclus curvatulus, and Rhizosolenia spp. (mostly Rhizosolenia hebetata f. semispina) are plotted near the top of the figure. Typically, diatom data are available every 5 cm in ODP 893A, corresponding to an interval between 8 and 32 years. 6.2. Silicoflagellates The relative abundance of selected silicoflagellate taxa in box core SBBC2902 and ODP 893A is presented in Fig. 4. Census data are in Tables 3 and 4 of the Supplementary data. Silicoflagellate taxa are arranged with warm-water taxa Dictyocha calida, Dictyocha perlaevis, Dictyocha stapedia aspinosa, Dictyocha stapedia, and the large form of D. stapedia on the bottom and cool-water and upwellingtaxa O. pulchra, Distephanus speculum speculum, D. speculum minutus, and D. octangulatus on the top of the figure. Dictyocha aculeata, a taxon that ranges from the tropical Pacific to the Gulf of Alaska, where it is associated with deeper thermocline conditions, is plotted in the center of Fig. 4. Due to poor preservation or low abundance in selected intervals, perhaps as a consequence of dissolution, the silicoflagellate data from ODP 893A are not as detailed as the diatom data, as gaps of relatively sparse data occur in intervals between wAD 1130 and 1400, wAD 1660 and 1760, and w1780 and 1840. 6.3. Comparison of assemblage with SST Fig. 5 shows the relative abundances of selected warm- and coldwater diatoms and silicoflagellates in SBBC2902 along with SST values over the period from AD 1940 to 2001. The ‘SST anomaly’ represents an updated Kaplan reconstruction for the 5  5 grid centered at 122.5 W, 32.5 N, in the California Current near the SBB for the 20th century (Field et al., 2006b). Box core samples are plotted at the older date of the two-year-long slabs of the varved sediment. SST values are averaged for the same two-year-period and plotted at the older year. The phases of the PDO are added after Mantua et al. (1997), http://www.atmos.washington.edu/wmantua/REPORTS/ PDO/PDO_egec.htm The warm-water diatoms F. doliolus and N. interrupteseriata have been chosen to compare with SST, as they are relatively common in both the SBBC2902 and ODP 893A records. Also, the relative abundance of F. doliolus has been shown previously to track changes in SST fairly well (Hemphill-Haley and Fourtanier, 1995; Barron and Bukry, 2007). The linear relationship between %F. doliolus and anomaly SST is relatively weak but significant (r2 ¼ 0.13; p < 0.05; n ¼ 31). When the relative abundance of the warm-water diatom N. interrupteseriata is added to that of F. doliolus, a clearly significant relationship with anomaly SST is obtained (r2 ¼ 0.21; p < 0.05; n ¼ 31). Conversely, the relative abundance of Rhizosolenia spp., which is dominated by R. hebetata f. semispina, has a weak, negative relationship with anomaly SST (r2 ¼ 0.11; n ¼ 31). While not significant, two peaks in % between AD 1960 and 1964 and between AD 1974 and 1976coincide with lower SST and negative phases of PDO. The standard deviations of the F. doliolus þ N. interrupteseriata, F. doliolus, and Rhizosolenia spp. relative abundance data (6.33, 7.10, and 9.87, respectively) are comparatively large compared to the amplitudes of the curves. This

J.A. Barron et al. / Quaternary International 215 (2010) 34–44

ODP 893A Diatoms

Percent

80

100

80 Rz.

T.ecc.

T.o.

Thx.

60

40

Thalassionema

40

SBCC2902 Diatoms

benthic

Ac. T.sp. T.pac.

60

fw & rw

bottom yr

tycho other plank.

Pseud.

100

39

Rop.

20

20

Cyc. Azp.

C.rad.

N.int. Az.nod.

F.dol.

0 -400

-200

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 1940 1950 1960 1970 1980 1990 2000

Year (AD)

BottomYear

Fig. 3. Relative abundance changes of selected diatoms in samples studied from SBBC2902 and ODP 893A. Refer to Appendix A for taxa abbreviations.

suggests that the moderately delicate valves of these taxa may be subject to differential preservation. The relationships of the warm-water silicoflagellate Dictyocha stapedia and the upwelling-silicoflagellates Distephanus speculum and O. pulchra with anomaly SST are not apparent (Fig. 5). The relative abundance of D. stapedia broadly follows SST anomalies with reduced values between AD 1958 and 1970 coincident with lower SST, but the relationship is not significant due to greater percentages in the early 1950s when SST is anomalously low. The combined D. speculum and O. pulchra relative abundances, which likely reflect enhanced upwelling, are slightly higher during the negative PDO interval between AD 1958 and 1976, but the relationship between these silicoflagellates and SST anomalies is not significant. The standard deviations of the D. stapedia, and D. speculum þ O. pulchra relative abundance data (22.05 and 7.59, respectively) are comparatively large compared to the amplitudes of the curves, possibly reflecting the low number of 50 specimens counted per sample.

are dominated by T. nitzschioides, a temperate to subtropical taxon that represents spring-season production within a broad region extending seaward from the coastal zone (Sancetta, 1992). Thalassionema nitzschioides is slightly less common in SBBC2902 (mean 22%, n ¼ 31) than it is in the longer ODP 893A record (mean 30%, n ¼ 72). The relative percentage of the diatom assemblage made up of benthic and other transported (tychopelagic, freshwater, and reworked) diatoms is similar (w20%) in both records, suggesting that the processes resulting in redeposition of sediments in the SBB have not changed substantially over time. Major constituents of silicoflagellates in each core are D. aculeata and D. stapedia. Major differences, however, are evident between the relative percents of the different species in the box core and ODP 893A diatom and silicoflagellate records. In general, there are greater percentages of subtropical species and lower portions of temperate and subpolar species in the recent box core. The warm-water diatoms F. doliolus and N. interrupteseriata are about twice as common in the box core record than they are in ODP 893A (Fig. 3). Similarly, the warm-water silicoflagellate D. stapedia, is more dominant in the SBBC2902 assemblages (mean 52%) than it is in ODP 892A (mean 36%). Conversely, Distephanus speculum speculum and D. speculum minutus, silicoflagellates indicative of cool, nutrient-rich waters that occur in regions of coastal upwelling (Poelchau, 1976; Takahashi et al., 1989), are much more common in the ODP 893A record (mean17% and 18%, respectively) than they are in SBBC2902 (mean both >10%) (Fig. 4). Dictyocha aculeata,

6.4. Comparison of assemblages between cores The percent variations in diatom and silicoflagellate assemblages of the SBBC2902 record (AD 1940–2001) and the ODP 893A record (w220 BC–AD 1880) have both similarities and differences. The major species constituents of both diatoms and silicoflagellates are similar in each of the two cores. Both diatom records

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Fig. 4. Relative abundance changes of selected silicoflagellates in samples studied from SBBC2902 and ODP 893A. Refer to Appendix A for taxa abbreviations.

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Anomaly SST (°C)

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a silicoflagellate that reflects deeper thermocline conditions that occur during the winter off California (Barron and Bukry, 2007), is generally more common in the box core record (mean 27%) than in the ODP 893A record (mean 20%), supporting the D. speculum evidence that upwelling (shallow thermocline) conditions were reduced in the modern record compared with that of the past. Distephanus octangulatus, a subarctic silicoflagellate, is absent from the SBBC2902 record, but has sporadic occurrences in the ODP 893A record, notably at w141 BC (10%), wAD 1418 (4%), and wAD 1866 (6%). The subarctic diatom Neodenticula seminae which makes up 5–10% of late Holocene diatom assemblages off northern California (Barron and Bukry, 2007), however, is absent from both SBBC2902 and the ODP 893A records. Octactis pulchra, a subtropical silicoflagellate associated with high biologic productivity, is relatively common (>5%) in selected intervals of SBBC2902: namely, in the samples representing the years AD 1940–1941; 1960–1961; 1962–1963; 1964–1965; 1966–1967; 1976– 1977; and 1986–1987. With the exception of AD 1940–1941 and 1986–1987 these years occur within an interval of negative PDO, supportive of increased upwelling. In the ODP 893A record, O. pulchra also occurs sporadically with relative abundances normally >5%. Greater abundances (10%) mark the samples corresponding to wAD 145, 256, 972, 1029, and 1644. Rhizosolenia spp., which are mostly R. hebetata semispina, typically make up an average of 9% of the diatom assemblages of the box core, but their relative numbers vary considerably, >1% in the AD 1944–1945 varves vs. 39% in the AD 1976–1977 varves. In

the ODP 893A record, the relative numbers of Rhizosolenia spp. are not as variable, averaging 5% of the assemblages, with percentage values rarely exceeding 10%. The box core samples containing higher % Rhizosolenia spp. contain clustered specimens that likely have been deposited in fecal pellets. These possible Rhizosolenia spp. blooms are concentrated in the interval from AD 1960 to 1977, a period of negative PDO and likely increased upwelling in the SBB. Comparable events may be recorded in ODP 893A at wAD 831, 1577, and 1844 by anomalously high percentage values of Rhizosolenia spp. Thalassiosira pacifica, a diatom that Sancetta (1992) considers to be typical of coastal diatom blooms, is sparse in both the box core (mean w1%) and the ODP 893A record (mean < 1%). Indistinguishable small Thalassiosira, tabulated as Thalassiosira spp. and likely including T. decipiens, T. lineata, and T. pacifica, average about 4% of the box core assemblages and 3% of the ODP 893A assemblages. These cool-water Thalassiosira are more common in Holocene records off central and northern California (Barron and Bukry, 2007). Dictyocha perlaevis, a subtropical silicoflagellate that is very common in late Holocene assemblages of the northern Gulf of California (27–60%; Bukry unpublished data), makes up w30% of the SBBC2902 silicoflagellate assemblages in the years AD 1940– 1943. It remains persistently at 2–17% levels in samples through AD 1955, where it is associated with moderate counts of opal phytoliths that are suggestive of offshore winds. In the ODP 893A record, D. perlaevis is usually more common and persistent in intervals

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where the strong upwelling-proxies D. speculum minutus and O. pulchra are less common. The presence of 34% R. tesselata in the ODP 893A sample from AD w1158 represents an anomalous, non-modern analog occurrence for the SBB. This subtropical diatom is sparse and sporadic in SBBC2902, making up 1% of the assemblage. In other ODP 893A samples studied, R. tesselata never exceeds 2%. The senior author has only observed such abundant occurrences of R. tesselata in the central Gulf of California (Barron et al., 2004, 2005), where they were interpreted to represent winter blooms under upwellingconditions (w20  C). 7. Discussion The correlations of species changes associated with two-year sampling intervals and the corresponding variations in observed SST confirm the utility of diatom and silicoflagellates as indicators of temperature changes in the California Current. However, neither the diatoms nor the silicoflagellates show agreement with the negative part of ‘anomaly SST’ curve between the years AD 1946 and 1954. Large differences in planktonic foraminifera and d18O values are also observed between AD 1946 and 1954 relative to the cooler phase of the PDO during the1960s and early 1970s (Field et al., 2006a). It is likely that a different combination of SST, transport of water masses within the California Current, thermocline structure, mixing and/or seasonality are responsible for the differences between these two different time periods that have similar signs of the PDO. Also, SST data used is centered outside of the SBB and the SST of the spring upwelling season is likely to be more important to diatoms than annual-averaged SST. Thus the diatom and silicoflagellate assemblages provide another proxy, albeit with their own limitations, to examine decadal- to centennial-scale variations from SBB sediments. 7.1. Paleoclimate of the past 1200 years Fig. 6 shows the diatom proxies for SST, %F. doliolus (red line) and %F. doliolus þ %N. interrupteseriata (green line) along with the silicoflagellate proxies %D. stapedia (warm) and % D. speculum þ %O. pulchra (cool/upwelling) for the past 1200 year at ODP 893A. Kennett and Kennett’s (2000) d18O records of G. bulloides has been updated on the bottom of Fig. 6, but modified to reflect the varve chronology rather than the radiocarbon-based chronology that results in ages that are as much 200 years older than the varve chronology (see Schimmelmann et al., 2006; Fisler and Hendy, 2008). Hendy has also updated the SST estimates for the d18O record using the equations of Bemis et al. (2002). Note that although the diatom and silicoflagellate records and the d18O G. bulloides data are both from ODP 893A, the sampling frequency and intervals differ. Shaded intervals in the diatom, silicoflagellate, and d18O record of G. bulloides depict intervals with inferred SST that is cooler than the long-term average. These are compared with Cook et al.’s (2004) drought curve and with two lengthy Sierra Nevada droughts documented by Stine (1994); AD 892–1112 and AD 1209–1350 on the top of Fig. 6. MacDonald and Case’s (2005) PDO index has been included for comparison below the drought records. For the most part, the wAD 900–1350 period of widespread MCA drought in the western US coincides with cooler proxy SSTs in the diatom, silicoflagellate, and d18O G. bulloides records (Kennett and Kennett, 2000) of the SBB. These, in turn, match an interval of extended negative PDO predicted by MacDonald and Case (2005). These four records thus support the hypothesis of Herweijer et al. (2006) that widespread drought in the American Southwest during ˜ a-like conditions in the Pacific. the MCA was linked with La Nin Diatoms and the PDO index suggest that the interval of reduced

41

drought conditions between wAD 1400 and 1800 coincided with warmer SST. The silicoflagellate and G. bulloides proxies, however, argue for more variable SST during this wAD 1400 and 1800 interval. Nevertheless, matches of individual data points between the three SBB proxy SST records are limited. Higher resolution studies on the same sample sets involving multiple proxies of SST, salinity, and nutrients are needed. 7.2. 20th century changes The greater (reduced) percentages of species associated with higher (lower) SSTs during the 20th century generally support the results of planktonic foraminifera and radiolarians that indicate an increase in the abundances of tropical and subtropical species in response to warming in the early 20th century (Weinheimer et al., 1999; Field et al., 2006a). The changes in % abundance of diatoms and silicoflagellates do not indicate an additionally unusual assemblage after the mid-1970s, as observed in some species of foraminifera and radiolarians. Rather, some of the principal changes had already occurred by 1940. In order to be more comparable with the older ODP 893 record, detailed box core studies must be extended into the latter half of the 19th century. The differences between the response of % changes in diatoms and silicoflagellates from other taxa are not unexpected since each taxon may have its own response to specific environmental conditions. Field et al. (2006a,b) observed that some changes in abundance of planktonic foraminifera in association with 20th century warming had occurred by the 1930s while other changes occurred in the mid-1970s. The results of the changes in diatoms and silicoflagellates reiterate that many aspects of the marine ecosystem that have been observed within the 20th century reflect a warmer climate period than prior decadal- and centennial-scale fluctuations. Moreover, the species composition of many marine taxa had already changed before many ecological studies began. 7.3. Occurrence of toxic diatoms The 15% occurrence of P. australis in the AD 2000–2001 sample of SBBC2902 (Fig. 4) is remarkable in that this toxic diatom typically makes up <1% of the SBB diatom assemblage prior to the year AD 2000; occurrences of 5% (years AD 1990–1991) and 3% (years AD 1986–1987) are exceptions. Pseudonitzschia australis was not tabulated separately in the ODP 893A samples, as it never made up 1% or more of the diatom assemblages. Although the samples here are presented only as percentage of the total diatom community, rather than an absolute flux rate, the results suggest that many of the historical outbreaks of Pseudonitzschia are atypical of variability in the California Current. Although the mechanisms responsible for these outbreaks are clear, they are likely related to anthropogenic activity. Fritz et al. (1992) were the first to show that an outbreak of domoic acid poisoning in seabirds in Monterey Bay, California, in 1991 coincided with a bloom of P. australis. The increase in the 1990–1991 sample may be a consequence of that bloom, which would have extended to other regions of the coastline. Fryxell et al. (1997) reviewed the occurrence of Pseudonitzschia on the West Coast between AD 1920 and 1996, noting that it produces domoic acid, which when concentrated by organisms higher in the food chains, can lead to sickness and mortality in sea mammals, seabirds and humans. Schnetzer et al. (2007) determined the abundances of Pseudonitzschia spp. in phytoplankton off the coasts of Los Angeles and Orange counties during the spring and summer of AD 2003 and 2004 and related these to

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% Drought Area (Cook et al., 2004)

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MCA Droughts Year (AD) Fig. 6. Comparison of Cook et al.’s (2004) western drought curve and the epic Sierra Nevada droughts of Stine (1994) with MacDonald and Case’s (2005) PDO index, diatom and silicoflagellate proxies for SST in the Santa Barbara Basin (this report), and Kennett and Kennett’s (2000) d18O G. bulloides record of SST (updated by Ingrid Hendy, 2008). Note that the shaded areas representing SST cooler than the long-term averages are concentrated between AD 800 and 1350, during the Medieval Climate Anomaly interval characterized by widespread drought in the western US.

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concentrations of domoic acid in particulate organic matter and recorded toxic events for higher organisms. These authors cited laboratory studies demonstrating that silica and phosphate stress caused increased domoic acid production by cells of Pseudonitzschia. They noted that Pseudonitzschia abundances did not correlate with chlorophyll a concentrations but did correlate with lower values of salinity such as are found in regions of river plumes. Greg W. Langlois of the California Department of Public Health noted (written comm., 2007) that there was a significant increase in domoic acid and Pseudonitzschia populations in the year 2000 in samples taken off the Santa Barbara coast. This phytoplankton data supports the abrupt increase of P. australis in the AD 2000–2001 sample of SBBC2902. Langlois’ collections began in the years AD 1992–1993, suggesting that a more quantitative box core study of Pseudonitzschia abundances prior to AD 1992 would prove useful in determining the magnitude of Pseudonitzschia increases off central California. 8. Conclusions Warm-water diatoms F. doliolus and N. interrupteseriata and the silicoflagellate D. stapedia are significantly much more common in box core assemblages of the years AD 1940–2001 than they are in ODP 893A assemblages representing the interval from w220 BC to AD 1880. These results suggest that the surface waters in the SBB during the past 60 years are significantly warmer than they were prior to the 20th century. The observation that the marine ecosystem had altered species assemblages by the mid-20th century in comparison with prior periods must be taken into consideration when comparing recent changes with respect to a long-term baseline in ecosystem variability. The combined relative abundance of F. doliolus and N. interrupteseriata in the box core varies with changes in SST anomalies, reinforcing their utility in SST reconstruction. When applied to the ODP 893A record, the F. doliolus–N. interrupteseriata SST proxy indicate a broad interval of cooler SSTs between wAD 800 and 1350 followed by an interval of warmer SSTs between wAD 1400 and 1800. These results support the SST interpretations of Kennett and Kennett (2000) that the California Current was cool during the MCA and warm during the LIA. The assemblage results also agree with published reconstructions of the Pacific Decadal Oscillation and the hypothesis that the widespread droughts of the Medieval Climate Anomaly (wAD 900– 1300) in the western US were associated with lower eastern Pacific SSTs (Herweijer et al., 2006; Graham et al., 2007). Pseudonitzschia australis, a diatom linked with domoic acid production, makes up 15% of the (Chaetoceros-free) diatom assemblage of the box core sample for AD 2000–2001, coinciding with a documented outbreak of shellfish poisoning in the SBB. Pseudonitzschia spp. were never observed at relative abundances of 1% or more in the ODP record of w220 BC–AD 1880, supporting the hypothesis that increase in bloom intensity and magnitude may be related to anthropogenic activity. Acknowledgments We are grateful to Amdt Schimmelmann and an anonymous reviewer for Quaternary International for their thorough reviews. Ingrid Hendy provided helpful comments on an earlier draft and updated the Kennett and Kennett (2000) isotope record. This manuscript benefited from reviews by Scott W. Starratt, Benita L. Murchey, and David Wahl of the USGS. We thank the Ocean Drilling Program for providing the samples from ODP 893.

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Appendix A

List of diatom taxa observed (abbreviations on Fig. 3): Actinocyclus curvatulus Janisch in Schmidt et al. 1878 (Ac) Actinoptychus spp., A. undulatus (Ku¨tzing) Ralfs in Pritchard 1861; Actinoptychus bipunctatus K.E. Lohman 1941 Alvelus marinus Kaczmarska et Fryxell 1996 Azpeitia africana (Janisch ex Schmidt) G. Fryxell et T. P. Watkins in Fryxell, Sims et Watkins 1986 Azpeitia nodulifera (Schmidt) G. Fryxell et P.A. Sims in Fryxell, Sims et Watkins 1986 (Az. nod.) Azpeitia tabularis (Grunow) G. Fryxell et P.A. Sims in Fryxell, Sims et Watkins 1986 Coscinodiscus radiatus Ehrenberg 1840 (C. rad.) Cyclotella spp. – C. stylorum Brightwell 1860 and C. striata (Ku¨tzing) Grunow in Cleve et Grunow 1880 (Cyc.) Delphineis spp. Fragilariopsis doliolus (Wallich) Medlin et Sims 1993 (F. dol.) Hemidiscus cuneiformis Wallich 1860 (H. cun.) Neodenticula seminae (Simonsen et Kanaya) Akiba et Yanagisawa 1985 Nitzschia interruptestriata Simonsen (N. int.) N. sp. cf. N. sicula (Castracane) Hustedt 1958 Paralia sulcata (Ehrenberg) Cleve 1873 Pseudonitzschia australis Frenguelli 1939; equivalent to Nitzschia pungens of Reimers et al. 1990. (Pseud.) Rhizosolenia spp. – mostly R. hebetata Bailey f. semispina (Hensen) Gran (Rz.) Roperia tesselata (Roper) Grunow ex Pelletan 1889 (Rop.) Stephanopyxis spp. – Stephanopyxis turris (Greville et Arnott) Ralfs in Pritchard 1861 and S. dimorpha Schrader 1973 Thalassionema nitzschioides (Grunow) Mereschkowsky 1902 includes T. bacillaris (Heiden et Kolbe) Kolbe 1955 and T. nitzschioides var. parva (Heiden) emend Moreno-Ruiz in Moreno-Ruiz et Licea 1995 (Thalassionema) Thalassiosira eccentrica (Ehrenberg) Cleve emend Fryxell et Hasle 1972 (T. ecc.) Thalassiosira oestrupii (Ostenfeld) Proschkina-Lavrenko ex Hasle 1960 (T.o.) Thalassiosira pacifica Gran et Angst 1930 (T. pac.) T. spp. – includes T. lineata Jouse´ 1968, T. aff. pacifica, and T. decipiens (Grunow) Jørgensen 1905 (T. sp.) Thalassiothrix longissima Cleve et Grunow in Grunow 1880 Other planktic – rare taxa including, Coscinodiscus plicatoides Simonsen 1974, Nitzschia bicapitata Cleve 1900 Benthic Freshwater Reworked List of silicoflagellate taxa observed (abbreviations on Fig. 4): Dictyocha aculeata (Lemmermann) Dumitrica, 1973 (D. acul.) Dictyocha calida Poelchau, 1976 (D. cal.) Dictyocha perlaevis Frenguelli, 1951 (D. per.) Dictyocha stapedia Haeckel, 1887 Dictyocha stapedia (large form) (D. st. l.) Dictyocha stapedia aspinosa Haeckel, 1887 (D. st. a.) Distephanus octangulatus Wailes, 1939 (Ds. oct.) Distephanus speculum minutus (Bachmann) Bukry, 1976. As emended by Bukry (1981), this form has an apical ring that in plan view is contiguous or overlaps the basal ring (Ds. spec. min.) Distephanus speculum speculum (Ehrenberrg) Haeckel, 1887 (Ds. spec. spec.) Octactis pulchra Schiller, 1925 (O. p.)

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Appendix B. Supplementary data Supplementary data associated with this article can be found in the online version, at doi:10.1016/j.envpol.2007.09.017. References Barron, J.A., Bukry, D., 2007. Development of the California Current during the past 12,000 years based on diatoms and silicoflagellates. Palaeogeography, Palaeoclimatology, Palaeoecology 248, 333–338. Barron, J.A., Bukry, D., Bischoff, J.L., 2004. High resolution paleoceanography of the Guaymas Basin, Gulf of California, during the past 15,000 years. Marine Micropaleontology 50 (3–4), 185–207. 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. Barron, J.A., Heusser, L., Herbert, T., Lyle, M., 2003. High resolution climatic evolution of coastal northern California during the past 16,000 years. Paleoceanography 18, doi:10.1029/2002PA000768. Bemis, B.E., Spero, H.J., Thunell, R.C., 2002. Using species-specific paleotemperature equations with foraminifera: a case study in the Southern California Bight. Marine Micropaleontology 46, 405–430. Black, D.E., Thunell, R.C., Tappa, E.J., 2001. Planktonic foraminiferal response to the ˜ o: a sediment-trap record from the Santa Barbara Basin. 1997–1998 El Nin Geology 29, 1075–1078. Bograd, S.J., Schwing, F.B., Mendelssohn, R., Green-Jessen, P., 2002. On the changing seasonality over the North Pacific. Geophysical Research Letters 29 (9), doi:10.1029/2001GL013790. Bukry, D., 1976. Silicoflagellate and coccolith stratigraphy, southwestern Pacific Ocean, Leg 34. Deep Sea Drilling Project, Initial Reports 34, pp. 715–735. ˜ o/Southern OscillaCobb, K.M., Charles, C.D., Cheng, H., Edwards, R.L., 2003. El Nin tion and tropical Pacific climate change during the last millennium. Nature 424, 271–276. Cook, E.R., Woodshouse, C.A., Eakin, C.M., Meko, D.M., Stahle, D.W., 2004. Long-term aridity changes in the western United States. Science 306, 1015–1018. Di Lorenzo, E., 2003. Seasonal dynamics of the surface circulation in the Southern California Current System. Deep Sea Research II 50, 2371–2388. Field, D.B., 2004. Variability in vertical distributions of planktonic foraminifera in the California Current: relationships in vertical ocean structure. Paleoceanography 19, PA2014, doi:10:1029/2003PA000970. Field, D.B., Baumgartner, T.R., Charles, C.D., Ferreira-Bartrina, V., Ohman, M.D., 2006a. Planktonic foraminifera of the California Current reflect 20th-Century warming. Science 311, 63–66. Field, D., Cayan, D., and Chavez, F., 2006b. Secular warming in the California Current and North Pacific. California Cooperative Ocean Fisheries Investigation (CalCOFI) Report 47, pp. 1–17. Fisler, J., Hendy, I.L., 2008. California Current System response to late Holocene climate cooling in southern California. Geophysical Research Letters 35, L09702, doi:10.1029/2008GL033902. Friddell, J.E., Thunell, R.C., Guilderson, T.P., Kashgarian, M., 2003. Increased northeast Pacific climatic variability during the warm middle Holocene. Geophysical Research Letters 30 (11), 1–4. Fritz, L., Quilliam, M.A., Wright, J.L.C., Beale, A.M., Work, T.M., 1992. An outbreak of domoic acid poisoning attributed to the pinnate diatom Pseudonitzschia australis. Journal of Phycology 28, 439–442. Fryxell, G.A., Villac, M.C., Sapiro, L.P., 1997. The occurrence of the toxic diatom genus Pseudo-nitzschia (Bacillariophyceae) on the West Coast of the USA, 1920–1996: a review. Phycologia 36, 419–437. Graham, N.E., Hughes, M.K., Ammann, C.M., Cobb, K.M., Hoerling, M.P., Kennett, D.J., Kennett, J.P., Rein, B., Stott, L., Wigand, P.E., Taiyi, Xu, 2007. Tropical Pacific – mid-latitude teleconnections in medieval times. Climate Change 83 (1–2), 241–285. Hardee, M.L., Thunell, R.C., 2006. 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