5.1. Millennial Timescale Variability of the Diatom Production in the Panama Basin
 MD02-2529 data for total diatom concentrations provide clear evidence for millennial variability in diatom production in the Panama Basin during the last glacial period (Figure 2). Minima of total diatom concentration broadly cooccurred with maxima in the oligotrophic assemblage from group 2 (Figures 3 and 4) and correspond to HEs (Figure 2; see also section 5.3) Such a pattern is also observed in the opal sedimentary content, a parameter not drastically affected by dissolution at MD02-2529 until ∼18.5 ka B.P., as relatively good valve preservation suggests (Figures 2b and 2c). All together these results indicate that millennial-scale changes in diatom production closely followed the Heinrich-Dansgaard/Oeschger climate variability already recorded in the tropical Pacific hydrological characteristics and presumably linked to latitudinal migrations of the ITCZ [Leduc et al., 2009]. Even though every millennial-scale event recognized in Greenland ice cores during the MIS3, 4 and 5 [Bond et al., 1993] is not clearly mirrored by diatom values, maxima in total diatom concentration occurring during early MIS 4 as well as also during the MIS 4/3 transition and MIS 3 mimic Bond cycles and/or the SSS reconstruction derived from planktonic foraminifera at MD02-2529 (Figure 2), indicating a clear linkage between diatom production in the coastal EEP and rapid climate changes in the high-latitude North Atlantic.
Figure 4. Summary of the peak occurrences of the diatom groups, as represented by horizontal color bars, at MD02-2529 during the time period 98–10 ka B.P. The horizontal shadings are Group 1 (gray), Group 2 (blue), Group 3 (pink), Group 4 (green), and Group 5 (orange). For the qualitative composition of each diatom group (factor) see Table 1. The measured color reflectance (550 nm) of Cariaco Basin sediments is from ODP Hole 1002C [Peterson et al., 2000]. The value Δδ18O (‰ versus SMOW) is a proxy for local seawater salinity at MD02-2529 [Leduc et al., 2007]. The vertical shadings denote Heinrich events (H, gray) and the Last Glacial Maximum (LGM, yellow.)
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 Diatom production in the Panama Basin is expected to respond to changes in surface water nutrient concentration as triggered by changes in wind conditions. The present-day diatom production along the southwestern Central American coast is heavily seasonal and strongly wind-dependant [Fiedler, 2002; Pennington et al., 2006]. During July through October, as the ITCZ moves northward [Amador et al., 2006], upwelling curl lifts the 10°N thermocline ridge across the entire Panama basin [Kessler, 2006] and the CRD develops west of MD02-2529 coring site (Figure 1). From November through January, the ITCZ migrates southward, and northeast trade winds blow strongly through Central American low-level mountain channels and trigger coastal upwelling northwest and southeast of MD02-2529 coring site [Amador et al., 2006; Fiedler, 2002; Kessler, 2006] (see also section 2). These two distinct modes of atmospheric circulation are responsible for two distinct winter and summer spikes in organic matter fluxes to the seafloor in the Panama Basin [Honjo, 1982], and both are candidates for explaining the diatom productivity patterns on longer timescales.
 The relationship between higher diatom concentration and northernmost migration of the ITCZ during the last glacial period, as shown by the reflectance record at ODP Site 1002 in the Cariaco Basin (Figure 4), may explain the variability of the diatom production in the Panama Basin. Such atmospheric variability translated into the modern-day seasonal cycle corresponds to times when the CRD shoals and seeds most of the Panama Basin, hence inducing seasonal chlorophyll concentration maxima over site MD02-2529 (Figure 1). Such atmospheric conditions associated with northward migrations of the ITCZ might have been responsible for upwelling during periods when northern Panama Basin surface salinity was lowest (Figure 4).
 In addition to the ITCZ, persistent regional-scale winds blow through low-level mountain channels in the Central American narrow continental band [Chelton et al., 2000; Kessler, 2006]. Opposite signs of wind stress curl generated by the Papagayo and Panama jets act to maintain local wind stress convergence at the MD02-2529 coring site, which is situated in the lee of the Talamanca Cordillera [Chelton et al., 2000]. This local atmospheric pattern prevents coastal upwelling at the core location, as further suggested by a local SST maximum bracketed by coastal upwelling situated east and west of the MD02-2529 site [see Pennington et al., 2006, Figure 6]. Such regional heterogeneity cannot change through time since it is linked to the topography of Central America. Therefore, increases in Papagayo and Panama coastal upwelling intensity as observed nowadays during winter may be valid for HEs since stronger northeastern trade winds are expected over these time periods [Pahnke et al., 2007]. These local, rapid atmosphere-ocean interactions provide a potential explanation for why diatom production may have decreased at the MD02-2529 core location at these times, and may explain the absence of a glacial/interglacial pattern of variability in the diatom production as shown in downcore records from pelagic areas of the EEP [Dubois et al., 2010].
5.2. Temporal Shifts of the Diatom Assemblage at Site MD02-2529
 The most prominent temporal trends in the diatom assemblage at MD02-2529, summarized now in Figure 4, are (1) the demise of diatom group 1 (predominantly coastal) after 62 ka B.P., (2) the highest contribution of diatom group 4 (coastal/pelagic) between 64 and 54 ka B.P., (3) the rise of diatom group 3 (pelagic/coastal) after 62 ka B.P., and (4) the rise of diatom group 5 (pelagic) after 27 ka B.P. The long-term succession of these four groups points to a longer trend from coastal diatoms, which predominantly occurred during MIS 5 and 4, toward pelagic diatoms, which occurred during MIS 3 and 2 in the Panama Basin (Figure 4). Apart from this first-order trend, there is a superimposed second-order variability displayed in the assemblages on a millennial timescale as well. This second-order pattern is expressed by increases in the relative contribution of diatom group 2 (coastal, oligo-mesotrophic) and cooccur with minima of opal content and diatom concentration that are concomitant with HEs (Figure 4; see also section 5.3).
 The prevalence of coastal diatoms between 98 and 62 ka B.P. (group 1, gray shading, Figure 4) is likely linked to the dominance of the CRCC over site MD02-2529 at these times. The neritic Thalassionema nitzschioides var. nitzschioides, which is the main component of group 1, represents boreal spring production in coastal areas of the low-latitude Pacific Ocean [Sancetta, 1992]; its dominance beyond surface waters of the upper continental slope indicates strong scattering of coastal waters into more pelagic areas [Romero et al., 2008; Romero and Armand, 2010]. Toward the late MIS 4, the increase in the dominance of coastal/pelagic group 4 (green shading, Figure 4) at the expense of the neritic group 1 suggests a reorganization of regional surface ocean dynamics over the MD02-2529 site. Around 62 ka B.P., the increase in the relative contribution of the pelagic/coastal group 3 (pink shading, Figure 4) likely reveals the shoreward spreading of pelagic and hemipelagic waters with moderate nutrient content. This trend into more pelagic waters over site MD02-2529 continued well into the MIS 2 and the last deglaciation.
 The decrease in the total diatom concentration recorded after 18.5 ka B.P. corresponds to times when the dominance of nutrient-depleted water diatoms are the most expressed in the assemblages (groups 5 and 2, Figure 4). Fragilariopsis doliolus, the main contributor to group 5, mainly occurs in warm, nutrient-poor waters located along the southwestern coast of North America [Romero and Armand, 2010, and references therein] and is associated with gyre waters moving shoreward when the California Current relaxes during late summer and early fall [Sancetta, 1992; Barron et al., 2003]. A similar decrease in diatom production after 20 ka B.P. has been already recorded in the Gulf of California [Sancetta, 1992]. Such features can probably be linked to similar features identified upstream along the California margin during periods of glacial maxima [Herbert et al., 2001]. The strong reduction in diatom values, the marked shift in species composition and the poor valve preservation after the LGM (Figures 2 and 4) together point toward the dominance of warmer waters and an increasingly stratified upper water column over the MD02-2529 site. Although opal fluxes at MD02-2529 are not Al- or Th-normalized, we assume here that the abrupt decrease in sedimentary opal content after the LGM differs from those recorded in pelagic EEP locations. Within the EEP, opal/Al values [Kienast et al., 2006; Dubois et al., 2010] and Th-normalized opal fluxes [Bradtmiller et al., 2006] suggest that diatom productivity was lower during LGM as compared to the deglaciation. Pelagic EEP sites, however, are not necessarily impacted by the same processes as at MD02-2529 site which is much closer to the Central American margin (see also discussion by Dubois et al. ).
 The abrupt decrease in the valve preservation recorded after the LGM in the coastal Panama Basin (Figure 2) agrees well with observations in the pelagic EEP at Site 849 (i.e., at ∼110°W near the equator). Based on the electron microscopy surveys of valves of the well-silicified Azpeitia nodulifera, Warnock et al.  observed stronger valve dissolution during the deglaciation than earlier during LGM. Our observations, however, contrast with those recently presented by Dubois et al. . Using geochemical proxies in a series of sediment cores from pelagic areas of the tropical eastern Pacific, Dubois et al.  offered evidence of enhanced opal preservation during deglaciation. This contradiction can be partly explained by the use of different proxies: while Warnock et al.  and the present study characterized the valve preservation with microscopy observations, while Dubois et al.  used biogeochemical proxies. Since only one study assessing the preservation of diatom remains in the pelagic EEP is available so far [Warnock et al., 2007], further studies of species composition and valves preservation at other EEP core locations are required to assess whether the deglaciation was more or less corrosive with respect to opal.
5.3. Global Hydrographic Changes and the Diatom Paleoproduction in the Panama Basin
 One of the most remarkable features observed in diatom production recorded in MD02-2529 sediments is the dramatic decrease observed during HEs. To explain the low diatom production recorded during HEs in the Northern Hemisphere oceans, two opposite hypotheses have been proposed. The first hypothesis interprets the low diatom concentration during HEs as a consequence of the drastic decrease in surface water productivity [Nave et al., 2007]. The second hypothesis, however, considers that diatom productivity increased, probably in response to enhanced nutrient input from the continents, and that this increase would not have been recorded in the sediments [Sancetta, 1992]. Drastic increases in nutrient-depleted water diatom assemblages coeval with decreases in diatom counts recorded at site MD02-2529 favor the hypothesis that the low diatom concentrations during HEs 6 to 3 mirrors abrupt decreases in the diatom production in the Panama Basin (Figure 3). It would additionally suggest a reduced CRD activity over these time intervals despite increased Tehuantepec, Papagayo and Panama northeast wind jets. Such a hypothesis is in line with a regional ocean-atmosphere model from Central America that accurately simulates the seasonal cycle of atmospheric and oceanic circulation in the region [Xie et al., 2007] and further points to increased wind jets and decreased CRD activity in water hosing experiments [Pahnke et al., 2007].
 Water hosing experiments using earth system models of intermediate complexity that incorporate changes in the carbon cycle, in nutrient availability and in the response of phytoplankton communities provide further diagnostics for changes in the EEP productivity during HEs [Schmittner et al., 2007; Menviel et al., 2008]. These models indicate that the reduction in the Atlantic Meridional Overturning Circulation (AMOC) caused a year-round halocline in surface waters overlying site MD02-2529, a reduction in the nutrients supplied by upwelled waters and strong variations in export production over broad geographic regions without significant time lags [Schmittner et al., 2007; Menviel et al., 2008]. Other simulations suggest that the upwelled water in equatorial regions of the oceans originated from the subtropics during HEs, as circulation in the Pacific Ocean was dominated by subtropical cells [Haarsma et al., 2008]. The global reduction in the amount of water upwelled together with changes in the source of upwelling water led to a reduction in the nutrient content in the upper 1000 m and to a nutrient accumulation in the deep ocean during HEs [Menviel et al., 2008].
 Such hydrological effects simulated for HEs agree well with abrupt changes in the dominance of diatom populations at MD02-2529. In addition to the decrease of diatom and opal concentrations during HEs, maxima of the well-silicified diatom C. litoralis (group 2, blue shading, Figure 4) reflect stratified, nutrient-limited waters. A present-day analog is given by a 3 year sediment trap study carried out in the Gulf of California where C. litoralis was dominant during summers, that is, when the lithogenic flux is high and opal values are low [Sancetta, 1995]. Evidence previously attained at MD02-2529 relates the southernmost migration of the ITCZ with SSS increases during HEs [Leduc et al., 2007, 2009]. Associated water column stratification over site MD02-2529 possibly limited the input of nutrients into the euphotic zone, leading to a diatom production decrease. Additionally, the poorest valve preservation during HEs (Figure 2) also suggests H4SiO4-depleted surface waters in the Panama Basin at these times.
 The predominant explanation for opal variations along the pelagic EEP has been the silica leakage hypothesis [e.g., Brzezinski et al., 2002; Matsumoto et al., 2002]. It has been speculated that during glacial periods the excess H4SiO4 was transported from the Southern Ocean to lower latitudes within Subantarctic Mode Waters [Matsumoto et al., 2002]. Subsequently, Chase et al.  suggested that high-latitude sea ice cover was the prime factor responsible for reduced Si utilization in glacial Antarctic surface waters. Compared to the pelagic locations along the EEP, piston core MD02-2529 was collected in an area affected by a variety of processes such as seasonal changes in upwelling and wind intensity, and depth variations of the thermocline (see section 2). The comparison of opal records from the pelagic EEP with that of MD02-2529 delivers some insights into the nutrient dynamics of the low-latitude Pacific as a whole. The weak temporal relationship between opal records from MD02-2529 and more pelagic zones [Kienast et al., 2006; Bradtmiller et al., 2006, 2010; Dubois et al., 2010] still needs to be tested and requires a denser net of cores situated between the Panama Basin and the pelagic EEP. In addition, it also requires the assessment of the quantitative variations in the diatom assemblages in pelagic EEP locations.
 The low correlation between total diatom concentration and valve preservation (R = 0.38) suggests that the preserved diatom signal at site MD02-2529 weakly depends on the species composition of the diatom assemblage. Pichevin et al.  proposed that the Si- and Fe-replete conditions in the EEP during the LGM led to lowered Si utilization and diatom silicification, which may further have favored dissolution of opal. Although this hypothesis might be valid for the pelagic EEP, there is no clear evidence for long-term opal burial changes on the glacial-interglacial timescale at site MD02-2529 prior to the LGM. Our results indicate that coastal areas surrounding the Panama Basin were less affected by large-scale process such as nutrient export from high latitudes into pelagic areas from the EEP during the last glacial cycle. We therefore suggest that local hydrological variability induced by atmospheric circulation changes, potentially exacerbated by other large-scale processes such as those identified in water hosing experiments, best explains our diatom records that differ from those recorded in more pelagic areas.