Sea Surface Temperature Reconstruction Research Articles

Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: <i>p</i>CO<sub>2</sub> and paleo productivity

Abstract. A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ13Cphytane), a biomarker for photosynthetic algae. The δ13Cphytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7‰) compared to existing C/T proto-North Atlantic δ13Cphytane records (3–6‰). Overall, the amplitude of the excursion of δ13Cphytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ13Cphytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ13Cphytane-records from Site 1260, we subsequently reconstructed pCO2 levels across the OAE-2 interval. Accordingly, pCO2 decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO2. Whereas the onset of OAE-2 coincided with increased pCO2, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO2 sequestration in black shales outcompeting CO2 input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO2 related to perturbations of the global carbon cycle.

Structural analysis and paleoenvironmental potential of dog cockle shells (Glycymeris glycymeris) in Brittany, northwest France

To compensate for the lack of high resolution data on variation in environmental parameters before the establishment of monitoring networks, we evaluated dog cockle shells (Glycymeris glycymeris; Mollusca: Bivalvia) as archives for temperature and climate perturbations such as the North Atlantic Oscillation (NAO) over the last 50years. Oxygen isotope compositions were measured from annual increments of shells collected from the Bay of Brest and the Chausey Islands in northwestern France. Simultaneously, sclerochronological analyses were performed on the same shells. Oxygen isotopic ratios measured in shells collected from the Bay of Brest were compared to in situ temperature and salinity data from monitoring stations and buoys in order to build a specific paleotemperature equation (r2=0.87; p<0.001) for the dog cockle. The specific equation was used to estimate sea surface temperatures around the Chausey Islands between 1966 and 1994. Standardized growth indices (SGI) calculated through sclerochronological methods were strongly correlated to oxygen isotope-derived temperatures. The SGI however did not capture the area's predominant climate pattern, the NAO. This study demonstrates that dog cockle shells can be used for long-term high-resolution reconstructions of past sea surface temperatures.

Early Pleistocene Variability of SST Record by Foraminifera Shell Mg/Ca Ratio at IODP Site U1313,North Atlantic

In this paper, we measured Mg/Ca ratio in planktonic foraminifera shell of Globigerinoides ruber (white) from early Pleistocene (2414.2~2253.77ka) North Atlantic deep sea sediment drilled at Site U1313, IODP Expedition 306, and tested 6 transform functions that used to reconstruct sea surface temperature (SST) with foraminifera shell Mg/Ca ratio, the result showed that the theoretical equation of Elderfield and Ganssen (2000) and the equation of Anand (2003) both are appropriate, but there is big difference between their results, so we use their average value as the final results to reflect SST in the North Atlantic area in early Pleistocene. The fluctuation of the SST we reconstructed and that of the benthic foraminifera oxygen isotopic are basically same, but in the stages of MIS 87 to 95, there exist 2.71ka to 19.54ka error, probably it was caused by the difference of the response to changes in water temperature between the planktonic foraminifera and the benthic foraminifera.

Enhanced warming over the global subtropical western boundary currents

Subtropical western boundary currents are warm, fast-flowing currents that form on the western side of ocean basins. They carry warm tropical water to the mid-latitudes and vent large amounts of heat and moisture to the atmosphere along their paths, affecting atmospheric jet streams and mid-latitude storms, as well as ocean carbon uptake1, 2, 3, 4. The possibility that these highly energetic currents might change under greenhouse-gas forcing has raised significant concerns5, 6, 7, but detecting such changes is challenging owing to limited observations. Here, using reconstructed sea surface temperature datasets and century-long ocean and atmosphere reanalysis products, we find that the post-1900 surface ocean warming rate over the path of these currents is two to three times faster than the global mean surface ocean warming rate. The accelerated warming is associated with a synchronous poleward shift and/or intensification of global subtropical western boundary currents in conjunction with a systematic change in winds over both hemispheres. This enhanced warming may reduce the ability of the oceans to absorb anthropogenic carbon dioxide over these regions. However, uncertainties in detection and attribution of these warming trends remain, pointing to a need for a long-term monitoring network of the global western boundary currents and their extensions.

Mid-Holocene sea-surface temperature reconstruction using fossil corals from Kume Island, Ryukyu, Japan

The relative warmth and stability of the Holocene was punctuated by several brief cold events. Whereas these cold events on a global scale are widely reported, the lack of records from regions such as the East China Sea (ECS) results in an incomplete understanding of the underlying cooling mechanism. Here, we present a coral-based paleo-SST (sea-surface temperature) reconstruction from the ECS to constrain Holocene variability in the Kuroshio Western Boundary Current and the East Asian Monsoon (EAM). Our new data confirm that cold conditions prevailed at 3.8 cal kyr BP, which is consistent with the previously-reported Pulleniatina Minimum Event (PME). While this previous reconstruction could not reveal seasonal differences, our high-resolution data indicate a differing seasonal SST response between summer and winter. This result provides an important insight into understanding the mechanism of the millennium scale cold event in the ECS, the region affected by EAM.

Regional Patterns of Tropical Indo-Pacific Climate Change: Evidence of the Walker Circulation Weakening

Regional patterns of tropical Indo-Pacific climate change are investigated over the last six decades based on a synthesis of in situ observations and ocean model simulations, with a focus on physical consistency among sea surface temperature (SST), cloud, sea level pressure (SLP), surface wind, and subsurface ocean temperature. A newly developed bias-corrected surface wind dataset displays westerly trends over the western tropical Pacific and easterly trends over the tropical Indian Ocean, indicative of a slowdown of the Walker circulation. This pattern of wind change is consistent with that of observed SLP change showing positive trends over the Maritime Continent and negative trends over the central equatorial Pacific. Suppressed moisture convergence over the Maritime Continent is largely due to surface wind changes, contributing to observed decreases in marine cloudiness and land precipitation there. Furthermore, observed ocean mixed layer temperatures indicate a reduction in zonal contrast in the tropical Indo-Pacific characterized by larger warming in the tropical eastern Pacific and western Indian Ocean than in the tropical western Pacific and eastern Indian Ocean. Similar changes are successfully simulated by an ocean general circulation model forced with the bias-corrected wind stress. Whereas results from major SST reconstructions show no significant change in zonal gradient in the tropical Indo-Pacific, both bucket-sampled SSTs and nighttime marine air temperatures (NMAT) show a weakening of the zonal gradient consistent with the subsurface temperature changes. All these findings from independent observations provide robust evidence for ocean–atmosphere coupling associated with the reduction in the Walker circulation over the last six decades.

Oceanographic changes through the last millennium off North Iceland: Temperature and salinity reconstructions based on foraminifera and stable isotopes

Abstract Temperature and salinity reconstructions for two 1000-year high-resolution sedimentary records, located at the boundary between Atlantic and Arctic surface waters on the North Icelandic shelf, are based on transfer functions and oxygen isotopes for planktonic and benthic foraminifera. There is a general increase of Arctic Water indicator species at the transition from the Medieval Warm Period into the Little Ice Age (LIA) and a subsequent return of Atlantic Water indicator species towards the end of the LIA and in the 20th century. The timing of the reconstructed temperature changes, both at the beginning and at the end of the LIA, appears to be slightly different for the different water masses. The earliest temperature change is seen in the bottom and subsurface waters, where a cooling is reconstructed as early as AD 1150–1200 at both locations, whereas previously published diatom-based and alkenone-based sea-surface temperature reconstructions show a change at AD 1300, coinciding with the air temperature shift in the area. Our results show the need of a thorough understanding of the oceanography in the study area, as well as the different living habitat for the biological proxies used for the temperature estimates.

Multidecadal variability and late medieval cooling of near‐coastal sea surface temperatures in the eastern tropical North Atlantic

Multidecadal variations in Atlantic sea surface temperatures (SST) influence the climate of the Northern Hemisphere. However, prior to the instrumental time period, information on multidecadal climate variability becomes limited, and there is a particular scarcity of sufficiently resolved SST reconstructions. Here we present an eastern tropical North Atlantic reconstruction of SSTs based on foraminiferal Mg/Ca ratios that resolves multidecadal variability over the past 1700 years. Spectral power in the multidecadal band (50 to 70 years period) is significant over several time intervals suggesting that the Atlantic Multidecadal Oscillation (AMO) has been influencing local SST. Since our data exhibit high scatter the absence of multidecadal variability in the remaining record does not exclude the possibility that SST variations on this time scale might have been present without being detected in our data. Cooling by ∼0.5°C takes place between about AD 1250 and AD 1500; while this corresponds to the inception of the Little Ice Age (LIA), the end of the LIA is not reflected in our record and SST remains relatively low. This transition to cooler SSTs parallels the previously reconstructed shift in the North Atlantic Oscillation toward a low pre‐20th century mean state and possibly reflects common solar forcing.

Sea surface temperature variability in the subpolar Atlantic over the last two millennia

A decadal resolution time series of sea surface temperature (SST) spanning the last two millennia is reconstructed by combining a proxy record from a new sediment sequence with previously published data from core MD99–2275, north of Iceland. The alkenone based SST reconstruction is validated with historic observational data and compared to a new similar temporal resolution reconstruction obtained from sediment core RAPiD21–3K, in the subpolar North Atlantic. The two SST paleorecords show consistent multidecadal scale coolings throughout the interval and similar expressions during the contrasted climatic periods ‘colloquially known’ as the Medieval Climatic Anomaly (MCA) and Little Ice Age (LIA). In order to further understand the temporal and spatial SST variations and investigate the influence of natural forcings on the observed SST changes during the last millennium, we compare our time series to simulations using the Institut Pierre‐Simon Laplace IPSLCM4‐v2 climate model. This comparison highlights the potential importance of volcanism as a natural forcing driving coherent abrupt cooling events captured in the subpolar North Atlantic records.

Soil pH impact on microbial tetraether lipids and terrestrial input index (BIT) in China

The distributions of microbial tetraether lipids, i.e. glycerol dialkyl glycerol tetraethers (GDGTs), were investigated in a variety of Chinese soils featured by a wide range of pH values in different climate zones. The bacterial branched GDGTs and archaeal isoprenoid GDGTs were found to be widely distributed in Chinese soils and their relative abundance was controlled primarily by soil pH. The bacterial bGDGTs usually dominate over archaeal iGDGTs in abundance when the soil pH is below 8.0. However, archaeal iGDGTs become dominant and the abundance of halophilic archaea characterized by GDGT V without cyclopentyl ring increases dramatically in the alkaline soil with pH greater than 8.0. The relative abundance of archaeal iGDGTs to bacterial bGDGTs was defined here as RAI index. The RAI index decreases with elevated soil pH and a linear correlation is found between them, indicating that the relative abundance of archaeal iGDGTs to bacterial bGDGTs increases with increased pH. The RAI index thus could be a novel proxy of the pH values of paleosol. The terrestrial input index, BIT (Branched and Isoprenoid Tetraethers Index), decreases with increased soil pH, suggesting that soil pH should be taken into consideration when the terrestrial contribution to marine sediments is estimated by BIT index. Moreover, the TEX86 index cannot be used to reconstruct sea surface temperature in the sediments featured by a large terrestrial input.

Variations of Indian monsoon precipitation during the last 32 kyr reflected in the surface hydrography of the Western Bay of Bengal

Hydrography of the Bay of Bengal is highly influenced by the river runoff and rainfall during the southwest monsoon. We have reconstructed δ 18Osw, sea surface salinity and sea surface temperature (SST) changes in the Bay of Bengal by using paired measurements of δ 18O and Mg/Ca in a planktonic foraminifera species Globigerinoides ruber from core SK218/1 in the western Bay of Bengal in order to understand the rainfall variability associated with southwest monsoon over the past 32 kyr. Our SST reconstructions reveal that Bay of Bengal was ∼3.2 °C cooler during the LGM as compared to present day temperature and a ∼3.5 °C rise in SST is documented from 17 to 10 ka. Both SST and δ 18Osw exhibit greater amplitude fluctuations during MIS 2 which is attributable to the variability of NE monsoon rainfall and associated river discharge into the Bay of Bengal in association with strong seasonal temperature contrast. On set of strengthening phase of SW monsoon was started during Bølling/Allerød as evidenced by the low δ 18Osw values ∼14.7 ka. δ 18Osw show consistently lower values during Holocene (with an exception around 5 ka), which suggests that the freshening of Bay of Bengal due to heavy precipitation and river discharge caused by strong SW monsoon. Results of this study signify that the maximum fluctuations of the NE monsoon rainfall during MIS 2 appear to be controlled by the strong seasonality and boundary conditions.

The effect of sea surface properties on shell morphology and size of the planktonic foraminifer Neogloboquadrina pachyderma in the North Atlantic

The variability in size and shape of shells of the polar planktonic foraminifer Neogloboquadrina pachyderma have been quantified in 33 recent surface sediment samples throughout the northern Atlantic Ocean and correlated with the properties of the ambient surface waters. The aim of the study was to determine whether any of the morphological features could be used to reconstruct sea surface properties in the polar realm of the North Atlantic, where most paleotemperature proxies appear to fail. The analyses revealed that shell morphology is only weakly controlled by habitat properties, whereas shell size showed a strong correlation with sea surface temperature. The regression of mean shell size on sea surface temperature revealed the presence of two trends among the sinistrally coiled shells: a continuous increase in shell size with decreasing SST in sediments deposited under polar water masses and a continuous increase in shell size with increasing SST in samples from transitional waters. The second trend mirrors the trend observed for dextrally coiled shells, which are frequent in the same samples and signal the presence of N. incompta. The identical mean shell size trends among the sinistral and dextral specimens in the temperate samples confirms the results of earlier genetic studies which indicated the existence of a small but distinct proportion of opposite coiling in N. incompta, to which the sinistral shells in the temperate samples could be attributed. The linear correlation between mean shell size and sea surface temperature in the polar domain (summer SST<9°C) has been used to develop an empirical formula for the reconstruction of past sea surface temperatures from shell sizes in fossil samples. The standard error of the residuals of the linear regression is 2.36°C (1 sigma), which implies a much larger error than for most paleothermometers, but enough precision to allow resolution between results by individual paleothermometers in the polar domain. The resulting regression model has been applied on two sediment cores spanning the interval from the Last Glacial Maximum (LGM) to the present day. The results from core PS1906-1 are consistent with ice-free conditions during the LGM in the Norwegian Sea. The SST estimates for the LGM inferred from N. pachyderma shell size are similar or slightly higher than those for the latest Holocene. The results do not indicate anomalously high SST during the glacial and the LGM reconstructions thus appear more consistent with the results from foraminiferal transfer functions and geochemical proxies. Both sediment cores show the highest reconstructed SST during the early Holocene insolation optimum.

Alkenone distribution in surface sediments of the southern Yellow Sea and implications for the $$ {\text{U}}^{{{\text{K}}\prime}}_{{37}} $$ thermometer

The $$ {\text{U}}^{{{\text{K}}\prime}}_{{37}} $$ index has been widely applied for sea surface temperature (SST) reconstruction in open ocean environments, but has inherently limited applications at smaller, regional scales including some marginal seas where both historical and reconstructed SST records are urgently needed for understanding regional climate evolution. We determined the spatial distribution of alkenone contents in surface sediments from the southern Yellow Sea to assess the regional $$ {\text{U}}_{{{37}}}^{{{\text{K'}}}} $$ —SST relationship for paleo-SST reconstructions. C37:2 and C37:3 alkenones were detected at all 36 sites covering most of the southern Yellow Sea. Alkenone content ranges from 17 to 1,063 ng/g, with high values (ca. 400 to 1,000 ng/g) at deep water sites and a decreasing trend shoreward. For six samples at shallower depths near the coast and further offshore, the values were too low for statistical evaluation. This spatial pattern of alkenone contents is consistent with existing knowledge on the spatial distribution and productivity of alkenone-producing coccolithophorid species in the region. There is a significant positive relationship ( $$ {\text{U}}_{{{37}}}^{{{\text{K'}}}} = 0.059{\text{SST}} - 0.350 $$ , R = 0.912, n = 30) between the $$ {\text{U}}_{{{37}}}^{{{\text{K'}}}} $$ values and satellite-derived annual mean SSTs (0 m) for the last 27 years, providing support for the application of a region-specific $$ {\text{U}}_{{{37}}}^{{{\text{K'}}}} $$ index as paleothermometer in the southern Yellow Sea. However, the slope of the southern Yellow Sea calibration (0.059) is considerably larger than that of the well-known global core-top calibration (0.033). This implies that global SST trends may not adequately encompass regional SST patterns and/or that environmental factors other than temperature may gain importance in explaining coccolithophore dynamics in marginal seas.

Cold tongue/Warm pool and ENSO dynamics in the Pliocene

Abstract. It has been suggested that a "permanent" El Niño climate state has existed in the warm Pliocene. One of the main pieces of evidence of such conditions is the small east-west sea surface temperature (SST) difference that is found in proxy temperature records of the equatorial Pacific. Using a coupled version of the Zebiak-Cane model of intermediate complexity for the tropical Pacific, we study the sensitivity of the time-mean Pacific background state and El Niño/Southern Oscillation (ENSO) variability to Pliocene climate changes. The parameters varied in this sensitivity study include changes in the trade wind strength due to a reduced equator-to-pole temperature gradient, higher global mean temperatures and an open Panama gateway. All these changes lead to a westward shift of the position of the cold tongue along the equator by up to 2000 km. This result is consistent with data from the PRISM3D Pliocene SST reconstruction. Our model further suggests that ENSO variability is present in the Pliocene climate with only slight changes as compared to today. A background climate that would resemble a "permanent" El Niño with weak to no east-west temperature difference along the equator is only found for very weak trade winds which seem unrealistic for the Pliocene climate.

Instrumental validation ofGlobigerinoides ruberMg/Ca as a proxy for NE Pacific summer SST

[1] Accurate reconstruction of sea surface temperature (SST) is a high research priority, given that it is such a crucial variable in the Earth's climate system. The Mg/Ca composition of Globigerinoides ruber (white) has been calibrated and applied for a number of tropical and extratropical paleo-SST reconstructions, though validation studies of the proxy against instrumental observations are relatively scarce. Here we present a validation of G. ruber Mg/Ca-derived SSTs against instrumental summer values, firstly from the modern seasonal water column perspective, and secondly from a 20th century observational time series. The study occurs in the San Lazaro Basin (SLB), one of the marginal basins in the NE Pacific known for very high sedimentation rates, excellent preservation, laminated sequences, and the ability to record upwelling processes on high-resolution timescales, from interannual climatic variability (El Nino / Southern Oscillation (ENSO)) to interdecadal (e.g., the Pacific Decadal Oscillation (PDO)). Results suggest that the proxy best reflects the summer season. The proxy-instrument time-series comparison for summer SSTs displays remarkable agreement, driven largely by ENSO cycles for the past century, with some events missing due to scarcity of foraminiferal specimens and/or lack of sufficient temporal resolution. This study validates the G. ruber Mg/Ca proxy for summer SSTs in this region, and suggests its high fidelity to reconstruct summer SST from SLB over longer timescales to record multi-decadal and multi-centennial variabilities.

Nutrient regime and upwelling in the northern Benguela since the middle Holocene in a global context &amp;ndash; a multi-proxy approach

The last 5500 years of climate change and environmental response in the northern Benguela Coastal Upwelling are reconstructed by means of three sediment cores from the inner shelf off central Namibia. The study is based on nutrient (&amp;delta;&lt;sup&gt;15&lt;/sup&gt;N, &amp;delta;&lt;sup&gt;13&lt;/sup&gt;C) and productivity proxies (accumulation rates of total organic carbon; AR&lt;sub&gt;TOC&lt;/sub&gt;). Reconstructed sea surface temperatures (alkenone-derived SST) and temperatures at subsurface depths (&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;&amp;delta;18O&lt;/sub&gt;; based on tests of planktonic foraminifers) reflect the physical boundary conditions. The selection of proxy indicators proved a valuable basis for robust palaeo-climatic reconstructions, with the resolution ranging from multi-decadal (NAM1) over centennial (core 178) to millennial scale (core 226620). The northern Benguela experienced pronounced and rapid perturbation during the middle and late Holocene, and apparently, not all are purely local in character. In fact, numerous correlations with records from the adjacent South African subcontinent and the northern hemisphere testify to global climatic teleconnections. The Holocene Hypsithermal, for instance, is just as evident as the Little Ice Age (LIA) and the Roman Warm Period. The marked SST-rise associated with the latter is substantiated by other marine and terrestrial data from the South African realm. The LIA (at least its early stages) manifests itself in intensified winds and upwelling, which accords with increased rainfall receipts above the continental interior. It appears that climate signals are transferred both via the atmosphere and ocean. The combined analysis of SST and &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;&amp;delta;18O&lt;/sub&gt; proved a useful tool in order to differentiate between both pathways. SSTs are primarily controlled by the intensity of atmospheric circulation features, reflecting changes of upwelling-favourable winds. &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;&amp;delta;18O&lt;/sub&gt; records the temperature of the source water and often correlates with global ocean conveyor speed due to varying inputs of warm Agulhas Water. It seems as though conveyor slowdown or acceleration not only affected the temperature of the source water but also its nutrient content. This relationship between source water quality and conveyor speed is already known from glacial times. &lt;br&gt;&lt;br&gt; doi:&lt;a href="http://dx.doi.org/10.1002/mmng.201100006" target="_blank"&gt;10.1002/mmng.201100006&lt;/a&gt;

Nutrient regime and upwelling in the northern Benguela since the middle Holocene in a global context - a multi-proxy approach

Abstract. The last 5500 years of climate change and environmental response in the northern Benguela Coastal Upwelling are reconstructed by means of three sediment cores from the inner shelf off central Namibia. The study is based on nutrient (δ15N, δ13C) and productivity proxies (accumulation rates of total organic carbon; ARTOC). Reconstructed sea surface temperatures (alkenone-derived SST) and temperatures at subsurface depths (Tδ18O; based on tests of planktonic foraminifers) reflect the physical boundary conditions. The selection of proxy indicators proved a valuable basis for robust palaeo-climatic reconstructions, with the resolution ranging from multi-decadal (NAM1) over centennial (core 178) to millennial scale (core 226620). The northern Benguela experienced pronounced and rapid perturbation during the middle and late Holocene, and apparently, not all are purely local in character. In fact, numerous correlations with records from the adjacent South African subcontinent and the northern hemisphere testify to global climatic teleconnections. The Holocene Hypsithermal, for instance, is just as evident as the Little Ice Age (LIA) and the Roman Warm Period. The marked SST-rise associated with the latter is substantiated by other marine and terrestrial data from the South African realm. The LIA (at least its early stages) manifests itself in intensified winds and upwelling, which accords with increased rainfall receipts above the continental interior. It appears that climate signals are transferred both via the atmosphere and ocean. The combined analysis of SST and Tδ18O proved a useful tool in order to differentiate between both pathways. SSTs are primarily controlled by the intensity of atmospheric circulation features, reflecting changes of upwelling-favourable winds. Tδ18O records the temperature of the source water and often correlates with global ocean conveyor speed due to varying inputs of warm Agulhas Water. It seems as though conveyor slowdown or acceleration not only affected the temperature of the source water but also its nutrient content. This relationship between source water quality and conveyor speed is already known from glacial times. doi: 10.1002/mmng.201100006

Siliceous microorganisms in the upwelling center off Concepción, Chile (36°S): Preservation in surface sediments and downcore fluctuations during the past ∼150 years

We analyzed the temporal patterns of siliceous microorganisms in the water column and their representation in the underlying surface sediments between September 2002 and November 2005, at a fixed station on the shelf off Concepción, Chile (Station 18, 36° 30.80′S and 73° 07.75′W), connecting our results to instrumental records of temperature, salinity, coastal upwelling and freshwater input. The goals of the study were to understand the seasonal representation of siliceous microorganisms in surface sediments and to evaluate the use of this information in the interpretation of past climatic/oceanographic conditions in the area. The implications of seasonal and preservational biases are discussed. Additionally, a 30-cm core collected at the same site and spanning the last ∼150 years of sedimentation was also studied in order to provide a record of historical siliceous productivity changes. Firstly, the analyses focused on diatoms since they were numerically the most prominent microorganisms in the water column and the surface sediments, independent of season and year. In both settings, maxima of the key diatom genera Chaetoceros and Skeletonema coincided with the spring–summer upwelling period. For the autumn–winter non-upwelling period, significant plankton–sediment discrepancies were observed, with enrichment of moderately robust taxa in the sediments, as well as freshwater diatoms and phytoliths tracing the increased river discharges in winter. Secondly, the downcore analysis revealed a marked decrease in total diatom accumulation rates since the late 19th and throughout the 20th century, which was accompanied by increasing concentrations of lithogenic particles and freshwater diatoms. An alkenone-based sea surface temperature reconstruction in the same core (past ∼150 years) and instrumental data show that these changes occurred simultaneously with a general trend of increasing temperatures in the upwelling area off Concepción. Taken together, these signals suggest an intensified influence of ENSO-like variability in the ocean-climate system off central-southern Chile.

The impact of devegetated dune fields on North American climate during the late Medieval Climate Anomaly

During the Medieval Climate Anomaly, North America experienced severe droughts and widespread mobilization of dune fields that persisted for decades. We use an atmosphere general circulation model, forced by a tropical Pacific sea surface temperature reconstruction and changes in the land surface consistent with estimates of dune mobilization (conceptualized as partial devegetation), to investigate whether the devegetation could have exacerbated the medieval droughts. Presence of devegetated dunes in the model significantly increases surface temperatures, but has little impact on precipitation or drought severity, as defined by either the Palmer Drought Severity Index or the ratio of precipitation to potential evapotranspiration. Results are similar to recent studies of the 1930s Dust Bowl drought, suggesting bare soil associated with the dunes, in and of itself, is not sufficient to amplify droughts over North America.

Western equatorial Pacific deep water carbonate chemistry during the Last Glacial Maximum and deglaciation: Using planktic foraminiferal Mg/Ca to reconstruct sea surface temperature and seafloor dissolution

We present a reconstruction of deep‐water carbonate saturation state (ΔCO32−) in the western equatorial Pacific for the Last Glacial Maximum (LGM) and deglaciation based on changes in the Mg/Ca ratio of planktic foraminifers with increased water depth. Our data suggest there have been changes in bottom water ΔCO32− over the past 25,000 years at water depths as shallow as 1.6 km. The ΔCO32− reconstruction for the LGM suggests ΔCO32− was similar or slightly higher than modern values between 1.6 and 2.0 km, shifting sharply to lower values (an average ∼30 μmol/kg lower) below 2.5 km. The shift in chemistry between 2.0 and 2.5 km supports a hypothesis that Pacific overturning circulation occurred deeper during the LGM with a slightly more ventilated water mass above 2.0 km. The data are not consistent with enhanced preservation in this region of the deep Pacific at depths greater than 2.5 km, suggesting that the long‐held view of better preservation throughout the glacial deep Pacific must be reevaluated. For the deglaciation, we have evidence of a ΔCO32− maximum that suggests enhanced deglacial preservation between 1.6 and 4.0 km in comparison to the Holocene and the LGM. The deglacial ΔCO32− was as much as 28 μmol/kg higher than modern between 1.6 and 4.0 km. Results suggest carbonate burial rates were 1.5 times greater during the deglacial than the over the past 5 kyr.