SST Estimates Research Articles

Sea Surface Temperatures Across the Coral Sea Over the Last Glacial‐Interglacial Cycle

AbstractSea surface temperature (SST) across the Coral Sea is tightly coupled to the regional and larger‐scale ocean circulation and climate. Continuous records that reflect past changes in Coral Sea SST in high resolution are missing, however. Here, we present Mg/Ca‐ and alkenone‐based SST reconstructions from the northwestern Coral Sea that cover the past 130 kyr. Our SST estimates vary in line with southern hemisphere high latitude climate variability, linked to atmospheric CO2. Combining the newly generated with published records, we find that the SST changes across the Coral Sea show a large spatial heterogeneity during the last glacial‐interglacial cycle. Particularly prior to 60 ka, precession exerts a discernible influence on the meridional SST gradients across the Coral Sea, as well as between the Coral Sea and southern Western Pacific Warm Pool. We posit that these changes are linked to the regional trade winds and the South Pacific subtropical gyre circulation, and/or to changes in the El Niño Southern Oscillation.

A new radiolarian transfer function for the Pacific Ocean and application to fossil records: Assessing potential and limitations for the last glacial-interglacial cycle

Abstract Climatic and oceanographic processes in the Pacific Ocean have global implications. Reliable sedimentary proxies in this region are thus necessary to understand the magnitude of past climate variability. Radiolarian assemblages are of particular relevance in some regions of the Pacific Ocean where other proxies are either poorly preserved or are biased towards a specific season. Previously published radiolarian-based transfer functions in the Pacific Ocean did not cover some regions, such as the Western Pacific marginal seas or high-latitude Southwestern Pacific. Filling these gaps in sample distribution and using a thorough methodology for finding the most important variable for radiolarian assemblages in surface sediment are both essential steps in quantitative studies. In this study, we use published radiolarian census counts from 801 core-tops distributed across the Pacific Ocean, to investigate which environmental factors drive the assemblages. We harmonized taxonomically the modern microfossil dataset, and using multivariate statistical analyses, we determined that sea surface temperature at 10 m (SST10) is the most important variable influencing the changes observed in the radiolarian assemblages. The calibration method weighted-mean modern analogue technique with five analogs (WMAT-K5) corrected for autocorrelation using a cut-off distance of 500 km, showed a performance of R2cv = 0.83; RMSEP = 3.8 °C. This calibration method was then applied to taxonomically harmonized radiolarian census counts from 31 cores located in the Bering Sea, Western Pacific marginal seas, and Southwest and Eastern Equatorial Pacific, with some of these records covering at least the last 165 ka. We assessed the analogue quality and significance of the downcore SST10 reconstructions in all of them. We found that temperatures at 10 and 200 m were the most significant variables for the fossil assemblages. Finally, we compare the temperature reconstructions to previously published radiolarian-based SST estimates for the same cores, or to other SST records based on other methodologies. We find some differences between our new temperature estimates compared to existing ones, in particular in the Eastern Equatorial Pacific. Comparison to other methods in the Japan and Bering Sea and Sea of Okhotsk show that radiolarian-based reconstructions provide robust temperature estimates compared to biogeochemical methods, which showed SST overestimation during glacial periods.

Morphometric analysis in the shells of the planktonic foraminifera Orbulina universa: a source for paleoceanographic information?

Abstract In this study, we describe morphometric analyses of Orbulina universa morphotypes presently found in the Western South Atlantic, and their relation to the upper water thermohaline indexes. We also investigate this relationship under glacial and interglacial conditions between 7.2 and 32 kyr BP. Ten plankton samples distributed between 32o55' and 26o20'S were selected for the analyses of modern O. universa specimens. Core SIS188 was retrieved at 29o22’S, 47o28’W enabling the analysis of 21 fossil samples. Shell diameter, pore diameter, pore density and porosity were measured using SEM images. Chronology of core SIS188 is based on four AMS 14C ages and the planktonic foraminifera oxygen isotope curve. Planktonic foraminifera relative abundances were converted into SST data using the Modern Analog Technique (MAT). Correlations between the morphometric parameters and temperature and salinity of surface Waters (modern samples) or paleotemperature (fossil samples) were calculated. Morphological characteristics of the analyzed shells allowed to differentiate two morphotypes of O. universa, namely Caribbean and Mediterranean in the studied region. For the Caribbean morphotype, the large pore density showed a negative correlation with both temperature and salinity. For the Mediterranean morphotype, no significant correlations were found. For fossil samples, no significant correlations were found between summer SST (0-50m) and morphometric parameters for the Caribbean or the Mediterranean morphotypes. Possibly, other genotypes do exist in the fossil record, so modern and fossil morphotypes would not have the same behavior. Another explanation for the absence of correlation in fossil samples are errors associated with the SST estimates. Furthermore, fossil samples could represent distinct and more complex oceanographic conditions compared to current configuration, with Plata Plume Water and Brazil Malvinas Confluence migration playing a most significant role in the past.

Comparación de los productos de TSM L3 generados a partir de los sensores AVHRR y MODIS frente al Golfo San Jorge, Argentina

<p>Since the early 1980s, the AVHRR sensor on board the NOAA satellites has provided SST estimates to the scientific community in general. Towards the end of the 1990s, a new generation of radiometers was added, combining a wider range of spectral measurements with improvements in technology, such as the MODIS sensor on board the Terra and Aqua satellites. The development of this last sensor was based on the years of NOAA/AVHRR series. Understanding the relationship between products derived from different sensors is critical to the continuous long-term monitoring of any variable and the construction of continuous time series. The objective of this work was to make a comparison of the monthly SST values for the 2003-2006 period calculated from data from the NOAA/ AVHRR series versus Aqua-MODIS on a sector of the middle Argentina Continental shelf off the San Jorge Gulf. For both sensors, the isotherms show a similar distribution pattern in all months. Pearson’s correlation coefficient was high, both in the seasonal (> 0.9) and monthly (> 0.8) analyzes. The comparative analysis of the temperature estimated by both sensors shows that SST products are similar and can be used to analyze large and continuous time series.</p>

The role of wind gusts in upper ocean diurnal variability

AbstractUpper ocean processes play a key role in air‐sea coupling, with variability on both short and long time scales. The diurnal cycle associated with diurnal solar insolation and nighttime cooling, may act, along with stochastic wind variability, on upper ocean temperatures and stratification resulting in a diurnal warm layer and a nonlinear rectified effect on longer time scales. This study describes diurnal changes in upper ocean temperature for a location in the equatorial Indian Ocean, using observations from the Dynamics of the Madden‐Julian Oscillation field campaign, a high vertical resolution 1‐D process model, and a diurnal cycling scheme. Solar forcing is the main driver of diurnal variability in upper ocean temperature and stratification. Yet except during nighttime convection, winds with variability on the order of hours (here referred to as “wind gusts”) regulate how fast surface water is mixed to greater depths when daily mean winds are weak. Wind gusts are much stronger than diurnal winds. Even using stochastic wind gusts but no diurnal winds as input in a 1‐D process model yields an estimate of diurnal temperature that compares well with observations. A new version of the Large and Caron (2015) scheme (LC2015) provides an estimate of upper ocean diurnal temperature that is consistent with observations. LC2015 has the advantage of being suitable for implementation in a climate model, with the goal to improve SST estimates, hence the simulated heat flux at the air‐sea interface. Yet LC2015 is not very sensitive to the inclusion or omission of the high‐frequency component of the wind.

Extended Reconstructed Sea Surface Temperature Version 4 (ERSST.v4): Part II. Parametric and Structural Uncertainty Estimations

Abstract Described herein is the parametric and structural uncertainty quantification for the monthly Extended Reconstructed Sea Surface Temperature (ERSST) version 4 (v4). A Monte Carlo ensemble approach was adopted to characterize parametric uncertainty, because initial experiments indicate the existence of significant nonlinear interactions. Globally, the resulting ensemble exhibits a wider uncertainty range before 1900, as well as an uncertainty maximum around World War II. Changes at smaller spatial scales in many regions, or for important features such as Niño-3.4 variability, are found to be dominated by particular parameter choices. Substantial differences in parametric uncertainty estimates are found between ERSST.v4 and the independently derived Hadley Centre SST version 3 (HadSST3) product. The largest uncertainties are over the mid and high latitudes in ERSST.v4 but in the tropics in HadSST3. Overall, in comparison with HadSST3, ERSST.v4 has larger parametric uncertainties at smaller spatial and shorter time scales and smaller parametric uncertainties at longer time scales, which likely reflects the different sources of uncertainty quantified in the respective parametric analyses. ERSST.v4 exhibits a stronger globally averaged warming trend than HadSST3 during the period of 1910–2012, but with a smaller parametric uncertainty. These global-mean trend estimates and their uncertainties marginally overlap. Several additional SST datasets are used to infer the structural uncertainty inherent in SST estimates. For the global mean, the structural uncertainty, estimated as the spread between available SST products, is more often than not larger than the parametric uncertainty in ERSST.v4. Neither parametric nor structural uncertainties call into question that on the global-mean level and centennial time scale, SSTs have warmed notably.

Time-series measurements of settling particulate matter in Alfonso Basin, La Paz Bay, southwestern Gulf of California

More than 200 sediment trap samples were collected at 310 or 360m depth during 2002–2009 from the center of the 410m-deep Alfonso Basin in La Paz Bay, an arid, subtropical embayment of the southern Gulf of California. Sample splits were analyzed for total mass flux (TMF), particulate organic carbon (POC), inorganic carbon (CaCO3) and opal (biogenic silica BioSi). The lithogenic (Litho) and biogenic fluxes (Biogen), but especially the particulate organic matter (POM) and CaCO3, fluxes are relatively high compared with those recorded in a number of other coastal basins at depths less than ~500m. The average yearly Litho fluxes (mean of 133±158gm−2yr−1) are well correlated with the frequency of strong wind gusts and reflect the relative proximity of the mooring to shore and the importance of eolian transport. The sedimentation patterns are influenced by monsoonal shifts—cool temperatures and strong northerly winds in late fall and winter sustain a well-mixed deep surface layer and high lithogenic fluxes, whereas progressive heating and relatively weak southerlies in summer and fall lead to a shallower, more-stratified surface layer with lower nutrient levels, limited productivity and generally low particulate fluxes. There is considerable interannual variability in the size of the various fluxes. This is likely related to a number of regional and internal factors whose timing, intensity and interactions remain to be resolved.Although there is an inverse relationship between SST and satellite-derived net primary production (NPP) estimates, no correlation was found between NPP and the POC, CaCO3 and BioSi fluxes (r2<0.05 and p>0.05). Significant correlations (p<0.001) do exist, however, between the POC flux and the total mineral flux as well as with the CaCO3, Litho and BioSi fluxes (r=0.86, 0.82, 0.79 and 0.69, respectively). As suggested by the ballast ratio hypothesis, the strength of these correlations follows the relative density of the individual ballast components. Among the ballast minerals, the Litho fraction correlates strongly with CaCO3 (r=0.84), both fluxes being important during the windy cool season. The lithogenic fraction accounts for almost half of the total mass flux and is clearly a significant ballasting agent in this continental margin basin.

Modern sea surface productivity and temperature estimations off Chile as detected by coccolith accumulation rates

Recent coccoliths from 74 surface sediment samples recovered from the southeastern Pacific off Chile were examined quantitatively to investigate modern regional gradients of sea surface productivity and temperature. All findings are based on coccolith accumulation rates. Therefore an approach was designed to estimate recent sedimentation rates based on 210Pb and bulk chemistry analyses of the same set of surface samples. Highest total coccolith accumulation rates were found off north-central Chile, where seasonal upwelling takes place. Based on a multiple linear regression between calculated coccolith accumulation rates and World Ocean Atlas derived sea surface temperatures, a calibration model to reconstruct annual average temperatures of the uppermost 75m of the water column is provided. The model was cross-validated and the SST estimates were compared with SST observed and SST estimates based on diatoms and planktonic foraminifera, showing a good correlation.

Including Uncertainties of Sea Surface Temperature in an Ensemble Kalman Filter: A Case Study of Typhoon Sinlaku (2008)

Abstract Sea surface temperature (SST) plays an important role in tropical cyclone (TC) life cycle evolution, but often the uncertainties in SST estimates are not considered in the ensemble Kalman filter (EnKF). The lack of uncertainties in SST generally results in the lack of ensemble spread in the atmospheric states near the sea surface, particularly for temperature and moisture. In this study, the uncertainties of SST are included by adding ensemble perturbations to the SST field, and the impact of the SST perturbations is investigated using the local ensemble transform Kalman filter (LETKF) with the Weather Research and Forecasting Model (WRF) in the case of Typhoon Sinlaku (2008). In addition to the experiment with the perturbed SST, another experiment with manually inflated ensemble perturbations near the sea surface is performed for comparison. The results indicate that the SST perturbations within EnKF generally improve analyses and their subsequent forecasts, although manually inflating the ensemble spread instead of perturbing SST does not help. Investigations of the ensemble-based forecast error covariance indicate larger scales for low-level temperature and moisture from the SST perturbations, although manual inflation of ensemble spread does not produce such structural effects on the forecast error covariance. This study suggests the importance of considering SST perturbations within ensemble-based data assimilation and promotes further studies with more sophisticated methods of perturbing SST fields such as using a fully coupled atmosphere–ocean model.

Alkenone unsaturation in surface sediments from the eastern equatorial Pacific: Implications for SST reconstructions

Significant uncertainties persist in the reconstruction of past sea surface temperatures in the eastern equatorial Pacific, especially regarding the amplitude of the glacial cooling and the details of the post‐glacial warming. Here we present the first regional calibration of alkenone unsaturation in surface sediments versus mean annual sea surface temperatures (maSST). Based on 81 new and 48 previously published data points, it is shown that open ocean samples conform to established global regressions of U37K′ versus maSST and that there is no systematic bias from seasonality in the production or export of alkenones, or from surface ocean nutrient concentrations or salinity. The flattening of the regression at the highest maSSTs is found to be statistically insignificant. For the near‐coastal Peru upwelling zone between 11–15°S and 76–79°W, however, we corroborate earlier observations that U37K′ SST estimates significantly over‐estimate maSSTs at many sites. We posit that this is caused either by uncertainties in the determination of maSSTs in this highly dynamic environment, or by biasing of the alkenone paleothermometer toward El Niño events as postulated by Rein et al. (2005).

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.

Comment on “A snapshot of climate variability at Tahiti at 9.5 ka using a fossil coral from IODP Expedition 310” by Kristine L. DeLong, Terrence M. Quinn, Chuan‐Chou Shen, and Ke Lin

[1] The Integrated Ocean Drilling Program (IODP) Expedition 310 recovered drill cores from the drowned reefs around the island of Tahiti (17°40′S, 149°30′W), many of which contained samples of massive corals from the genus Porites. Herein we report on one well-preserved fossil coral sample: a 13.6 cm long Porites sp. dated by uranium series techniques at 9523 ± 33 years. Monthly δ18O and Sr/Ca determinations reveal nine clear and robust annual cycles. Coral δ18O and Sr/Ca determinations estimate a mean temperature of ∼24.3°C (∼3.2°C colder than modern) for Tahiti at 9.5 ka; however, this estimate is viewed with caution since potential sources of cold bias in coral geochemistry remain to be resolved. The interannual variability in coral δ18O is similar between the 9.5 ka coral record and a modern record from nearby Moorea. The seasonal cycle in coral Sr/Ca is approximately the same or greater in the 9.5 ka coral record than in modern coral records from Tahiti. Paired analysis of coral δ18O and Sr/Ca indicates cold/wet (warm/dry) interannual anomalies, opposite from those observed in the modern instrumental record.

Middle Pleistocene sea surface temperature in the Brazil-Malvinas Confluence Zone: Paleoceanographic implications based on planktonic foraminifera

We reconstructed Middle Pleistocene surface hydrography in the western South Atlantic based on planktonic foraminiferal assemblages, modern analog technique and Globorotalia truncatulinoides isotopic ratios of core SP1251 (38o29.7´S / 53o40.7´W / 3400 m water depth). Biostratigraphic analysis suggests that sediments were deposited between 0.3 and 0.12 Ma and therefore correlate to Marine Isotopic Stage 6 or 8. Faunal assemblage-based winter and summer SST estimates suggest that the western South Atlantic at 38oS was 4-6oC colder than at present, within the expected range for a glacial interval. High relative abundances of subantarctic species, particularly the dominance of Neogloboquadrina pachyderma (left), support lower than present SSTs throughout the recorded period. The oxygen isotopic composition of G. truncatulinoides suggests a northward shift of the Brazil-Malvinas Confluence Zone and of the associated mid-latitude frontal system during this Middle Pleistocene cold period, and a stronger than present influence of superficial subantarctic waters and lowering in SSTs at our core site during the recorded Middle Pleistocene glacial.

Cooling upwelling regions along the California margin during the early Pliocene: evidence for a shoaling thermocline

The early Pliocene is the most recent time in Earth history when average global temperatures were warmer than they are today. Previous work has shown that coastal upwelling regions had a particularly strong response to early Pliocene warmth, with sea surface temperature records in the southern California margin indicating SST was 9-10°C warmer compared to today. Warmer SST in upwelling regions could have been an important factor in maintaining Pliocene warmth. However, the cause for the warmer conditions in upwelling regions and the transition to colder SST has remained elusive. We present alkenone derived SST estimates from a northern California margin location, ODP Site 1022 (40°N, 125°W, 1950m water depth), from the early Pliocene (4. 2-2.8 Ma). The 2.8°C cooling during this time interval is similar to that observed at two southern California margin sites (ODP Sites 1014 and 1012), indicating the three sites were responding to the same forcing. However, SST was only 3°C warmer than today at ODP Site1022 while the southern sites were 9-10°C warmer than today, thereby increasing the latitudinal temperature gradient along the California margin dramatically during the early Pliocene compared to today. This indicates that the tropical warm pool expansion during the early Pliocene was not felt along the Northern California margin. We quantified diatom assemblages at ODP Site 1022 to constrain changes in oceanographic conditions during early to middle Pliocene. The percent of diatoms indicative of subarctic waters is unchanged from 4.2 to 3 Ma, suggesting that a change in the strength of the California current cannot explain the cooling SST trend. We also found no trend in the percent of upwelling-indicative diatoms, which is consistent with records of alkenone mass accumulation rate, diatom preservation, biosilica, and total organic carbon, none of which show evidence that the upwelling of nutrient-rich subsurface water increased as SST cooled. The fact that upwelling of nutrient-rich water was occurring even when SST were warmer can be explained by a deeper early Pliocene thermocline.

Pliocene Atlantic molluscan assemblages from the Mondego Basin (Portugal): Age and palaeoceanographic implications

The Pliocene molluscan assemblage from the Mondego Basin (Portugal, Western Iberia) plays a particularly important role in the understanding of the palaeobiogeography of Neogene–Quaternary molluscs of the Atlantic Frontage of Europe and the western Mediterranean. The importance of these Portuguese molluscan deposits is stressed, as it is the only assemblage representative of the southern portion of the Pliocene French–Iberian biogeographical Province. The Pliocene marine fossiliferous deposits of the Mondego Basin (central-west Portugal) are dated using their nannofossil and molluscan assemblages, as well as Strontium dating. The results suggest a late Zanclean to early Piacenzian age. Chronologically they are equivalent to the Mediterranean Pliocene Molluscan Unit 1 (MPMU1). However, due to the more northern geographical location of the Mondego Basin assemblages, their molluscan content is closer to that of MPMU2 than to that of MPMU1 in the Mediterranean. The presence of a stock of thermophilic taxa in the Mondego assemblage, no longer existent in European waters, enabled us to suggest a palaeoenvironmental reconstruction for mid-Pliocene SSTs in the region. We put forward the hypothesis that the SSTs at the latitude of Mondego, during late Zanclean to early Piacenzian, would be characterized by a yearly SST pattern analogous to that of present-day Cape Blanc (West Africa). Consequently, whilst subtropical conditions existed in the Atlantic Zanclean to mid-Piacenzian at Mondego latitude in the Mediterranean fully tropical conditions prevailed at that time. The Mondego SST estimates correlate with those estimated for MPMU2 in the Mediterranean. The global palaeoenvironmental reconstruction of mid-Pliocene SSTs in the PRISM2 Project suggests, for western Iberia, at Mondego latitude, an August SST of about 23 °C, and a February temperature of about 17 to 18 °C. Our hypothesis suggests similar August SST differing in only half a degree Celsius (23.5 vs. 23 °C) and February SSTs slightly higher (19 vs. 17–18 °C).

Systematic pattern in [formula omitted] – Temperature residuals for surface sediments from high latitude and other oceanographic settings

Twenty-seven multicores, collected in late summer 2004 from coastal and inland marine sites throughout SE Alaska (∼55 to 61°N), were analyzed for C 37 to C 39 alkenones of specific haptophyte origin. This biomarker signal, detected in core-top sediments from 20 of the sites, was quantitatively most significant in organic carbon and biogenic silica-rich sediments deposited south of Icy Strait (∼58°N), along the outer coastline of Baranof and Prince of Wales islands. U 37 K ′ -based water temperature estimates derived using a standard calibration equation ( U 37 K ′ = 0.034T + 0.039) were uniform throughout the study area, averaging 11.9 ± 1.0 °C, a value ∼4 °C warmer than mean annual SST (maSST) for the region. Epoxides, indicative of selective alkenone degradation by an aerobic bacterial process, were also detected and quantified in all sediments containing alkenones. Semi-quantitative accounting of the impact that compound-selective degradation can impose on alkenone unsaturation patterns indicates the standard U 37 K ′ -based temperature estimates could be too warm by a variable amount (0.7–2.4 °C) depending upon the site, with an overall average of ∼1.4 °C. Even after adjustment for this potential diagenetic ‘warming’ effect, estimates still best match summer and not maSST in this subarctic region. Re-examination of alkenone signatures preserved in sediment from the North Atlantic and on the continental margin located south of ∼45°S along the Chilean coast indicates the apparent summer signal in U 37 K ′ -derived SST estimates is not unique to SE Alaskan sediments but rather a common finding when alkenone thermometry is applied to sediments from high latitude environments. Examination of model residuals from analysis of surface sediment samples collected at lower latitude settings along the west coast of the Americas also identifies other sites where significant biogeographic patterns seem apparent and viable oceanographic explanations can be offered.

Organic geochemical changes in Pliocene sediments of ODP Site 1083 (Benguela Upwelling System)

The intensification of the Northern Hemisphere Glaciation (INHG) was a major event in the development of the current climate state, and as one of the most productive regions in the world's oceans, the behaviour of the Benguela Upwelling System (BUS) following the INHG is of wide interest. To investigate post-INHG changes in productivity and organic matter accumulation, total organic carbon and biomarker accumulation rates were determined for sediments from ODP Site 1083 and compared to alkenone-derived sea surface temperatures and nitrogen isotopic compositions. These data indicate that the interval between 2.6 and 2.4 Ma was characterized by dramatic changes in upwelling intensity and organic carbon export on the northern edge of the modern BUS. The upwelling is reflected by significant changes in alkenone-derived SST estimates between glacial and interglacial intervals, with a total variability of 16 °C. The studied interval is also characterized by large changes in organic matter export as reflected by changes in TOC and biomarker accumulation rates, which show maxima during OIS 98 and during the transition from OIS 97 to 96. Intervals of elevated TOC are also characterized by elevated concentrations of sedimentary microbial biomarkers and lower %CaCO 3, suggesting that enhanced delivery of labile organic matter to the seafloor resulted in enhanced remineralisation with released CO 2 being consumed by CaCO 3 dissolution. However, in apparent contrast to recent Pleistocene sediments at the same site, organic matter export after the INHG was not solely driven by upwelling intensity. Of the three Pliocene glacial–interglacial cycles examined (OIS 101 to 96), each is unique with respect to the timing and magnitude of changes in organic matter accumulation. Each is also characterized by different algal assemblages as inferred from biomarker distributions, with OIS 97 and 96 particularly dominated by diatoms. We suggest that these differences reflect the important but evolving role of Southern Ocean waters in the Pliocene BUS: nutrient depletion of SO waters occurred during parts of Pliocene glacial intervals such that even intense upwelling did not persistently result in enhanced organic matter accumulation rates.

Palaeoenvironmental records from fossil corals: The effects of submarine diagenesis on temperature and climate estimates

The geochemistry of coral skeletons may reflect seawater conditions at the time of deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However the precipitation of cements in the primary coral skeleton during diagenesis may significantly affect bulk skeletal geochemistry. We used secondary ion mass spectrometry (SIMS) to measure Sr, Mg, B, U and Ba concentrations in primary coral aragonite and aragonite and calcite cements in fossil Porites corals from submerged reefs around the Hawaiian Islands. Cement and primary coral geochemistry were significantly different in all corals. We estimate the effects of cement inclusion on climate estimates from drilled coral samples, which combine cements and primary coral aragonite. Secondary 1% calcite or ∼2% aragonite cement contamination significantly affects Sr/Ca SST estimates by +1 °C and −0.4 to −0.9 °C, respectively. Cement inclusion also significantly affects Mg/Ca, B/Ca and U/Ca SST estimates in some corals. X-ray diffraction (XRD) will not detect secondary aragonite cements and significant calcite contamination may be below the limit of detection (∼1%) of the technique. Thorough petrographic examination of fossils is therefore essential to confirm that they are pristine before bulk drilled samples are analysed. To confirm that the geochemistry of the original coral structures is not affected by the precipitation of cements in adjacent pore spaces we analysed the primary coral aragonite in cemented and uncemented areas of the skeleton. Sr/Ca, B/Ca and U/Ca of primary coral aragonite is not affected by the presence of cements in adjacent interskeletal pore spaces i.e. the coral structures maintain their original composition and selective SIMS analysis of these structures offers a route to the reconstruction of accurate SSTs from altered coral skeletons. However, Mg/Ca and Ba/Ca of primary coral aragonite are significantly higher in parts of skeletons infilled with high Mg calcite cement. We hypothesise this reflects cement infilling of intraskeletal pore spaces in the primary coral structure.

Evaluating the efficacy of planktonic foraminifer calcite δ 18O data for sea surface temperature reconstruction for the Late Miocene

This study examines the efficacy of published δ 18O data from the calcite of Late Miocene surface dwelling planktonic foraminifer shells, for sea surface temperature estimates for the pre-Quaternary. The data are from 33 Late Miocene (Messinian) marine sites from a modern latitudinal gradient of 64°N to 48°S. They give estimates of SSTs in the tropics/subtropics (to 30°N and S) that are mostly cooler than present. Possible causes of this temperature discrepancy are ecological factors (e.g. calcification of shells at levels below the ocean mixed layer), taphonomic effects (e.g. diagenesis or dissolution), inaccurate estimation of Late Miocene seawater oxygen isotope composition, or a real Late Miocene cool climate. The scale of apparent cooling in the tropics suggests that the SST signal of the foraminifer calcite has been reset, at least in part, by early diagenetic calcite with higher δ 18O, formed in the foraminifer shells in cool sea bottom pore waters, probably coupled with the effects of calcite formed below the mixed layer during the life of the foraminifera. This hypothesis is supported by the markedly cooler SST estimates from low latitudes—in some cases more than 9 °C cooler than present—where the gradients of temperature and the δ 18O composition of seawater between sea surface and sea bottom are most marked, and where ocean surface stratification is high. At higher latitudes, particularly N and S of 30°, the temperature signal is still cooler, though maximum temperature estimates overlap with modern SSTs N and S of 40°. Comparison of SST estimates for the Late Miocene from alkenone unsaturation analysis from the eastern tropical Atlantic at Ocean Drilling Program (ODP) Site 958—which suggest a warmer sea surface by 2–4 °C, with estimates from oxygen isotopes at Deep Sea Drilling Project (DSDP) Site 366 and ODP Site 959, indicating cooler than present SSTs, also suggest a significant impact on the δ 18O signal. Nevertheless, much of the original SST variation is clearly preserved in the primary calcite formed in the mixed layer, and records secular and temporal oceanographic changes at the sea surface, such as movement of the Antarctic Polar Front in the Southern Ocean. Cooler SSTs in the tropics and sub-tropics are also consistent with the Late Miocene latitude reduction in the coral reef belt and with interrupted reef growth on the Queensland Plateau of eastern Australia, though it is not possible to quantify absolute SSTs with the existing oxygen isotope data. Reconstruction of an accurate global SST dataset for Neogene time-slices from the existing published DSDP/ODP isotope data, for use in general circulation models, may require a detailed re-assessment of taphonomy at many sites.

Constraints on SST estimates for the northern North Atlantic/Nordic Seas during the LGM

A map of estimated calcification temperatures of the planktic foraminifer Neogloboquadrina pachyderma sinistral ( T Nps) for the Nordic Seas and the northern North Atlantic for the Last Glacial Maximum was produced from oxygen isotopes with support of Mg/Ca ratios. To arrive at the reconstruction, several constraints concerning the plausible salinity and δ 18 O -fields were employed. The reconstruction indicates inflow of temperate waters in a wedge along the eastern border of the Nordic Seas and at least seasonally ice-free waters. The reconstruction from oxygen isotopes shows similarities with Mg/Ca based paleotemperatures in the southern and southeastern sector, while unrealistically high Mg/Ca values in the central Nordic Seas prevent the application of the method in this area. The oxygen isotope based reconstruction shows some agreement with temperature reconstructions based on the modern analogue technique, but with somewhat lower temperatures and a stronger internal gradient inside the Nordic Seas. All told, our results suggest a much more ice-free and dynamic high latitude ocean than the CLIMAP reconstruction.