Sea Surface Temperature Proxy Research Articles

Natural and Anthropogenic Climate Variability Signals in a 237‐Year‐Long Coral Record From the Philippines

AbstractBoth proxy and model studies conducted to understand anthropogenic warming have revealed historical variations in sea‐surface temperature (SST) since the industrial revolution. However, because of discrepancies between observations and models in the late nineteenth century, the timing and degree of anthropogenic warming remain unclear. In this study, we reconstructed a 237‐year‐long record of SST and salinity using a coral core collected from Bicol, southern Luzon, Philippines, which is located at the northern edge of the western Pacific warm pool. The SST record showed volcanic cooling after several volcanic eruptions, including the 1815 Tambora eruption, but the pattern of change differed. Decadal SST variations at Bicol are connected to Pacific Decadal Variability (PDV). Therefore, it is suggested that the PDV conditions at the time of the eruption may have influenced marine conditions, such as the degree and duration of cooling and/or salinity, after the eruptions. Although there were discrepancies in SST variations among the modeled, observed, and proxy SST data from the late nineteenth to early twentieth centuries, SST data from the late twentieth century showed globally coherent anthropogenic warming, especially after 1976. In particular, summer SST in the northwestern Pacific has become more sensitive to anthropogenic forcing since 1976.

Assessing Seasonal and Inter‐Annual Marine Sediment Climate Proxy Data

AbstractThree recently published papers including Napier et al. (2022, https://doi.org/10.1029/2021PA004355) utilize novel microanalytical approaches with varved marine sediments to demonstrate the potential to reconstruct seasonal and inter‐annual climate variability. Obtaining paleoclimate data at a resolution akin to the observational record is vitally important for improving our understanding of climate phenomena such as monsoons and modes of variability such as the El Niño Southern Oscillation, for which appraisals of past inter‐annual variability is critical. The ability to generate seasonal and inter annual resolution sea surface temperature proxy time series spanning a thousand years or more is revolutionary and has the potential to fill gaps in our knowledge of climate variability. Although generally limited to sediments from regions with oxygen depleted bottom waters, there is great potential to integrate shorter seasonal resolution climate “snap shots” from other archives such as annually banded corals into composite time series. But as paleoceanographic data are used more by the observational and modeling fields, we make the case for conducting a thorough case‐by‐case assessment of the processes that influence the climate signal recovered from proxies, using careful replication to validate new approaches. Understanding or exploring the potential influence of processes which effectively filter the climate signal will lead to more quantitative paleoceanographic data that will better serve the broader climate science community.

Climate and oceanic condition changes influence subsistence economic adaptation through intensification on the Central Andean coasts

Understanding the causes of subsistence economic adaptation remains a critical topic in archaeology. Here we explore one potential causal phenomenon, climate change, to understand how shifting ecological conditions incentivized adaptation through subsistence economic intensification along the Central Andean coasts. To do so we couple 775 archaeological individuals that have dietary stable isotope data (collagen δ13C and δ15N, hydroxyapatite δ13C) with spatio-temporal core-based proxies of oceanic sea surface temperature and El Niño Southern Oscillation frequency estimates. Using an ensemble machine learning model, we evaluate hypotheses that changes in ocean conditions resulting in decreased marine productivity correspond with isotopic signals of increasing terrestrial resource reliance over the past ∼7000 years. Results support the hypotheses, and prior work, showing isotopic signatures of diet across the coastal Central Andes reflect greater incorporation of resources indicative of intensification during times when marine productivity was likely depressed. As near-shore marine productivity declined, people adapted in manners that may have both increased their resiliency to climate change and improved their overall subsistence returns, but at higher investment costs. The overall findings support theoretical intensification expectations, suggesting adaptation through intensification represents one of the key factors in understanding broader behavioral transformation in the face of climate change.

The CoralHydro2k database: a global, actively curated compilation of coral δ18O and Sr ∕ Ca proxy records of tropical ocean hydrology and temperature for the Common Era

Abstract. The response of the hydrological cycle to anthropogenic climate change, especially across the tropical oceans, remains poorly understood due to the scarcity of long instrumental temperature and hydrological records. Massive shallow-water corals are ideally suited to reconstructing past oceanic variability as they are widely distributed across the tropics, rapidly deposit calcium carbonate skeletons that continuously record ambient environmental conditions, and can be sampled at monthly to annual resolution. Climate reconstructions based on corals primarily use the stable oxygen isotope composition (δ18O), which acts as a proxy for sea surface temperature (SST), and the oxygen isotope composition of seawater (δ18Osw), a measure of hydrological variability. Increasingly, coral δ18O time series are paired with time series of strontium-to-calcium ratios (Sr/Ca), a proxy for SST, from the same coral to quantify temperature and δ18Osw variability through time. To increase the utility of such reconstructions, we present the CoralHydro2k database, a compilation of published, peer-reviewed coral Sr/Ca and δ18O records from the Common Era (CE). The database contains 54 paired Sr/Ca–δ18O records and 125 unpaired Sr/Ca or δ18O records, with 88 % of these records providing data coverage from 1800 CE to the present. A quality-controlled set of metadata with standardized vocabulary and units accompanies each record, informing the use of the database. The CoralHydro2k database tracks large-scale temperature and hydrological variability. As such, it is well-suited for investigations of past climate variability, comparisons with climate model simulations including isotope-enabled models, and application in paleodata-assimilation projects. The CoralHydro2k database is available in Linked Paleo Data (LiPD) format with serializations in MATLAB, R, and Python and can be downloaded from the NOAA National Center for Environmental Information's Paleoclimate Data Archive at https://doi.org/10.25921/yp94-v135 (Walter et al., 2022).

Warm, not cold temperatures contributed to a Late Miocene reef decline in the Coral Sea

Evidence shows that in the modern ocean, coral reefs are disappearing, and these losses are tied to climate change. However, research also shows that coral reefs can adapt rapidly to changing conditions leading some researchers to suggest that some reef systems will survive future climate change through adaptation. It is known that there were changes in the area covered by coral reefs in the past. Therefore, it is important to investigate the long-term response of coral reefs to environmental changes and high sea-surface temperatures (SSTs). However, because of diagenetic issues with SST proxies in neritic, metastable carbonate-rich environments, there is an incomplete and sometimes even incorrect understanding of how changes in SSTs affect carbonate reef systems. A good example is the Queensland Plateau offshore northeast Australia next to the threatened Great Barrier Reef. In the Late Miocene, between 11 and 7 Ma, a partial drowning caused the reef area on the Queensland Plateau to decline by ~ 50% leading to a Late Miocene change in platform geometry from a reef rimmed platform to a carbonate ramp. This reef decline was interpreted to be the result of SSTs at the lower limit of the modern reef growth window (20–18 °C). This article presents a new Late Miocene—ased SST record from the Coral Sea based on the TEX86H molecular paleothermometer, challenging this long held view. Our new record indicates warm tropical SSTs (27–32 °C) at the upper end of the modern reef growth window. We suggest that the observed temperatures potentially exceeded the optimal calcification temperatures of corals. In combination with a low aragonite supersaturation in the ocean, this could have reduced coral growth rates and ultimately lowered the aggradation potential of the reef system. These sub-optimal growth rates could have made the coral reefs more susceptible to other stressors, such as relative sea-level rise and/or changes in currents leading to reef drowning. Given that these changes affected coral reefs that were likely adapted to high temperature/low aragonite saturation conditions suggests that reefs that have adapted to non-ideal conditions may still be susceptible to future climate changes due to the interaction of multiple stressors associated with climate change.

Geochemical proxies of sea surface temperature from MIS 5e marine fossils in the Taranto Gulf (Ionian Sea, Southern Italy)

Well preserved fossil specimens of Thetystrombus latus, Spondylus gaederopus, Venus verrucosa, Pinna nobilis and Cladocora caespitosa, all coming from a MIS 5e fossiliferous calcarenite outcropping along the Mar Piccolo of Taranto coastline, were analyzed for trace elements and ∂18O and ∂13C isotopic composition in the carbonates of skeletons; with the aim to estimate the Sea Surface Temperature (SST) using different published equations. The adopted methods gave close values of the mean SST, even if with wide standard deviation values: 21.6 °C and of 21.1 °C, derived respectively from the Mg/Ca ratio of P. nobilis and from the Sr/ Ca ratio of C. caespitosa; 19.6 °C derived from the of C. caespitosa growth rate; 21.0 °C derived from the ∂18O. The similarity of these mean results could indicate that a moderate warming with an average of 2.8 °C, in respect to today, is the most likely climatic scenario of the Mar Piccolo of Taranto during the MIS 5e.

Proxy‐Model Comparison for the Eocene‐Oligocene Transition in Southern High Latitudes

AbstractThe Eocene‐Oligocene transition (EOT) marks the shift from greenhouse to icehouse conditions at 34 Ma, when a permanent ice sheet developed on Antarctica. Climate modeling studies have recently assessed the drivers of the transition globally. Here we revisit those experiments for a detailed study of the southern high latitudes in comparison to the growing number of mean annual sea surface temperature (SST) and mean air temperature (MAT) proxy reconstructions, allowing us to assess proxy‐model temperature agreement and refine estimates for the magnitude of the pCO2 forcing of the EOT. We compile and update published proxy temperature records on and around Antarctica for the late Eocene (38–34 Ma) and early Oligocene (34–30 Ma). Compiled SST proxies cool by up to 3°C and MAT by up to 4°C between the timeslices. Proxy data were compared to previous climate model simulations representing pre‐ and post‐EOT, typically forced with a halving of pCO2. We scaled the model outputs to identify the magnitude of pCO2 change needed to drive a commensurate change in temperature to best fit the temperature proxies. The multi‐model ensemble needs a 30 or 33% decrease in pCO2, to best fit MAT or SST proxies respectively. These proxy‐model intercomparisons identify declining pCO2 as the primary forcing of EOT cooling, with a magnitude (200 or 243 ppmv) approaching that of the pCO2 proxies (150 ppmv). However individual model estimates span a decrease of 66–375 ppmv, thus proxy‐model uncertainties are dominated by model divergence.

Development of a novel sea surface temperature proxy based on bacterial 3-hydroxy fatty acids

Gram-negative bacterial 3-hydroxy fatty acids (3-OH-FAs) have received recent attention for their potential as palaeoclimate proxies. A novel temperature proxy, the ratio of anteiso to normal C13 3-OH-FA (RAN13), has been proposed for sea surface temperature (SST) reconstruction in the North Pacific Ocean. However, whether this newly proposed temperature proxy is applicable to marginal seas with significant terrigenous input or tropical oligotrophic seas requires further investigation. Here, we analyzed the composition and distribution of 3-OH-FAs and evaluated the possible impact of various environmental parameters (SST, water depth, dissolved oxygen, salinity and nutrient concentration) on their distribution in marine surface sediments from the Bohai Sea (BS) and the South China Sea (SCS). In the BS, the potential source proxy, fractional abundance of anteiso 3-OH-FAs (average 17%), indicates 3-OH-FA geochemical signature are not greatly overprinted by terrigenous inputs. The relative abundance of long-chain 3-OH-FAs (C15-C18) are higher in the SCS (average 41%) compared to those in other seas (average 33% for all marine samples). Massive inputs of terrigenous organic matter to the BS likely result in overestimation of SSTs based on the RAN13 proxy, and limited abundance of anteiso and normal C13 3-OH-FAs in the oligotrophic SCS may increase the uncertainty of the RAN13 estimated SSTs. More importantly, we find that most short-chain 3-OH-FAs are temperature dependent, especially the fractional abundance of i-C12, a-C13, i-C14 and n-C14 with a high determination coefficient (R2 > 0.60). Based on these newly found correlations, we propose a novel proxy: RANs. The RANs index shows a strong linear relationship with SST (R2 = 0.92, p< 0.001, n = 85) and more accurate prediction than the RAN13, especially in tropical samples. Furthermore, the RANs proxy is significantly correlated with TEX86, and RANs-based SSTs are approximate to LDI derived temperature in the SCS, which support the reliability of RANs as a temperature proxy. These findings further suggest 3-OH-FA based proxies have potential for paleo-SST reconstruction, especially at higher and lower ends of the ocean temperature spectrum and even in cases where marginal inputs of terrestrial organic matter and nutrients are high.

Unraveling the mechanisms and implications of a stronger mid-Pliocene Atlantic Meridional Overturning Circulation (AMOC) in PlioMIP2

Abstract. The mid-Pliocene warm period (3.264–3.025 Ma) is the most recent geological period in which the atmospheric CO2 concentration was approximately equal to the concentration we measure today (ca. 400 ppm). Sea surface temperature (SST) proxies indicate above-average warming over the North Atlantic in the mid-Pliocene with respect to the pre-industrial period, which may be linked to an intensified Atlantic Meridional Overturning Circulation (AMOC). Earlier results from the Pliocene Model Intercomparison Project Phase 2 (PlioMIP2) show that the ensemble simulates a stronger AMOC in the mid-Pliocene than in the pre-industrial. However, no consistent relationship between the stronger mid-Pliocene AMOC and either the Atlantic northward ocean heat transport (OHT) or average North Atlantic SSTs has been found. In this study, we look further into the drivers and consequences of a stronger AMOC in mid-Pliocene compared to pre-industrial simulations in PlioMIP2. We find that all model simulations with a closed Bering Strait and Canadian Archipelago show reduced freshwater transport from the Arctic Ocean into the North Atlantic. This contributes to an increase in salinity in the subpolar North Atlantic and Labrador Sea that can be linked to the stronger AMOC in the mid-Pliocene. To investigate the dynamics behind the ensemble's variable response of the total Atlantic OHT to the stronger AMOC, we separate the Atlantic OHT into two components associated with either the overturning circulation or the wind-driven gyre circulation. While the ensemble mean of the overturning component is increased significantly in magnitude in the mid-Pliocene, it is partly compensated by a reduction in the gyre component in the northern subtropical gyre region. This indicates that the lack of relationship between the total OHT and AMOC is due to changes in OHT by the subtropical gyre. The overturning and gyre components should therefore be considered separately to gain a more complete understanding of the OHT response to a stronger mid-Pliocene AMOC. In addition, we show that the AMOC exerts a stronger influence on North Atlantic SSTs in the mid-Pliocene than in the pre-industrial, providing a possible explanation for the improved agreement of the PlioMIP2 ensemble mean SSTs with reconstructions in the North Atlantic.

First-order analyses on the role of surface wind in the long-term contraction of the Indo-Pacific warm pool

Due to its high evaporation rate, the Indo-Pacific Warm Pool (IPWP) is one of the most important drivers of Indonesian weather and climate. Previous studies, based on the Sea Surface Temperature (SST) proxy records, suggest that IPWP in the mid-Holocene era (~6000 years ago) underwent a contraction (colder on its east-west perimeter and warmer on its center) compared to today’s condition. In this research, the role of surface wind in contracting the IPWP was analyzed by checking the coherency between changes in SST, wind-stress magnitude, and evaporation. The Climate Community System Model version 4 (CCSM4) simulated these three physical quantities under the pre-Industrial and mid-Holocene scenarios. In these simulations, an anti-phase relation between SST and wind-stress magnitude indicates an important role for a weaker surface wind in warming the SST in the center of the IPWP (South China Sea and Banda Sea), mainly during boreal autumn. However, a weaker surface wind did not seem to have simultaneously suppressed ocean evaporation to warm the SST, as shown by the phase-lag relation in their monthly climatology. On the other hand, colder SSTs on the east-west perimeter of the IPWP (western coast of Sumatra and northern coast of Papua) are unlikely to be associated with changes in the surface wind following a weak correlation between their SST and wind-stress magnitude

3-Hydroxy fatty acids as proxies for seawater temperature and pH in the eastern China marginal seas

3-Hydroxy fatty acids (3-OH-FAs) have recently been proposed as proxies for terrestrial, lacustrine and marine environment reconstruction. However, their application in the latter is not yet fully understood. Here we analyzed 3-OH-FAs in surface sediments from the eastern China marginal seas (ECMS), aiming to explore the application of 3-OH-FA-based proxies in marine temperature and pH reconstruction. Results showed that both the empirical mean annual air temperature (MAAT) proxies RAN15 and RAN17 (ratio of anteiso to normal isomers for C15 and C17 homologues, respectively), and the mean annual sea surface temperature (SST) proxy RAN13 were significantly correlated with SST (with RAN15 showing the strongest correlation and highest amplitude response), indicating the applicability of 3-OH-FA-based proxies developed in terrestrial and marine settings in the ECMS. The strongest correlation of RAN15 with SST is probably attributable to the relatively higher abundance and larger variation range of anteiso C15 homologues in the sediments. Moreover, the above proxies were also significantly correlated with mean annual bottom water temperature (BWT) with slightly stronger correlation than with SST, suggesting an important contribution of bottom layer to sedimentary 3-OH-FAs. In particular, the temperature proxies showed marked seasonal variation and a stronger correlation with BWT during warm seasons. A novel temperature proxy (RAI15, the ratio of anteiso to iso C15 isomers) provided strong correlations with BWT in summer and autumn, suggesting that RAI15 is a robust potential BWT proxy for warm seasons in the ECMS. Based on the current data, the sedimentary 3-OH FAs could be a mixture of lipids both produced from the surface and the bottom water layers, and which layer sea temperature the proxies better reflect need to be further explored although a slightly stronger correlation with BWT than SST was found in the present study. In addition, the empirical soil 3-OH-FA-based pH proxies were not suitable in this study, although the influence of pH on marine 3-OH-FAs was confirmed based on a locally developed calibration. Our findings contribute to the understanding of the potential use of bacterial 3-OH-FAs as environmental proxies in temperate marginal seas.

Spatial and temporal distributions of glycerol dialkyl glycerol tetraethers in suspended materials in the northern East China Sea: Applicability as a paleotemperature proxy

The spatial and temporal distributions of the glycerol dialkyl glycerol tetraether (GDGT) concentrations in suspended materials were investigated to determine whether the GDGT-based TEX86 can be used as a proxy for annual mean sea surface temperature in the northern East China Sea (ECS). Seawater samples were collected seasonally at three-month intervals from the surface and subsurface of the northern ECS. GDGT in the surface seawater was detected consistently across all seasons. However, the TEX86 temperatures of the surface seawater samples differed significantly from the in situ temperatures over the entire study area; in particular, TEX86 temperatures were higher (4–7 °C) than in situ temperatures in winter, and lower (9–12 °C) than in situ temperatures in summer. The occurrence of a high concentration of GDGTs near the bottom, with marked differences between TEX86 and in situ temperatures in the shallow water (~30 m) supports the possibility of sediment resuspension near the bottom in the western part and subsequent transportation eastward. GDGTs may have been resuspended owing to vertical mixing by strong winds, such as those from the monsoon (winter) and typhoons (summer), and increased turbulence near the bottom caused by tides across all seasons in the shallow waters (30–50 m). In contrast, the vertical distribution of GDGT concentration in the eastern part of the study area deeper than 100 m indicated that maximum GDGT concentrations occurred at depths of between 20 m and 50 m in all seasons. The TEX86 temperatures of the subsurface samples were similar to or warmer than in situ temperatures between 20 m and 50 m, suggesting that the main depth at which GDGT was produced in the eastern area could be the subsurface in that same depth range. A new local calibration equation was established based on the subsurface samples from the east. Our results indicate that sediment resuspension and subsurface production of GDGTs can have a significant impact on the use of GDGT-based TEX86 as a SST proxy in continental shelf area.

Warmer western tropical South Atlantic during the Last Interglacial relative to the current interglacial period

The Last Interglacial (LIG, 129–116 thousand years ago) is an excellent case study for global warming scenarios and a target for proxy-model comparisons. The LIG global average sea surface temperature (SST) was ~0.5 °C higher than pre-industrial (PI). Contrary to the global average, tropical SST proxy compilations and model simulations show a negative anomaly in LIG SST relative to PI. Here, we present a LIG SST record from marine sediment core GL-1180 retrieved from the western tropical South Atlantic (WTSA). The SST record is based on Mg/Ca ratios of planktonic foraminifera Globigerinoides ruber (white). Our results indicate a warmer LIG in the WTSA relative to PI and Holocene conditions. We show that a positive LIG SST anomaly in tropical regions can be explained by polar sea ice loss during the LIG, which warms the ocean surface all the way into the tropics. The disagreement between proxy results and model simulations from the fourth phase of the Paleoclimate Modelling Intercomparison Project could result from uncertainties in our proxy for SST and/or limitations of numeric models in capturing transient forcings and feedbacks in Earth's climate system and imprecise boundary conditions. Additional studies are warranted to better constrain the LIG SST evolution in tropical regions.

Comparing age model techniques for the last glacial cycle: A case study of ten Iberian Margin sediment cores

Because interpretations of paleoclimate records from ocean sediment cores rely on age models to estimate age as a function of core depth, age model uncertainty can affect the conclusions drawn from paleoclimate data. To assess the age model uncertainty associated with three methods, we compare the age models they generate over the last glacial cycle for high-resolution data from ten western and southern Iberian Margin cores. The first method uses radiocarbon data and the Bayesian statistical software Bacon [Blaauw and Christen, 2011]. The second method aligns benthic δ18O to a target core using the probabilistic alignment algorithm HMM-Match [Lin et al., 2014]. The third method is alignment of planktonic δ18O or sea surface temperature (SST) using the deterministic alignment software Match [Lisiecki and Lisiecki, 2002]. Where three or more age model types overlap in a core, the average range of age estimates is 1.7 kyr, and 90% of age ranges are less than 3 kyr. For the two probabilistic methods, HMM-Match and Bacon, age estimates agree to within uncertainty, and the highest resolution records yield 95% confidence interval widths of 1–2 kyr. However, HMM-Match appears to underestimate alignment uncertainty near gaps in benthic δ18O data in one core, as indicated by disagreement with the core's planktonic δ18O and SST alignments. Planktonic δ18O appears to provide the most precise age models for Iberian Margin cores due to strong millennial-scale variability and good correlation between aligned cores. Probabilistic alignment algorithms should be developed for planktonic δ18O and SST proxies to improve estimates of the age uncertainty for these alignments.

A palaeothermometer of ancient Indigenous fisheries reveals increases in mean temperature of the catch over five millennia

Climate change is altering the distribution and composition of marine fish populations globally, which presents substantial risks to the social and economic well-being of humanity. While deriving long-term climatic baselines is an essential step for detecting and attributing the magnitude of climate change and its impacts, these baselines tend to be limited to historical datasets and palaeoecological sediment records. Here, we develop a method for estimating the ‘ancient Mean Temperature of the Catch’ (aMTC) using Indigenous fisheries catch records from two archaeological sites in the northeast Pacific. Despite different catch compositions, we observe an increase in aMTC over a 5,000-year period at two contemporaneously occupied archaeological sites in southwestern British Columbia, Canada. We document cooler catches from 5,000 to 3,000 cal yr BP and comparatively warmer catches from 1,800 to 250 cal yr BP. These warmer temperatures are broadly consistent with palaeoceanographic sea surface temperature proxies from British Columbia and Alaska. Because this method requires converting measures of fish bones into estimates of fish size structure, abundance, biomass, and finally aMTC, opportunities exist to account for both variation and uncertainty at every step. Nevertheless, given that preindustrial fisheries data are ubiquitous in coastal archaeological sites, this method has the potential to be applied globally to broaden the temporal and geographic scale of ocean temperature baselines.

The Long chain Diol Index: A marine palaeotemperature proxy based on eustigmatophyte lipids that records the warmest seasons

Long chain 1,13- and 1,15-diols are lipids which are omnipresent in marine environments, and the Long chain Diol Index (LDI), based on their distributions, has previously been introduced as a proxy for sea surface temperature. The main biological sources for long chain 1,13- and 1,15-diols have remained unknown, but our combined lipid and 23S ribosomal RNA (23S rRNA) analyses on suspended particulate matter from the Mediterranean Sea demonstrate that these lipids are produced by a marine eustigmatophyte group that originated before the currently known eustigmatophytes diversified. The 18S rRNA data confirm the existence of early-branching marine eustigmatophytes, which occur at a global scale. Differences between LDI records and other paleotemperature proxies are generally attributed to differences between the seasons in which the proxy-related organisms occur. Our results, combined with available LDI data from surface sediments, indicate that the LDI primarily registers temperatures from the warmest month when mixed-layer depths, salinity, and nutrient concentrations are low. The LDI may not be applicable in areas where Proboscia diatoms contribute 1,13-diols, but this can be recognized by enhanced contributions of C28 1,12 diol. Freshwater input may also affect the correlation between temperature and the LDI, but relative C32 1,15-diol abundances help to identify and correct for these effects. When taking those factors into account, the calibration error of the LDI is 2.4 °C. As a well-defined proxy for temperatures of the warmest seasons, the LDI can unlock important and previously inaccessible paleoclimate information and will thereby substantially improve our understanding of past climate conditions.

Performance of temperature and productivity proxies based on long-chain alkane-1, mid-chain diols at test: a 5-year sediment trap record from the Mauritanian upwelling

Abstract. Proxies based on long-chain alkane-1, mid-chain diols (diol for short) are obtaining increasing interest to reconstruct past upper ocean temperature and productivity. Here we evaluate performance of the sea surface temperature proxies (long-chain diol index (LDI), diol saturation index (DSI), and diol chain length index (DCI)), productivity and upwelling intensity proxies (two diol indices DIR and DIW and the combined diol index (CDI)), and the nutrient diol index (NDI) as a proxy for phosphate and nitrate levels. This evaluation is based on comparison of the diols in sediment trap samples from the upwelling region off NW Africa collected at 1.28 km water depth with daily satellite-derived sea surface temperatures (SSTs), subsurface temperatures, productivity, the plankton composition from the trap location, monthly phosphate and nitrate concentrations, wind speed, and wind direction from the nearby Nouadhibou airport. The diol-based SST reconstructions are also compared the long-chain-alkenone-based SST reconstructions. The alkenone SSTs correlate best with satellite SST (r2= 0.60). Amplitude and absolute values agree very well as do the flux-corrected time series averages. For the diol proxies the situation is more complicated. Diol proxies including 1,14 diols lag trade wind speed by 30 d. Since wind is nearly always from the NNE to NNW and induces the upwelling, we relate the variability in these proxies to upwelling-induced processes. Correlation with the abundance of upwelling species and wind speed is best for the NDI and the 1,14 diol-based DCI and DSI. The DIR, DIW, and CDI perform comparatively poorly. A negative correlation between DSI and wind speed may suggest that the DSI reflects wind-speed-forced upwelling-related reductions in temperature rather than irradiation-induced temperatures. The nutrient proxy NDI shows no significant correlation to monthly phosphate and nitrate concentrations in the upper waters and a negative correlation with both wind-induced upwelling (r2=0.28 and lagging 32 d) and the abundance of upwelling species (r2=0.38). It is suggested that this proxy reflects upwelling intensity rather than upper ocean nutrient concentrations. At the trap site, satellite SST lags wind-speed-forced upwelling by about 4 months. The 1,13 and 1,15 diol-based LDI-derived SSTs lag satellite SSTs by 41 d but correlate poorly (r2= 0.17). Absolute as well as flux-corrected LDI SSTs are on average 3 ∘C too high and rather reflect values prevailing during the more oligotrophic summer period. We attribute outliers to low LDI SST to 1,13 diols added during short upwelling-related events. The use of the LDI in regions with higher productivity is therefore not recommended. It appears thus that at the trap site the 1,14 diols primarily reflect conditions relating to upwelling whereas the 1,15C30 and to a lesser extent the 1,13 diols seem to reflect the conditions of the more oligotrophic ocean.

A Simple Monte Carlo Approach to Estimate the Uncertainties of SST and δ18Osw Inferred From Coral Proxies

AbstractThe skeletons of massive corals inhabiting tropical and subtropical oceans grow over centuries. The chemical and isotopic compositions of their skeletons reflect changes in the marine environment and provide an archive of sea surface temperature (SST) and salinity at a high temporal resolution spanning decades to centuries. Stable oxygen isotope ratios in coral skeletons (δ18Oc) are an excellent proxy for SST and the stable oxygen isotopic composition in seawater (δ18Osw). Variations in δ18Osw serve as a hydrological proxy and are routinely estimated by subtracting the SST component from δ18Oc. The SST component of δ18Oc is inferred from coral Sr/Ca (SST proxy). Both Sr/Ca and δ18Oc are converted to SST units based on Sr/Ca‐ and (δ18Oc–δ18Osw)‐SST calibrations. However, the exact slope values of the Sr/Ca‐SST and (δ18Oc–δ18Osw)‐SST calibrations are challenging to assess, which hampers the reliable estimation of SST, δ18Osw, and their uncertainties. Here, we present a simple Monte Carlo approach to estimate variations in SST and δ18Osw together with their confidence intervals. Our approach improves on previous error propagation methods by including intercolonial differences of calibration slopes in addition to analytical errors. In addition, Monte Carlo experiments using adjusted errors suggest that uncertainties in the calibration slopes have a minor influence on estimates of SST and δ18Osw.

Limited Lateral Transport Bias During Export of Sea Surface Temperature Proxy Carriers in the Mediterranean Sea.

Some lipid‐biomarker‐based sea surface temperature (SST) proxies applied in the modern Mediterranean Sea exhibit large offsets from expected values, generating uncertainties in climate reconstructions. Lateral transport of proxy carriers along ocean currents prior to burial can contribute to this offset between reconstructed and expected SSTs. We perform virtual particle tracking experiments to simulate transport prior to and during sinking and derive a quantitative estimate of transport bias for alkenones and glycerol dibiphytanyl glycerol tetraethers (GDGTs), which form the basis of the UK’ 37 and TEX86 paleothermometers, respectively. We use a simple 30‐day surface advection scenario and sinking speeds appropriate for the export of various proxy carriers (6, 12, 25, 50, 100, 250, 500, and 1000 md−1). For the assessed scenarios, lateral transport bias is generally small (always <0.85°C) within the Mediterranean Sea and does not substantially contribute to uncertainties in UK’ 37‐ or TEX86‐based SSTs.

Coccolith Sr/Ca is a robust temperature and growth rate indicator that withstands dynamic microbial interactions.

Coccolithophores are a diverse group of calcifying microalgae that have left a prominent fossil record on Earth. Various coccolithophore relics, both organic and inorganic, serve as proxies for reconstruction of past oceanic conditions. Emiliania huxleyi is the most widely distributed representative of the coccolithophores in modern oceans and is known to engage in dynamic interactions with bacteria. Algal–bacterial interactions influence various aspects of algal physiology and alter algal alkenone unsaturation (UK’ 37), a frequently used organic coccolithophore‐derived paleo‐temperature proxy. Whether algal–bacterial interactions influence inorganic coccolithophore‐derived paleo‐proxies is yet unknown. A commonly used inorganic proxy for past productivity and sea surface temperature is the Sr/Ca ratio of the coccolith calcite. Interestingly, during interactions between bacteria and a population of calcifying algae, bacteria were shown to physically attach only to non‐calcified algal cells, suggesting an influence on algal calcification. In this study, we explore the effects of algal–bacterial interactions on calcification and coccolith Sr/Ca ratios. We find that while bacteria attach only to non‐calcified algal cells, coccolith cell coverage and overall calcite production in algal populations with and without bacteria is similar. Furthermore, we find that Sr/Ca values are impacted only by water temperature and algal growth rate, regardless of bacterial influences on algal physiology. Our observations reinforce the robustness of coccolith Sr/Ca ratios as a paleo‐proxy independent of microbial interactions and highlight a fundamental difference between organic and inorganic paleo‐proxies.