Sea Surface Temperature Reconstruction Research Articles

Surface and deep water variability in the Western Mediterranean (ODP Site 975) during insolation cycle 74: High-resolution calcareous plankton and molecular biomarker signals

We reconstructed changes in productivity and surface/subsurface and deep-water dynamics in the Western Mediterranean through a multi-proxy study of Ocean Drilling Program Site 975 between late Marine Isotope Stage (MIS) 20 and early interglacial MIS 19. Our high-resolution study (down to ~200-year resolution) combines calcareous plankton assemblages (coccolithophores and foraminifera), biomarkers (C37-alkenones, n-alkanes, n-alcohols) and elemental proxies (total organic carbon, total nitrogen, calcium carbonate). Surface water conditions are derived (i) from high-resolution δ18O and δ13C records obtained from the planktonic foraminifer Globigerina bulloides, and (ii) from summer and winter, foraminifera-based sea surface temperature reconstructions (SSTJAS-foram, SSTJFM-foram) achieved through transfer function. The integration of the whole dataset makes it possible to identify in the Balearic Sea, and to accurately characterize for the first time, an Organic Rich Layer (ORL) during latest MIS 20-early MIS 19, close to i-cycle 74. Its presence is marked firstly by higher values of total nitrogen (TN) and an increase of total C37-alkenone and total organic carbon (TOC) preserved in the sediments. The multi-proxy approach reveals that the deglacial phase played a prominent role for ORL formation that was characterized by centennial scale phases. The alcohol preservation index (API) suggests that the shoaling of the circulation, which boosted marine productivity, started in the deglaciation and, in combination with freshening by Atlantic water inflow/riverine input and surface water buoyancy during sea level rising, culminated during the ORL event. At this time calcareous plankton proliferated on subsurface-surface waters, benefiting from ameliorating conditions, which promoted maximum marine productivity and higher organic matter preservation on the seafloor.

The quality of the reproduction of El Nino and La Nina events using different reconstructed sea surface temperature data sets

In this paper, using various data sets of the reconstructed ocean surface temperature — HadISST, COBE SST2 and ERSSTv5 — the differences are showed in the identification of El Nino and La Nina events with their subsequent separation on the East Pacific (EP) and Central Pacific (CP) types from 1870 to 2018. The SST data of the TOGA-TAO buoys from 1981 to 2018, the Terra satellite (MODIS) from 2001 to 2018 and the NOAA IO SSTv2 from 1982 to 2018 were used for a verification of the quality of SST reconstructions in the equatorial Pacific. The obtained results in this work demonstrate differences in the number of EP and CP types of El Nino and La Nina for the early period from 1870 to 1900, when the data were less reliable, and for the entire time series, including the last two decades. According to the study results, it was shown that ERSSTv5 overestimates the number of CP El Nino by 42,8 % relative to HadISST and by 28,6 % relative to COBE SST2 and underestimates the number of EP El Nino by 23,8 % relative to HadISST and by 20 % relative to COBE SST2. The choice between the COBE SST2 and HadISST data sets as a result of comparing of the selected El Nino and La Nina events showed the possibility to use of both arrays is almost equivalent.

Endolithic Algae Affect Modern Coral Carbonate Morphology and Chemistry

While burial diagenetic processes of tropical corals are well investigated, current knowledge about factors initiating early diagenesis remains fragmentary. In the present study we focus on recent Porites microatolls, growing in the intertidal zone. This growth form represents a model organism for elevated sea surface temperatures and provides important but rare archives for changes close to the seawater/atmosphere-interface with exceptional precision on sea level reconstruction. As other coral growth forms, microatolls are prone to the colonization by endolithic green algae. In this case the algae can facilitate earliest diagenetic alteration of the coral skeleton. Algae metabolic activity not only results in secondary coral porosity due to boring activities, but may also initiate reprecipitation of secondary aragonite within coral pore space, a process not exclusively restricted to microatoll settings. In the samples of this initial study, we quantified a mass transfer from primary to secondary aragonite of around 4% within endolithic green algae bands. Using δ18O, δ13C, Sr/Ca, U/Ca, Mg/Ca, and Li/Mg systematics suggest that the secondary aragonite precipitation followed abiotic precipitation principles. According to their individual distribution coefficients the different isotope and element ratios showed variable sensitivity to the presence of secondary aragonite in bulk samples, with implications for microatoll-based sea surface temperature (SST) reconstructions. The secondary precipitates formed on an organic template, presumably originating from endolithic green algae activity. Based on laboratory experiments with the green algae Ostreobium quekettii we propose a conceptual model that secondary aragonite formation is potentially accelerated by an active intracellular calcium transport through the algal thallus from the location of dissolution into coral pore spaces. The combined high-resolution imaging and geochemical approach applied in this study shows that endolithic algae can possibly act as a main driver for earliest diagenesis of coral aragonite starting already during a coral’s life span.

Reductions in Strong Upwelling‐Favorable Wind Events in the Pliocene

AbstractProxy records of the Pliocene show significant warm anomalies of sea surface temperatures (SST) near midlatitude upwelling sites at the eastern boundaries of the Pacific and Atlantic Oceans. Weaker upwelling‐favorable mean winds or a deeper thermocline have been proposed as explanations, yet the mechanisms involved are still not clear and the dramatic warmings of up to 9°C are only partially accounted for. Here we quantify the response of strong “transient” upwelling‐favorable wind events, defined to have a wind velocity over 5 m s−1 and a duration of at least 3 days, to two reconstructions of Pliocene SST. Such events are responsible for much of the modern upwelling SST signal and may therefore be a more relevant measure than the climatological winds. We find that both the amplitude and duration of upwelling‐favorable events are reduced in the Pliocene scenarios. Using Pliocene Reconstruction Interpretation and Synoptic Mapping SST, we find an annual reduction in upwelling flux between 7.4% and 13.2%, depending on model resolution. With an idealized Pliocene SST, the reduction in upwelling flux is between 46.7% and 50.3%. This reduction is shown to be closely related to changes in anticyclonic atmospheric activity, associated in turn with a weaker large‐scale meridional SST gradient.

Reconstruction of sea-surface temperatures in the Canary Islands during Marine Isotope Stage 11

AbstractThe study of paleoclimates enables us to improve and better constrain climate models in order to forecast future climate variations. Marine Isotope Stage 11 (MIS11), which began around 425,000 yr BP and lasted about 65,000 yr, is a warm isotope stage of paramount importance, because the astronomical configuration was similar to the one characterizing the Holocene. Therefore, this warm isotope stage is the most appropriate analog to present-day climate known to date. This study aims to provide new data on sea-surface temperatures (SSTs) inferred from the carbon and oxygen isotope compositions of skeletal carbonates of marine invertebrates preserved in two marine deposits of the Canary Islands located at Piedra Alta, Lanzarote, and Arucas, Gran Canaria. According to published isotopic fractionation equations the marine deposit from Arucas recorded SSTs of 15.9 ± 2.2°C on average, while the tsunamite from Piedra Alta recorded SSTs of 21.2 ± 1.9°C on average. Absolute dating, mollusc assemblages, and calculated marine temperatures suggest that the Arucas marine deposit corresponds to the beginning of MIS11, while the Piedra Alta tsunamite was formed during MIS11c. These results show that low latitudes also experienced sizable SST changes during interglacial stages.

Using Late Pleistocene sea surface temperature reconstructions to constrain future greenhouse warming

Future greenhouse warming projections conducted with coupled climate models still exhibit a substantial spread in response to a given anthropogenic greenhouse gas concentration scenario. In order to constrain this spread and to provide robust warming projections, our understanding of Earth's global-mean surface temperature response to radiative forcing (referred to as climate sensitivity) needs to be further refined. Here we estimate an averaged glacial/interglacial climate sensitivity using 25 transient Earth system model simulations of the Last Glacial Cycle and a global-mean sea surface temperature (SST) reconstruction derived from 64 globally-distributed paleo-proxies of SST. Our results document that Earth's averaged Late Pleistocene equilibrium climate sensitivity is in the order of ∼4.2 K per CO2 doubling. Using the Representative Concentration Pathway 8.5 for future greenhouse radiative forcing, this value translates into a global-mean surface warming of ∼5.0 K by the year 2100 relative to pre-industrial levels. This estimate is in excellent agreement with the ensemble-mean projection of climate simulations conducted as part of the Coupled Model Intercomparison Project Phase 5 (CMIP5). Our uncertainty analysis reveals further that the lack of robust reconstructions of glacial aerosol forcing is a key contributor to the overall uncertainty of paleo-based estimates of climate sensitivity.

Postglacial paleoceanography of the western Barents Sea: Implications for alkenone-based sea surface temperatures and primary productivity

The increasing influence of Atlantic Water (AW) in the Barents Sea, a process known as “Atlantification”, is gradually decreasing sea ice cover in the region. Ongoing global climate warming is likely to be one of its drivers, but to further understand the role of natural variability and the biogeochemical impacts of the inflow of AW into the western Barents Sea, we reconstructed sea surface temperatures (SSTs) and primary productivity in Storfjordrenna, a climatically sensitive area south of Spitsbergen, between approximately 13,950 cal yr BP and 1300 cal yr BP. The alkenone U37K* proxy has been applied to reconstruct SSTs, and the alkenone accumulation rate in marine sediments has been used to infer changes in primary productivity. Our data show that the SST increase was concomitant with the progressive loss of sea ice cover and an increase in primary productivity in the western Barents Sea. We interpret these changes as reflecting the increasing influence of AW in the area as the ice sheets retreated in Svalbard. The transition from the Arctic to the Atlantic domain first occurred after 11,500 cal yr BP, as the Arctic Front moved eastward of the study site but with considerable variability in surface ocean conditions. High SSTs at approximately 6400 cal yr BP may have led to limited winter surface cooling, likely inhibiting convective mixing and the return of nutrients to the euphotic zone and/or enhanced organic matter consumption by zooplankton due to an earlier light signal in the ice-free Storfjordrenna. During the late Holocene (3600-1300 cal yr BP), low insolation facilitated sea ice formation and thus brine production. The former may have launched convective water mixing and increased nutrient resupply to the sea surface, consequently enhancing primary productivity in Storfjordrenna. We propose that, on the basis of the paleoceanographic evidence, the modern increasing inflow of warm AW and the disappearance of pack ice on the Eurasian continental shelf are likely to weaken convective water mixing and decrease primary production in the region.

Reconstruction of Large-Scale Sea Surface Temperature and Salinity Fields Using Sub-Regional EOF Patterns From Models

A method for reconstruction of gridded fields of sea surface variables from time-dependent observations, using sub-regional EOF (Empirical Orthogonal Functions) patterns from models, is presented and tested. Covariance fields, calculated from the model results over long enough time span, are used to find EOF modes. The gravest “observational” amplitudes and their first temporal derivatives are determined from the least-square minimization of fitting errors in relation to the observed values. The field is reconstructed by superposition of continuous model-based mode patterns multiplied by observational amplitudes that meet adopted statistical limits. If the observational amplitude exceeds the limits, gridded fields for this and higher modes are not produced. We applied the method in the northeastern Baltic over the model time series 2010-2015. Daily averages of sea surface temperature (SST) and salinity (SSS) from the high-resolution (grid step 0.5 nautical miles) sub-regional HBM model were spatially averaged over bins of 5x5 nautical miles. Three first modes cover 99% of variance of temperature and 61.4% of salinity. As shown by experiments with pseudo-observations (model values at these points reconstructed to the model grid and then compared with the original model data), reconstruction performance depends on the configuration of the observation points in the model domain. Still, a few first modes usually produce acceptable results. When removing the SST seasonal cycle prior to EOF analysis, spatial patterns of leading modes remained practically unchanged, share of variance of the three first modes was reduced to 88,6% and reconstruction errors were reduced by about 25%. Sufficient spatial data coverage of the larger basin with ship-born observations usually takes quite long time – of the order of month; therefore, time correction of the amplitudes using the found temporal derivatives improves the accuracy of reconstruction. The method is compared with the Optimal Interpolation (OI) by using the pseudo-observations. Results show that, for SST reconstruction, the OI method is significantly worse than the EOF method. For SSS, OI is slightly better than EOF. The superiority of EOF is that the remote correlation patterns can be used in the reconstruction, which is important when the observations are sparse.

Tree-ring indicators of winter-spring temperature in Central China over the past 200 years

Long-term, high-resolution proxy records containing temperature signals are relatively scarce over Central China, limiting our understanding of regional climate and the potential driving forces. In this study, a robust tree-ring width chronology is developed based on Pinus massoniana woods from two sites on the northeast Wuling Mountains, spanning from 1815 to 2014. The presented chronology is used to reconstruct regional long-term winter-spring (January-May) temperature changes over the past two centuries in Central China. The reconstruction reveals that cold epochs occurred in 1830 s–1840 s, 1850 s–1860 s, 1900 s–1910 s and 1960 s–1970 s. The correlation patterns between the reconstruction and global sea surface temperature (SST) are suggestive of the influence of SST of the western North Pacific Ocean (especially the Japan Current region) and the North Atlantic Ocean on regional temperature. In addition, persistent close matches are observed between the chronology and the Atlantic Multidecadal Oscillation (AMO) reconstructions over the past two centuries, suggesting the AMO may have been a key modulator of long-term temperature variations over Central China.

Coupling Influences of ENSO and PDO on the Inter-Decadal SST Variability of the ACC around the Western South Atlantic

The Antarctic Circumpolar Current (ACC) plays an important role in the climate as it balances heat energy and water mass between the Pacific and Atlantic Oceans through the Drake Passage. However, because the historical measurements and observations are extremely limited, the decadal and long-term variations of the ACC around the western South Atlantic Ocean are rarely studied. By analyzing reconstructed sea surface temperatures (SSTs) in a 147-year period (1870–2016), previous studies have shown that SST anomalies (SSTAs) around the Antarctic Peninsula and South America had the same phase change as the El Niño Southern Oscillation (ENSO). This study further showed that changes in SSTAs in the regions mentioned above were enlarged when the Pacific Decadal Oscillation (PDO) and the ENSO were in the same warm or cold phase, implying that changes in the SST of higher latitude oceans could be enhanced when the influence of the ENSO is considered along with the PDO.

Influences of Temperature and Secondary Environmental Parameters on Planktonic Foraminiferal Mg/Ca: A New Core‐Top Calibration

AbstractThe accuracy of the Mg/Ca paleothermometer is contested over the influences of secondary environmental parameters such as salinity and pH. Recent calibrations based on compiled sediment trap and laboratory culture data suggest moderate influences from salinity and pH. Core tops are the best analogues to downcore samples used for paleoceanographic reconstruction with well‐constrained environmental parameters and thus can be used to validate sediment trap and laboratory culture calibrations. Here, we calibrate new core‐top Mg/Ca data in Globigerinoides ruber (white) and Trilobatus sacculifer (without final sac‐like chamber) with sea surface temperature, salinity, and pH. Part of these coretops were previously used to argue for a large salinity effect on G. ruber (w) Mg/Ca (Arbuszewski et al., 2010, http://10.0.3.248/j.epsl.2010.10.035). Our new G. ruber (w) Mg/Ca data are on average 12% lower than the previous results. Our calibrations yield Mg/Ca‐temperature sensitivities of 8.1 ± 0.7%/°C for G. ruber (w) and 6.6 ± 0.8%/°C for T. sacculifer (w/o sac), and Mg/Ca salinity effects of 4.7 ± 2.4%/‰ for G. ruber (w) and 5.5 ± 2.3%/‰ for T. sacculifer (w/o sac). These results agree well with culture experiments but discount the large salinity effects reported in previous core‐top studies. Our data reveal insignificant pH effects on Mg/Ca in both species. Overall, our core‐top calibrations for G. ruber (w) and T. sacculifer (w/o sac) lend strong support to previous calibrations, strengthening our confidence in the use of planktonic Mg/Ca as a reliable proxy for sea surface temperature reconstructions.

Global Core Top Calibration of δ18O in Planktic Foraminifera to Sea Surface Temperature

AbstractThe oxygen isotopic composition of planktic foraminiferal calcite ( ) is one of the most prevalent proxies used in the paleoceanographic community. The relationship between , temperature, and seawater oxygen isotopic composition ( ) is firmly rooted in thermodynamics, and experimental constraints are commonly used for sea surface temperature (SST) reconstructions. However, in marine sedimentary applications, additional sources of uncertainty emerge, and these uncertainty constraints have not as of yet been included in global calibration models. Here, we compile a global data set of over 2,600 marine sediment core top samples for five planktic species: Globigerinoides ruber, Trilobatus sacculifer, Globigerina bulloides, Neogloboquadrina incompta, and Neogloboquadrina pachyderma. We developed a suite of Bayesian regression models to calibrate the relationship between and SST. Spanning SSTs from 0.0 to 29.5 °C, our annual model with species pooled together has a mean standard error of approximately 0.54‰. Accounting for seasonality and species‐specific differences improves model validation, reducing the mean standard error to 0.47‰. Example applications spanning the Late Quaternary show good agreement with independent alkenone‐based estimates. Our pooled calibration model may also be used for reconstruction in the deeper geological past, using modern planktic foraminifera as an analog for non‐extant species. Our core top‐based models provide a robust assessment of uncertainty in the paleothermometer that can be used in statistical assessments of interproxy and model‐proxy comparisons. The suite of models is publicly available as the Open Source software library bayfox, for Python, R, and MATLAB/Octave.

A 45-year sub-annual reconstruction of seawater temperature in the Bay of Brest, France, using the shell oxygen isotope composition of the bivalve Glycymeris glycymeris

A reconstruction of sea surface temperature (SST) spanning 45 years (1966–2011) was developed from δ18O obtained from the aragonitic shells of Glycymeris glycymeris, collected from the Bay of Brest, France. Bivalve sampling was undertaken monthly between 2014 and 2015 using a dredge. In total, 401 live specimens and 243 articulated paired valves from dead specimens were collected, of which 24 individuals were used to reconstruct SST. Temperatures determined using the palaeotemperature equation of Royer et al. compared well with observed SST during the growing season between 1998 and 2010 (Pearson’s correlation: p = 0.002, r = 0.760). Furthermore, a significant negative correlation was found between the reconstructed SST and the North Atlantic Subpolar Gyre (SPG) index ( p = 0.001, r = −0.50), and a significant positive correlation was found with the East Atlantic Pattern (EAP) index when the reconstructed SST was lagged by 1 year ( p = 0.002, r = 0.46). This led to the conclusion that EAP and SPG are major influences on SSTs in the Bay of Brest. As the SPG controls air temperature in Northern Europe and the EAP controls water temperature in Southern Europe, this suggests that the Bay of Brest is an interaction area between these two climate systems. As such, this locality is interesting as the δ18O of the shells can be used as a proxy for both the SPG and EAP, and temperature reconstructions can provide a unique insight into how these climate systems interacted prior to the instrumental era.

Temperature Evolution of the Indo‐Pacific Warm Pool Over the Holocene and the Last Deglaciation

AbstractThe Indo‐Pacific Warm Pool (IPWP) contains the warmest surface ocean waters on our planet making it a major source of heat and moisture to the atmosphere. Changes in the extent and position of the IPWP likely impacted the tropical and global climate in the past and may also do in the future. With the aim to put recent ocean changes into a longer temporal context, we present new paleoceanographic sea surface temperature reconstructions from the heart of the Western Pacific Warm Pool, which is the warmest region within the IPWP, across the last 17,000 years. To provide an improved spatial and temporal regional context we use new and published sea surface temperature records from the IPWP and update previous compilation efforts (Linsley et al., 2010, https://doi.org/10.1038/ngeo1920). We similarly conclude that the IPWP was warmer in the early Holocene compared to the late Holocene. However, with the new data we are able to argue against a western displacement/expansion of the IPWP and suggest a warmer southern IPWP in the early Holocene. We explore the potential drivers of the Holocene IPWP cooling and propose a combination of processes including changes in the monsoonal winds associated with the position of the rain belt, the tropical Pacific mean climate, and upper water column mixing. The proposed climatic processes differentially impacted the IPWP subregions resulting in spatially diverse trends. Additionally, the late deglacial section of the records mostly show a gradual IPWP warming similar in structure to the atmospheric CO2 and/or Antarctica rising temperatures.

Correction Factors for δ18O-Derived Global Sea Surface Temperature Reconstructions From Diagenetically Altered Intervals of Coral Skeletal Density Banding

Reconstruction of sea surface temperature (SST) from the δ18O and Sr/Ca composition of coral skeletal density banding (CSDB), identified with x-ray diffraction and micro computed tomography, provides invaluable centuries-long records of ocean circulation and climate change. Comparison with age-equivalent instrument measurements of SST over the last 125 years has proven these δ18O-derived SST reconstructions to be generally reliable. However, notable exceptions occur within discrete CSDB stratigraphic intervals that yield δ18O-derived SST underestimates of as much as 9°C with respect to instrument measured SST. Here we combine high-resolution optical and electron microscopy with geochemical modeling to establish correction factors for the impact of marine seafloor physical, chemical, and biological alteration (diagenesis) within these altered intervals of CSDB stratigraphy. Four cores were collected from Porites coral heads across a 4-24 m water depth bathymetric transect at Myrmidon Reef, Great Barrier Reef, Australia. Precise mapping of diagenetic aragonite cementation was completed within CSDB patterns digitally overlaid on 35 petrographic thin sections fully covering 2.1 m of core. The vast majority of core skeletal material exhibited little to no diagenetic aragonite cementation. However, extensive diagenetic alteration was observed within discrete CSDB intervals near the base of the two deeper water Porites heads. This diagenesis serves to modify skeletal density and CSDB stratigraphy in these intervals, as well as structurally reinforce the coral skeleton. Reliable δ18O-based SST correction factors for these diagenetically altered CSDB intervals are established here by applying the percent mixing of diagenetic aragonite cement to a binary mixing model. This approach, with quantitative extents of mixing established with both microscopy and existing globally distributed coral δ18O and Sr/Ca data sets, accurately restores modern and fossil coral δ18O-derived SST records. Results indicate that as little as 5% mixing of diagenetic aragonite cement with original coral skeleton will cause δ18O-based SST anomalies of 0.9°C.

Holocene hydrography evolution in the Alboran Sea: a multi-record and multi-proxy comparison

Abstract. A new high-resolution deglacial and Holocene sea surface temperature (SST) reconstruction is presented for the Alboran Sea (western Mediterranean), based on Mg∕Ca ratios measured in the planktonic foraminifera Globigerina bulloides. This new record is evaluated by comparison with other Mg∕Ca SST records and previously published alkenone SST reconstructions from the same region for both the Holocene and glacial periods. In all cases there is a high degree of coherence between the different Mg∕Ca SST records but strong discrepancies when compared to the alkenone SST records. We argue that these discrepancies are due to differences in the proxy response during deglaciation which we hypothesize to reflect a resilience strategy of G. bulloides, changing its main growth season, and consequently Mg∕Ca records a shorter deglacial warming than alkenones. In contrast, short-term Holocene SST variability is larger in the Mg∕Ca SST than in the alkenone SST records. We propose that the larger Mg∕Ca SST variability is a result of spring temperatures variability, while the smoothed alkenone SST variability represents averaged annual temperatures. The Mg∕Ca SST record differentiates the Holocene into three periods: (1) the warmest SST values occurred during the Early Holocene (11.7–9 cal. kyr BP), (2) a continuous cooling trend occurred during the Middle Holocene that culminated in the coldest Holocene SST having a double cold peak structure centred at around 4.2 cal. kyr BP, and (3) the Late Holocene (4.2 cal. kyr BP to present) did not follow any clear cooling/warming trend although millennial-scale oscillations were enhanced. This SST evolution is discussed in the context of the changing properties in the Atlantic inflow water associated with North Atlantic circulation conditions and also with local hydrographical and atmospheric changes. We propose that a tight link between North Atlantic circulation patterns and the inflow of surface waters into the Mediterranean played a major role in controlling Holocene climatic variability of this region.

Multi-proxy reconstruction of sea surface and subsurface temperatures in the western South Atlantic over the last ∼75 kyr

Millennial-scale oscillations are known to be important in the climatic evolution of the Atlantic basin, but which internal processes originates these oscillations are still uncertain. In this study, we investigated how the Greenland and Antarctic climates affect the SW Atlantic through basin-wide oceanographic features (such as the NADW formation and the Agulhas leakage). We reconstructed sea surface and subsurface temperatures (SST and subT) using three lipid-based biomarker proxies (UK’37, TEX86 and LDI indexes) from a sediment core (NAP 63-1) retrieved from the SW Atlantic slope (24.8°S, 44.3°W). This location allowed us to evaluate the temperature oscillations of the Brazil Current without any terrigenous or upwelling-derived biases. Both TEX86-based and LDI-based estimates represent the mean annual SST, while the UK’37-based estimates represent the subT (around 30 m water depth). The periods with the most well-mixed water column were observed during intervals of cooling orbital trends due to the time required to transfer the surface cooling to the subsurface. The temperature reconstructions showed a general colder MIS 3 when compared to the MIS 4. They also showed evidence of a late response to deglaciation, with its onset in the SW Atlantic occurring in the middle of the Last Glacial Maximum. Based on these reconstructions, the NAP 63-1 SST orbital-scale trend seems to be linked to the Antarctic climate, influenced by local insolation changes. These temperature records also presented a clear millennial periodicity around 8 kyr. On this timescale, the millennial oscillations in the SW Atlantic's SST are likely linked to the NADW formation.

Sensitivity to species selection indicates the effect of nuisance variables on marine microfossil transfer functions

Abstract. The species composition of many groups of marine plankton appears well predicted by sea surface temperature (SST). Consequently, fossil plankton assemblages have been widely used to reconstruct past SST. Most applications of this approach make use of the highest possible taxonomic resolution. However, not all species are sensitive to temperature, and their distribution may be governed by other parameters. There are thus reasons to question the merit of including information about all species, both for transfer function performance and for its effect on reconstructions. Here we investigate the effect of species selection on planktonic foraminifera transfer functions. We assess species importance for transfer function models using a random forest technique and evaluate the performance of models with an increasing number of species. Irrespective of using models that use the entire training set (weighted averaging) or models that use only a subset of the training set (modern analogue technique), we find that the majority of foraminifera species does not carry useful information for temperature reconstruction. Less than one-third of the species in the training set is required to provide a temperature estimate with a prediction error comparable to a transfer function that uses all species in the training set. However, species selection matters for paleotemperature estimates. We find that transfer function models with a different number of species but with the same error may yield different reconstructions of sea surface temperature when applied to the same fossil assemblages. This ambiguity in the reconstructions implies that fossil assemblage change reflects a combination of temperature and other environmental factors. The contribution of the additional factors is site and time specific, indicating ecological and geological complexity in the formation of the sedimentary assemblages. The possibility of obtaining multiple different reconstructions from a single sediment record presents a previously unrecognized source of uncertainty for sea surface temperature estimates based on planktonic foraminifera assemblages. This uncertainty can be evaluated by determining the sensitivity of the reconstructions to species pruning.

Shelf exposure influence on Indo-Pacific Warm Pool climate for the last 450,000 years

The Indo-Pacific Warm Pool is the largest source of heat and rainfall on Earth, but the mechanisms driving long-term climate variability in this region remain uncertain. Some studies suggest that precessional variations in insolation exert a primary role; alternatively, sea level change may be more important, with exposure of the Sunda and Sahul shelves leading to weakened Walker circulation across the Indian Ocean during glacial periods. The limited number of paleoclimate records from the warm pool that extend beyond the Last Glacial Maximum makes it difficult to test these competing hypotheses. Here, we present a 450,000-year long reconstruction of sea surface temperatures, sub-surface temperatures, and regional vegetation from southern Sumatra. We show that the dominant signal in each record is the 100 ky glacial cycle. We find that both surface and sub-surface temperatures cool during glacial periods, but the sub-surface cools more, reflecting a shoaling of the thermocline. Southern Sumatra and western Java consistently exhibit increased C4 grass coverage and an enhanced dry season during glacial periods. The observed changes in the thermocline and rainfall seasonality are consistent with the shelf exposure mechanism, suggesting that sea level changes exert a prominent role in warm pool climate over glacial-interglacial timescales.

Commentary: Reconstructing Four Centuries of Temperature-Induced Coral Bleaching on the Great Barrier Reef

Coral reefs are spectacular ecosystems found along tropical coastlines where they provide goods and services to hundreds of millions of people. While under threat from local factors, coral reefs are increasingly susceptible to ocean warming from anthropogenic climate change. One of the signature disturbances is the large-scale, and often deadly, breakdown of the symbiosis between corals and dinoflagellates. This is referred to as mass coral bleaching and often causes mass mortality. The first scientific records of mass bleaching date to the early 1980s (Hoegh-Guldberg et al., 2017). Kamenos and Hennige (2018, hereafter KH18), however, claim to show that mass coral bleaching is not a recent phenomenon, and has occurred regularly over the past four centuries (1572–2001) on the Great Barrier Reef (GBR), Australia. They support their claim by developing a putative proxy for coral bleaching that uses the suggested relationship between elevated sea surface temperatures (SSTs) and reduced linear extension rates of 44 Porites spp. coral cores from 28 GBR reefs. If their results are correct, then mass coral bleaching events have been a frequent feature for hundreds of years in sharp contrast to the vast majority of scientific evidence. There are, however, major flaws in the KH18 methodology. Their use of the Extended Reconstructed Sea Surface Temperature (ERSST) dataset (based on ship and buoy observations) for reef temperatures from 1854 to 2001, ignores the increasing unreliability of these data which become sparse, less rigorous, and more interpolated going back in time. To demonstrate how the quality of these data degrades, we plot the average number of SST observations per month that contribute to each 200 x 200 km ERSST pixel (Figure 1A, black line). Note that from 1854 to 1900 the four ERSST pixels used by KH18 averaged only 0.85 observations per month, and 82% of these months had no observations at all. Given the heterogeneous nature of SST at local and regional levels, using such broad-scale data as ERSST, is likely to produce substantial errors at reef scales (Figure 1A, red line prior to 1900).