Sea Surface Temperature Proxy Research Articles

Controls on stable strontium isotope fractionation in coccolithophores with implications for the marine Sr cycle

The controls on stable Sr isotope fractionation into the calcite produced by the coccolithophore species Emiliania huxleyi, Coccolithus pelagicus spp. braarudii and Gephyrocapsa oceanica are investigated. Each species has been cultured under controlled laboratory conditions at a range of temperatures (10–25°C) to test the potential of δ88/86Sr as a proxy for growth rate and/or sea surface temperature. Coccolithophores are one of the most abundant pelagic calcifiers; since Sr substitutes to some degree for Ca in their calcite coccoliths, coccolithophores represent a significant output of Sr from seawater, potentially influencing the δ88/86Sr mass balance in the modern oceans. The coccoliths are investigated for their δ88/86Sr, Sr/Ca, and δ18O measured as function of temperature. As temperature is increased all species show a negative stable Sr isotopic fractionation, related to a temperature controlled growth rate. We infer the shift of δ88/86Sr to lighter values as indicative of a kinetic control on the isotope fractionation, revealing the potential of δ88/86Sr as a proxy for growth rate, which in these experiments is primarily dictated by temperature. The results from these experiments indicate that coccolithophore calcite incorporates Sr with a very light δ88/86Sr isotope composition as low as ∼0‰.

Early Paleogene evolution of terrestrial climate in the SW Pacific, Southern New Zealand

We present a long‐term record of terrestrial climate change for the Early Paleogene of the Southern Hemisphere that complements previously reported marine temperature records. Using the MBT′‐CBT proxy, based on the distribution of soil bacterial glycerol dialkyl glycerol tetraether lipids, we reconstructed mean annual air temperature (MAT) from the Middle Paleocene to Middle Eocene (62–42 Ma) for southern New Zealand. This record is consistent with temperature estimates derived from leaf fossils and palynology, as well as previously published MBT′‐CBT records, which provides confidence in absolute temperature estimates. Our record indicates that through this interval, temperatures were typically 5°C warmer than those of today at such latitudes, with more pronounced warming during the Early Eocene Climate Optimum (EECO; ∼50 Ma) when MAT was ∼20°C. Moreover, the EECO MATs are similar to those determined for Antarctica, with a weak high‐latitude terrestrial temperature gradient (∼5°C) developing by the Middle Eocene. We also document a short‐lived cooling episode in the early Late Paleocene when MAT was comparable to present. This record corroborates the trends documented by sea surface temperature (SST) proxies, although absolute SSTs are up to 6°C warmer than MATs. Although the high‐calibration error of the MBT′‐CBT proxy dictates caution, the good match between our MAT results and modeled temperatures supports the suggestion that SST records suffer from a warm (summer?) bias, particularly during times of peak warming.

Northern and southern hemisphere controls on seasonal sea surface temperatures in the Indian Ocean during the last deglaciation

Different proxies for sea surface temperature (SST) often exhibit divergent trends for deglacial warming in tropical regions, hampering our understanding of the phase relationship between tropical SSTs and continental ice volume at glacial terminations. To reconcile divergent SST trends, we report reconstructions of two commonly used paleothermometers (the foraminifera G. ruber Mg/Ca and the alkenone unsaturation index) from a marine sediment core collected in the southwestern tropical Indian Ocean encompassing the last 37,000 years. Our results show that SSTs derived from the alkenone unsaturation index (UK′37) are consistently warmer than those derived from Mg/Ca by ~2–3°C except for the Heinrich Event 1. In addition, the initial timing for the deglacial warming of alkenone SST started at ~15.6 ka, which lags behind that of Mg/Ca temperatures by 2.5 kyr. We argue that the discrepancy between the two SST proxies reflects seasonal differences between summer and winter rather than postdepositional processes or sedimentary biases. The UK′37 SST record clearly mimics the deglacial SST trend recorded in the North Atlantic region for the earlier part of the termination, indicating that the early deglacial warming trend attributed to local summer temperatures was likely mediated by changes in the Atlantic Meridional Overturning Circulation at the onset of the deglaciation. In contrast, the glacial to interglacial SST pattern recorded by G. ruber Mg/Ca probably reflects cold season SSTs. This indicates that the cold season SSTs was likely mediated by climate changes in the southern hemisphere, as it closely tracks the Antarctic timing of deglaciation. Therefore, our study reveals that the tropical southwestern Indian Ocean seasonal SST was closely linked to climate changes occurring in both hemispheres. The austral summer and winter recorded by each proxy is further supported with seasonal SST trends modeled by Atmosphere–ocean General Circulation Models for our core site. Our interpretation that the alkenone and Mg/Ca SSTs are seasonally biased may also explain similar proxy mismatches observed in other tropical regions at the onset of the last termination.

Sr/Ca–sea surface temperature calibration in the coral Porites lutea from subtropical northern South China Sea

Strontium to calcium (Sr/Ca) ratios in annual bands of Porites corals are commonly used as past sea surface temperature proxy in tropical oceans, whereas studies that refer to Sr/Ca–SST calibration in high-latitude corals are rather rare in general. Here we present a 50-year (1961–2010) near-fortnightly Sr/Ca record in two Porites lutea corals living near the northern limit of reef-building corals in the subtropical northern South China Sea, where winter temperatures are extreme for hermatypic corals. The Sr/Ca time series shows a clear annual cyclicity, with marked sharp/narrow winter peaks. Comparison with an instrumental SST record (since 1961) indicates that this Sr/Ca record is chiefly modulated by SST. Two different methods are discussed for calibrating Sr/Ca with SST. The correlation between interpolated Sr/Ca and SST is significant, but the coefficient is relatively low, mainly attributed to extreme cold winter SSTs (<~16°C) which cause a significant decrease or even cessation of coral skeletal extension. Another modified method using only annual low Sr/Ca values which could clearly be matched with the same features in the instrumental SST record is more reliable. The modified regression equation does not encompass SSTs below 24°C. The fidelity of the coral-based Sr/Ca–SST calibration was assessed and the precision of the method is estimated to be 0.31±0.62°C based on the calibration–verification exercise. The modified method is suggested in marginal coral areas with extreme intra-annual SST variation (e.g., from 13 to 30°C).

A low latitude paleoclimate perspective on Atlantic multidecadal variability

Traces of environmental conditions found in natural archives can serve as proxies for direct climate measurements to extend our knowledge of past climate variability beyond the relatively short instrumental record. Such paleoclimate proxies have demonstrated significant multidecadal climate variability in the Atlantic sector since at least the mid-1700s. However, Atlantic multidecadal climate variability is primarily defined by fluctuations in sea surface temperature (SST) and the proxy evidence comes from a variety of sources, many of which are terrestrial and are not directly recording sea surface temperature. Further analysis into the causes and consequences of Atlantic multidecadal climate variability requires development of a spatial network of decadal resolution proxy SST records with both low and high latitude contributions. An initial attempt at a low latitude Atlantic SST reconstruction found only 4 sites with ≤5year resolution data, demonstrating the paucity of appropriate data available. The 4-site average correlated significantly with instrumental average SST and the Atlantic Multidecadal Oscillation (AMO). The full record, 1360–2000C.E., and a shortened version 1460–1850C.E., had significant multidecadal variability centered at a 60-year period. Comparing our reconstruction with reconstructions of SST anomalies in the North Atlantic shows that there is no consensus yet on the history of Atlantic multidecadal variability.

Comparison of organic (UK'37, TEXH86, LDI) and faunal proxies (foraminiferal assemblages) for reconstruction of late Quaternary sea surface temperature variability from offshore southeastern Australia

Several proxies have been developed to reconstruct past sea surface temperature (SST), but different proxies may reflect temperatures of different seasons and each proxy is characterized by certain uncertainties. Therefore, a multiproxy approach is preferred to precisely reconstruct SST. Here, we reconstruct SST of the ocean offshore southeastern Australia (Murray Canyons area) for the last ~135 ka using three independent organic proxies (TEXH86 based on glycerol dialkyl glycerol tetraethers (GDGTs), UK'37 based on alkenones, and LDI based on long‐chain diols) in addition to foraminiferal faunal assemblages. The organic proxy records show similar trends, with the highest temperature (21°C for UK'37 and TEXH86, and 25°C for LDI) during the last interglacial and lowest temperature (8°C for TEXH86, 10°C for UK'37, and 12°C for LDI) during the Last Glacial Maximum. However, the differences in absolute SST estimates obtained by the organic proxies varied over time with differences of up to 9°C between LDI and TEXH86. The seasonal SST reconstructions based on the modern analogue of foraminiferal assemblages also show similar trends as the organic proxies with highest temperatures during the last interglacial (23°C for the warmest month SST, 20°C for mean annual, and 18°C for the coolest month) and lowest temperature during the Last Glacial Maximum (14°C for the warmest month, 11°C for mean annual, and 9°C for the coolest month). Down core comparison between the reconstructed SSTs of the organic and inorganic proxies shows that LDI‐inferred temperatures compare well with the temperature of the warmest month, TEXH86 with the temperature of the coolest month, and UK'37 with mean annual temperature. An increase in TEXH86 SST estimates relative to those of other proxies during deglaciations and interglacials suggests that either winter temperatures rapidly warmed, possibly due to an invigoration of the Leeuwin Current over the core site, or there was a change in the growth season of the Thaumarchaeota, the source organism of GDGTs. Our study shows the benefits of a multiproxy approach in the interpretation of SST proxies, leading to a more robust knowledge of past ocean temperature changes.

A skeletal Sr/Ca record preserved in Dipsastraea (Favia) speciosa and implications for coral Sr/Ca thermometry in mid‐latitude regions

A core (900 mm long) of the scleractinian coral Dipsastraea (Favia) speciosa was collected from Iki Island (∼33°48′N), Japan, one of the highest latitude coral reefs known to exist at present, where winter monthly mean sea surface temperature (SST) drops to 13°C. The Sr/Ca profile was constructed using a bulk sampling method for the uppermost 280 mm interval of the core, which grew between 1966 and 2007, to test whether it could act as a suitable proxy for SST in a harsh environmental setting where reef‐building coral do not usually survive. The Sr/Ca‐SST relationship derived from the annual Sr/Ca and SST extremes predicted the observed monthly averaged summer SST extremes within an error range of ±1.1°C (1 s.d., n = 40). The obtained Sr/Ca‐SST calibration was also found to be valid for subtropical Dipsastraea (Favia) corals, proving its broad applicability. However, low‐amplitude winter peaks were observed in the slow‐growing intervals, which we confirmed (using individual spot analysis along a continuous growth line) result from the mixing of theca grown at different times. Our bulk sampling approach, across multiple growth lines in the skeleton of D. (F.) speciosa, led to the mixing of asynchronous skeletal part. At the study site, D. (F.) speciosa grows continuously, even during the cold season, suggesting that the skeletal Sr/Ca obtained from specimens of D. (F.) speciosa can be used as an SST proxy in the northwest Pacific marginal seas.

Seemingly divergent sea surface temperature proxy records in the central Mediterranean during the last deglaciation

Abstract. Sea surface temperatures (SSTs) were reconstructed over the last 25 000 yr using alkenone paleothermometry and planktonic foraminifera assemblages from two cores of the central Mediterranean Sea: the MD04-2797 core (Siculo–Tunisian channel) and the MD90-917 core (South Adriatic Sea). Comparison of the centennial scale structure of the two temperature signals during the last deglaciation period reveals significant differences in timing and amplitude. We suggest that seasonal changes likely account for seemingly proxy record divergences during abrupt transitions from glacial to interglacial climates and for the apparent short duration of the Younger Dryas (YD) depicted by the alkenone time series, a feature that has already been stressed in earlier studies on the Mediterranean deglaciation.

What do SST proxies really tell us? A high-resolution multiproxy (UK′37, TEXH86 and foraminifera δ18O) study in the Gulf of Taranto, central Mediterranean Sea

We present a multiproxy reconstruction of sea surface temperatures (SST) and coastal environmental changes covering the last 600 years on sediments from the Gulf of Taranto, central Mediterranean Sea. The reconstruction is based on UK′37 (alkenones from haptophytes), TEXH86 (membrane lipids of marine crenarchaeota), and δ18O and δ13C of Globigerinoides ruber (white and pink) and of Uvigerina mediterranea. The amplitudes of the temperature signals reconstructed from δ18O of G. ruber (white), TEXH86 and UK′37 exceed the amplitudes observed in other reconstructions of local and Northern Hemisphere temperature. UK′37-based SSTs reflect mainly winter/spring conditions with an additional influence of changing nutrient supplies related to water column mixing and runoff. TEXH86-based temperatures reflect SSTs of the oligotrophic summer season, while influences from near-coastal areas may complicate its interpretation. Co-variation between both lipid-based SST records suggests a common environmental mechanism during the last 600 years. δ18O of G. ruber (white) also reflects summer conditions and is amplified by changes in salinity and nutrient availability, which are caused by variations in the relative influence of the Western Adriatic Current (WAC) and of the Ionian Surface Waters (ISW). The combination of SSTTEXH86 and δ18O of G. ruber (white) shows that the circulation in the Gulf of Taranto underwent significant changes during the last 600 years.

Abrupt variability of the last 24 ka BP recorded by coccolithophore assemblages off the Iberian Margin (core MD03‐2699)

ABSTRACTA high‐resolution coccolithophore study allowed reconstructing sea surface conditions during the last 24 ka BP off the Iberian Margin. Variations in nannoplankton accumulation rate, cold‐water species, Florisphaera profunda, Umbilicosphaera sibogae and Coccolithus pelagicus ssp. azorinus trends suggest the occurrence of substantial changes in surface water dynamics. Palaeoproductivity shows changes during the glacial and the interglacial transition from intense glacial upwelling to persistent interglacial stratification. A drastic productivity decline occurred between ∼18 and 13 ka BP related to the arrival of subpolar waters indicated by the presence of the subarctic subspecies C. pelagicus pelagicus and increased percentages of tetra‐unsaturated alkenones. Abrupt variability at core MD03‐2699 during the last 24 ka BP was observed in palaeoproductivity and sea surface temperatures proxies. Increases of reworked species fluxes were observed during the Last Glacial Maximum and during the transition to the Holocene, likely connected to turbidity currents and/or contourites formed by Mediterranean outflow intensification. In addition, C. pelagicus ssp. pelagicus presence, Gephyrocapsa muellerae and tetra‐alkenone percentages reveal rapid coolings coeval with the Holocene Bond cycles and rapid climate coolings recognized for the Northern Hemisphere between 11.02 and 1.7 ka BP and associated with subpolar water arrival at the site. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Changing patterns of marine resource exploitation by hunter-gatherers throughout the late Holocene of Argentina are uncorrelated to sea surface temperature

During the late Holocene, the coastal marine resources of southern South America were exploited by both marine hunter-gatherers with aquatic mobility in the Beagle Channel and the Chilean archipelago and terrestrial hunter-gatherers who inhabited various coastal settings in Argentina. Although the two cultures differed in technology and in historical exploitation patterns, they both used otariids, molluscs, fishes and sea birds as sources of food and raw materials for centuries. Ultimately, their use of these resources, particularly of otariids, declined strongly at different times. Overexploitation has been suggested as the main reason for this pattern, at least in the Beagle Channel, but similar declines in the north Pacific have been attributed to an increase in sea surface temperature (SST). The present paper tests the latter hypothesis in southern South America by using the δ18O of bivalve shells (Aulacomya atra atra and Mytilus edulis) collected at archaeological sites as a proxy for SST and comparing the patterns of δ18O with the patterns of resource exploitation by hunter-gatherers. Samples were collected from the Beagle Channel and the central-northern Patagonian coast (north to 43° S) to generate two comparable datasets. The results suggest that SST increased in both areas at the beginning of the late Holocene and was slightly higher than at present during most of that period, except during the Little Ice Age, when values similar to those recorded at the end of the middle Holocene were found. The relative importance of otariids, mainly Arctocephalus australis, in the economy of the inhabitants of the Beagle Channel declined as SST increased, but otariid exploitation did not intensify again during the Little Ice Age. On the contrary, the intensity of otariid exploitation in central-northern Patagonia, mainly Otaria flavescens, was unrelated to the changes in δ18O. Thus, changes in SST are unlikely to be the major driver of these resource-exploitation patterns.

Comparison and implication of TEX86 and U37K' temperature records over the last 356 kyr of ODP Site 1147 from the northern South China Sea

Proxy records from the South China Sea (SCS) have played an important role in understanding late Quaternary SCS environmental changes and the East Asian Monsoon (EAM) variations, but few long sea surface temperature (SST) records have been generated from the SCS. The recently established TEX86 index provides an opportunity to generate long SST records, especially for tropical oceans, but the applicability and climatic interpretation of the TEX86 index for the SCS need further study. We report and compare orbital scale resolution (ca. 5.3kyr) TEX86 and U37K' temperature records of the past 356kyr for ODP Site 1147 from the northern SCS. Both records display clear and similar glacial–interglacial patterns. The U37K' SST fluctuated between 22.3°C and 28.3°C, with a glacial–interglacial difference of 4–5°C; while the TEX86 temperature fluctuated between 20.3°C and 25.4°C, with a slightly smaller glacial–interglacial difference of ca. 4°C. The TEX86 temperature has been consistently lower than the U37K' temperature over the past 356kyr, consistent with previous studies suggesting that the U37K' index is a proxy for SST but the TEX86 index is a proxy for subsurface temperature. Thus, it is proposed that the temperature difference (ΔT) between the U37K' and TEX86 records reflects mixed layer depth (MLD) changes in the northern SCS, which fluctuated between 0.3 and 4.0°C with high values mostly during interglacials and lower values mostly during glacial stages. MLD changes in the northern SCS are interpreted to be driven by EAM changes, with stronger winter monsoon resulting in a deeper MLD and a smaller ΔT. Thus, the combination of U37K' and TEX86 records and their ΔT can be used to reconstruct East Asian winter monsoon variations, as well as their influences on the hydrographic structure of the SCS.

Oxygen isotopes from Phorcus (Osilinus) turbinatus shells as a proxy for sea surface temperature in the central Mediterranean: A case study from Malta

The marine topshell, Phorcus (Osilinus) turbinatus, is a common component of many archaeological sites in the Mediterranean. This species has been successfully used as a palaeoclimate proxy in Italy. To test whether δ18O from P. turbinatus shells can serve as a reliable palaeoclimate archive for other regions of the Mediterranean, we collected live P. turbinatus from the northeast coast of Malta each month for a year. The δ18OSHELL values of the outermost growth increments of these live-collected shells ranged between −0.4 and +2.4‰. These values correspond to growing temperatures calculated from shell edge δ18O of between 15°C and 27°C. Calculated shell edge sea surface temperatures are highly correlated with instrumental records of sea surface temperature recorded over the period of collection. The individuals analysed for this study are smaller than P. turbinatus from populations studied elsewhere in the Mediterranean. Nonetheless, δ18OSHELL provides a robust record of sea surface temperatures, suggesting that smaller/younger shells in archaeological deposits can still provide reliable palaeothermometry records. This study extends the upper growth limit P. turbinatus by 2°C compared with the previous studies of P. turbinatus in the Mediterranean and suggests that, contrary to the previous studies, growth shutdown does not occur in all P. turbinatus when sea surface temperatures exceed 25°C. This may reflect the higher sample resolution that can be obtained from smaller/faster growing shells, or it may reflect actual higher growth tolerances of P. turbinatus populations in Malta. By showing that P. turbinatus precipitate their shells in δ18O equilibrium with surrounding sea water, this study reinforces the potential for the stable isotope chemistry of P. turbinatus shells preserved in Mediterranean archaeological sites to provide a window into the climate and seasonality regimes of the past.

Hydroxylated isoprenoidal GDGTs in the Nordic Seas

Archaea modify their membrane lipid composition to maintain integrity and exchange capabilities across a wide temperature range. The number of rings in the tetraether structure increases with higher ambient temperatures. Recently, novel glycerol dialkyl glycerol tetraethers (GDGTs) with an additional hydroxyl group on the alkyl chain were described (OH-GDGTs). Here we show that at a regional scale in the Nordic Seas (western Barents, Norwegian, Greenland, and Iceland Seas), the OH-GDGTs' contribution to the total archaeal GDGT pool as well as the ratio of specific OH-GDGTs versus crenarchaeol are higher in colder waters, especially in polar water masses. We also observe a shift in the number of rings of the OH-GDGTs with temperature. Thus, both the contribution of the OH-GDGTs to the total isoprenoidal GDGT pool and the number of rings in OH-GDGTs could be used as potential indicators of polar waters and might complement the currently used paleo sea surface temperature proxies in polar regions.

The construction of causal networks to estimate coral bleaching intensity

Current metrics for predicting bleaching episodes, e.g. NOAA's Coral Reef Watch Program, do not seem to apply well to Brazil's marginal reefs located in Bahia state and alternative predictive approaches must be sought for effective long term management. Bleaching occurrences at Abrolhos have been observed since the 1990s but with a much lower frequency/extent than for other reef systems worldwide. We constructed a Bayesian Belief Network (BN) to back-predict the intensity of bleaching events and learn how local and regional scale forcing factors interact to enhance or alleviate coral bleaching specific to Abrolhos. Bleaching intensity data were collected for several reef sites across Bahia state coast (∼12°–20°S; 37°–40°W) during the austral summer 1994–2005 and compared to environmental data: sea surface temperature (SST), diffuse light attenuation coefficient at 490 nm (K490), rain precipitation, wind velocities, and El Niño Southern Oscillation (ENSO) proxies. Conditional independence tests were calculated to produce four specialized BNs, each with specific factors that likely regulate bleaching intensity. All specialized BNs identified that a five-day accumulated SST proxy (SSTAc5d) was the exclusive parent node for coral bleaching producing a total predictive rate of 88% based on SSTAc5d state. When SSTAc5d was simulated as unknown, the Thermal-Eolic Resultant BN kept the total predictive rate of 88%. Our approach has produced initial means to predict beaching intensity at Abrolhos. However, the robustness of the model required for management purposes must be further (and regularly) operationally tested with new in situ and remote sensing data.

Sr/Ca Proxy Sea-Surface Temperature Reconstructions from Modern and HoloceneMontastraea faveolataSpecimens from the Dry Tortugas National Park, Florida, U.S.A.

Flannery, J.A. and Poore, R.Z., 2013. Sr/Ca proxy sea-surface temperature reconstructions from modern and Holocene Montastraea faveolata specimens from the Dry Tortugas National Park In: Brock, J.C.; Barras, J.A., and Williams, S.J. (eds.), Understanding and Predicting Change in the Coastal Ecosystems of the Northern Gulf of Mexico, Journal of Coastal Research, Special Issue No. 63, pp. 20–31, Coconut Creek (Florida), ISSN 0749-0208.Sr/Ca ratios from skeletal samples from two Montastraea faveolata corals (one modern, one Holocene, ∼6 Ka) from the Dry Tortugas National Park were measured as a proxy for sea-surface temperature (SST). We sampled coral specimens with a computer- driven triaxial micromilling machine, which yielded an average of 15 homogenous samples per annual growth increment. We regressed Sr/Ca values from resulting powdered samples against a local SST record to obtain a calibration equation of Sr/Ca = -0.0392 SST 10.205, R = -0.97. The resulting calibration was used to generate a 47-year modern (1961–2008) and a 7-year Holocene (∼6 Ka) Sr/Ca subannually resolved proxy record of SST. The modern M. faveolata yields well-defined annual Sr/Ca cycles ranging in amplitude from ∼0.3 and 0.5 mmol/mol. The amplitude of ∼0.3 to 0.5 mmol/mol equates to a ∼10–15°C seasonal SST amplitude, which is consistent with available local instrumental records. Summer maxima proxy SSTs calculated from the modern coral Sr/ Ca tend to be fairly stable: most SST maxima from 1961–2008 are 29°C ± 1°C. In contrast, winter minimum SST calculated in the 47-year modern time-series are highly variable, with a cool interval in the early to mid-1970s.The Holocene (∼6 Ka) Montastraea faveolata coral also yields distinct annual Sr/Ca cycles with amplitudes ranging from ∼0.3 to 0.6 mmol/mol. Absolute Sr/Ca values and thus resulting SST estimates over the ∼7-year long record are similar to those from the modern coral. We conclude that Sr/Ca from Montastraea faveolata has high potential for developing subannually resolved Holocene SST records.

Early mid‐Holocene SST variability and surface‐ocean water balance in the southwest Pacific

We present early mid‐Holocene records of Sr/Ca,δ18O and δ18Osw from marine archives collected in Vanuatu: two Porites sp. corals (6.7–6.5 ka BP) and a Tridacna maxima giant clam (6.2–6.0 ka BP). Sr/Ca, δ18O, and δ18Osw were used as proxies for sea surface temperature (SST) and sea surface salinity (SSS). The fossil geochemical records were compared to modern Porites sp. and T. maxima records. Reconstructed mean SSTs from the two fossil Porites sp.and from the modern coral are similar, implying that the Western Pacific Warm Pool (WPWP)' southern edge had reached its modern location by 6.7–6.5 ka BP. The post‐glacial SST rise in the Southwest Pacific was thus completed by the early mid‐Holocene. The two early mid‐Holocene corals and the giant clam recorded saltier conditions than modern related to 1) a decoupling between the precipitation regime and the SPCZ due to a northerly position of this climatic feature and 2) an increase of the moisture transport to the extra‐tropics, driven by a strengthened or extended Hadley cell. The longestδ18O coral profile displays an El Niño Southern Oscillation (ENSO) signal reduced by 20–30% compared to the period 1928–1992, in concordance with the reduced ENSO variability observed in the Pacific area during the first half of the Holocene. However, the decoupling between the SPCZ and the precipitation regime may have also contributed to the weak ENSO signal recorded in the early mid‐Holocene coralδ18O profile.

Reconstruction of the late-Holocene changes in the Sub-Arctic Front position at the Reykjanes Ridge, north Atlantic

Changes in the dynamics of the North Atlantic subpolar gyre are involved in the modulation of the northward salinity and heat transport in the northern North Atlantic via the North Atlantic Current (NAC). Variations in the strength of the East Greenland Current (EGC) can influence the gyre dynamics by impacting deep convection in the Labrador Sea. Oxygen isotope data of three planktonic foraminiferal species (surface water Neogloboquadrina pachyderma dextral coiling and Globigerina bulloides; thermocline recorder Globorotalia inflata) from a site located close to the present Sub-Arctic Front at the Reykjanes Ridge suggest significant strengthening or shifting of the Sub-Arctic Front throughout the late Holocene. The oxygen isotope based inferences are supported by Mg/Ca-derived temperature reconstructions from Neogloboquadrina pachyderma dextral coiling, alkenone-derived sea surface temperature and other paleoclimatic proxy data. The late-Holocene strengthening/shift of the Sub-Arctic Front appears caused by an increasingly more defined and fresher EGC. The proposed subpolar gyre changes may modulate the northward heat transport, and explain the geographically different long-term climatic trends in the North Atlantic during the late Holocene, i.e. a cooling of the EGC-influenced regions and a warming of the NAC-influenced areas from c. 4 to 5 ka. This mechanism cannot, however, explain the simultaneous occurrence of millennial-scale events at c. 5.6, 3.9, 2.7, 1.3 ka and the ‘Little Ice Age’ in both areas. Noteworthy is the steadily increasing amplitude of these cold events at the Reykjanes Ridge, likely induced by drift ice and/or EGC-influence culminating in the ‘Little Ice Age’. A widespread pronounced warming at 2.0 ka seems to represent the ‘Roman Warm Period’ and reflects the warmest period of the late Holocene.

Sedimentation and burial of organic and inorganic temperature proxies in the Mozambique Channel, SW Indian Ocean

Paleoceanographic studies strongly rely on proxies to reconstruct past environmental conditions. However, several factors influence the reliability of these proxies, particularly during sedimentation and burial. In this study, we measured both inorganic (δ 18O and Mg/Ca on three species of planktonic foraminifera ( Globigerinoides ruber, Globigerinoides trilobus and Neogloboquadrina dutertrei) and organic ( U 37 k ′ and TEX 86) sea surface temperature (SST) proxies on core top sediment taken from an east-west transect across the Mozambique Channel. We contrast our findings with previously published modern time-series temperature proxy data from a sediment trap moored in the mid-channel. The coretop sediment was analyzed for 14C, 210Pb and excess (xs) 234Th. While 234Th xs data indicate a flux of fresh particulate matter to the bottom sediment, radiocarbon dating shows that the core top sediments are composed of material that is on average about 1000 years old. The fine organic carbon is consistently (even though only slightly) younger than the coarser foraminiferal calcite, which is likely caused by preferential downcore mixing of the fine fraction. Besides vertical mixing by bioturbation, stable lead isotope ratios from the time-series particle fluxes indicate episodic lateral transport of old particles from the shelf to the deep Mozambique Channel as an additional source of pre-aged material in core tops. Core top temperature proxies show warmer values close to the channel flanks while colder values are found in the mid-channel. These could be associated with higher maximum summer temperatures in modern coastal waters in contrast to the mid-channel. Additionally, we find an offset in all foraminiferal proxies between core top samples and time-series data that corresponds to 1–3 °C, which probably reflects climate variability over the past ∼2000 years. However, this temperature difference is not observed in the organic proxies U 37 k ′ and TEX 86, which may result from current transport of unconsolidated organic matter that can homogenize the organic proxy signal prior to burial.

Biogeochemical controls on glycerol dialkyl glycerol tetraether lipid distributions in sediments characterized by diffusive methane flux

The TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms) is a proxy for sea surface temperature (SST) based on the distribution of isoprenoidal glycerol dialkyl glycerol tetraether (GDGT) membrane lipids synthesized by marine pelagic Thaumarchaeota. One of the caveats of this proxy is the production of additional GDGTs by sedimentary Euryarchaeota involved in anaerobic oxidation of methane (AOM) that occurs at deep-sea methane seeps but is also widespread in many continental shelf settings. Here, GDGT distributions are investigated through the sulfate-methane transition zone (SMTZ) in Aarhus Bay, Denmark, to examine the extent the TEX86 proxy is compromised in such a continental shelf setting where AOM is characterized by a diffusive rather than rapid advective methane flux. Both free extractable and non-extractable lipid fractions were analyzed as it was expected that pelagic-derived and sediment-derived GDGTs could become incorporated into the molecular and sedimentary matrix to a different extent. The results show a large change of TEX86 values mainly due to the relative high amounts of GDGT-2 produced by AOM-related Archaea, both in the free and non-extractable lipid fractions. This additional GDGT input renders calculation of SSTs based on TEX86 inappropriate at the SMTZ. The AOM-related GDGT signature, however, did not persist into deeper sediments, perhaps reflecting rapid remineralization of the GDGTs at the SMTZ. Although the process of AOM at Aarhus Bay might not be representative for all continental margin settings, it illustrates that AOM in a variety of settings, not just cold seeps, can influence sedimentary GDGT distributions.