Paleo Sea Surface Temperature Research Articles

Picking up the pieces: Assessing alternative methods for reconstructing whole red abalone (Haliotis rufescens) shell size from fragments

Dense layers of red abalone shells (“red abalone middens”) have been heavily studied along the California coast for their implications for pre-contact subsistence, paleo-sea surface temperatures, and nearshore ecology. However, shells in these deposits are typically broken which lessens their scientific value. To compensate for this problem, we evaluate four alternative methods for predicting the size of whole abalone shells from fragments. The first was a template made up of the outlines of complete modern and archaeological shells. The others were based on regression formulas that rely on measurements of shell features that potentially track with shell growth: (1) maximum diameter of the last developed aperture, (2) distance between the last two developed apertures, (3) maximum diameter of the muscle scar. We conducted a blind study with archaeological laboratory volunteers to evaluate the accuracy of these methods, and found that the template outperformed the allometric methods, and, of the latter, the muscle scar measurement was the most accurate. Our results provide a framework for future studies to assess variation in abalone size from fragmented shells as a proxy for past climate change and harvesting pressure. They also provide a methodological approach for reconstructing sizes of other shell species found in midden deposits elsewhere.

Encyclopedia of Geochemistry

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Intact polar glycosidic GDGTs in sediments settle from water column as evidenced from downcore sediment records

The origins of intact polar lipid glycerol dialkyl glycerol tetraethers (IPL-GDGTs) with glycosidic headgroups that are present in deep-sea sediments are still unclear. Here, we tried to clarify this issue by comparing variations in TEX86 temperatures and compositions of IPL (dihexose (DH) and monohexose (MH) head groups) and core lipid (CL) GDGTs in a sediment core covering a period since the last glacial time in the northern South China Sea (SCS). Haptophyte-derived alkenones were also analyzed to provide an independent paleo-sea surface temperature (SST) record. The sedimentary DH-GDGTs were composed mainly of DH-GDGT-1, -2, -3, and the crenarchaeol isomer. In contrast, both the sedimentary MH- and CL-GDGTs were composed mainly of crenarchaeol and GDGT-0. However, in each pool of GDGTs, crenarchaeol correlated significantly with other GDGT compounds, suggesting a shared origin. The DH-TEX86-derived temperature values were systematically higher than, but well correlated with CL-TEX86 and UK'37-derived SST values, showing an expected glacial-interglacial increase. This finding suggests that the DH-GDGTs could have originated mainly from the upper water column and may have been well preserved in sediments for a long time. However, the MH-TEX86-derived temperature did not display such a climatic pattern but exhibited unexpected higher values during the last glacial period. We propose that the elevated glacial MH-TEX86-derived temperature was likely associated with the ammonia limitation and subsequently reduced ammonia oxidation rate in seawater during the last glacial period. This study indicates that the sedimentary glycosidic GDGTs could settle mainly from the water column rather than being produced in situ.

Cretaceous calcareous dinoflagellate cysts as recorder of δ44/40Caseawater and paleo-temperature using Sr/Ca thermometry

We evaluate the potential of calcareous dinoflagellates as archives for Sr/Ca-based paleo-temperature reconstructions and δ44/40Caseawater fluctuations on sediments from Ocean Drilling Program Expedition 113 (Hole 690C, Weddell Sea, Southern Ocean). Between 73 and 68 Ma, Sr/Ca ratios of two Cretaceous dinoflagellate species, Pirumella krasheninnikovii and Orthopithonella globosa show a pronounced decrease, consistent with a significant drop in sea-surface temperature as reflected by the δ18O of planktic foraminifers. The apparent temperature sensitivity of the dinoflagellate cysts' Sr/Ca is ~0.06 and ~0.08 mmol/mol °C−1, using δ18O-derived paleo sea-surface temperatures, which is significant and large enough to resolve paleoenvironmental temperature changes at current analytical precision. As the chemical composition of the cyst calcite appears to have a good preservation, the Sr/Ca of calcareous dinoflagellates has a potential to serve as paleo-temperature proxy, although the chemical composition of the Cretaceous seawater and potential impacts on the cyst geochemistry is pending further inspection.The Ca isotope composition of the two dinoflagellate species shows identical trends of increasing δ44/40Ca between 73 and 67 Ma. The planktic foraminifer Archaeoglobigerina australis and the benthic foraminifer Nuttallides truempyi reveal the same increase of about 0.4‰ but are offset relative to the dinoflagellates by about +0.5‰, presumably due to species-specific Ca isotope fractionation. Bulk carbonate sediment shows significant scatter, likely caused by changes in faunal composition and does not reproduce the trend revealed by the dinoflagellate and foraminifer records. These observations demonstrate the importance of taxon-specific records and careful determination of fractionation factors of selected archives and highlight complications arising from utilizing less suitable archives, such as bulk sediments, for δ44/40Caseawater reconstructions. Our records indicate strong changes in the marine Ca cycle associated with the global temperature decrease towards the end of the Cretaceous.

Reduced‐dimension reconstruction of the equatorial Pacific SST and zonal wind fields over the past 10,000 years using Mg/Ca and alkenone records

We develop a multiproxy, reduced-dimension methodology to blend magnesium-calcium (Mg/Ca) and alkenone (U 37k) paleo sea surface temperature (SST) records from the eastern and western equatorial Pacific, to recreate snapshots of full field SSTs and zonal winds from 10 to 2 ka B.P. in 2000 year increments. Single-proxy reconstructions (Mg/Ca only versus U 37K′ only) reveal differences in the timing and duration of maximum cooling across the east-central equatorial Pacific. The largest zonal temperature differences (average west Pacific SST minus average east Pacific SST) occur at 6 ka B.P. for the Mg/Ca-only reconstruction (0.61°C) and at 10 and 4 ka for the U 37K′-only reconstruction (0.55°C and 0.47°C, respectively). Disagreements between SST trends suggested by each proxy call for methods that can resolve the common patterns between each and have motivated the work presented in this study. In combining inferences from these proxies, we treat both Mg/Ca and U 37K′ reconstructions of SST as annual average values, but we recognize that they may be sensitive to different seasons. In the multiproxy reconstruction, the zonal SST difference is largest at 10 ka (0.26°C), with coldest SST anomalies of ∼ −0.9°C in the eastern equatorial Pacific and concurrent easterly maximum zonal wind anomalies of 7 m s−1 throughout the central Pacific. From 10 to 2 ka, the entire equatorial Pacific warms, but at a faster rate in the east than the west, and the average central Pacific easterly winds weaken gradually to approximately 2 m s−1. These patterns are broadly consistent with previous inferences of reduced El Nino-Southern Oscillation variability associated with a “La Nina-like” state during the early to middle Holocene.

Neogloboquadrina pchyderma(s)のMg/Ca比古水温計による氷期南大洋太平洋セクターの水温復元

Neogloboquadrina pchyderma(s)のMg/Ca比古水温計による氷期南大洋太平洋セクターの水温復元

Lipid and DNA Evidence of Dominance of Planktonic Archaea Preserved in Sediments of the South China Sea: Insight for Application of the TEX86 Proxy in an Unstable Marine Sediment Environment

The marine planktonic archaea are dominated by Thaumarchaeotal Marine Group I, which is characterized by the lipid biomarker thaumarchaeol. The marine benthic archaea are characterized by greater diversity of currently unknown species whose lipid biomarker signatures are uncertain. In this study, a sediment core from the northwestern part of the South China Sea (SCS) (water depth 1474 m) was analyzed using molecular DNA and lipid biomarker approaches. While 16S rRNA gene analysis showed changing archaeal community structures with sediment depth, this change had little impact on the fossil record of archaeal lipids that are characteristic of the planktonic community. As a result, the fossil archaeal lipids recorded paleo sea surface temperature of the SCS since the last glacial maxima by the TEX86 proxy, which agreed generally with the winter temperature recorded by planktonic foraminiferas collected from the same area of the SCS that hosted mass-transported deposits. This suggests that this deep water deposit may have partially preserved paleoclimate record reflecting seasonal temperature variation in a near shore setting, which is in contrast to annual sea surface temperature or sub sea surface temperature variation recorded by TEX86 in the open ocean.

Salinity changes in the Agulhas leakage area recorded by stable hydrogen isotopes of C<sub>37</sub> alkenones during Termination I and II

Abstract. At the southern tip of Africa, the Agulhas Current reflects back into the Indian Ocean causing so-called "Agulhas rings" to spin off and release relatively warm and saline water into the South Atlantic Ocean. Previous reconstructions of the dynamics of the Agulhas Current, based on paleo-sea surface temperature and sea surface salinity proxies, inferred that Agulhas leakage from the Indian Ocean to the South Atlantic was reduced during glacial stages as a consequence of shifted wind fields and a northwards migration of the subtropical front. Subsequently, this might have led to a buildup of warm saline water in the southern Indian Ocean. To investigate this latter hypothesis, we reconstructed sea surface salinity changes using alkenone δD, and paleo-sea surface temperature using TEXH86 and UK'37, from two sediment cores (MD02-2594, MD96-2080) located in the Agulhas leakage area during Termination I and II. Both UK'37 and TEXH86 temperature reconstructions indicate an abrupt warming during the glacial terminations, while a shift to more negative δDalkenone values of approximately 14‰ during glacial Termination I and II is also observed. Approximately half of the isotopic shift can be attributed to the change in global ice volume, while the residual isotopic shift is attributed to changes in salinity, suggesting relatively high salinities at the core sites during glacials, with subsequent freshening during glacial terminations. Approximate estimations suggest that δDalkenone represents a salinity change of ca. 1.7–1.9 during Termination I and Termination II. These estimations are in good agreement with the proposed changes in salinity derived from previously reported combined planktonic Foraminifera δ18O values and Mg/Ca-based temperature reconstructions. Our results confirm that the δD of alkenones is a potentially suitable tool to reconstruct salinity changes independent of planktonic Foraminifera δ18O.

Unprecedented separation of long chain alkenones from gas chromatography with a poly(trifluoropropylmethylsiloxane) stationary phase

Long chain alkenones (LCAs) have played a central role in reconstructing paleo-sea surface temperature throughout much of the world oceans. The standard method for LCA separation and quantification utilizes gas chromatography (GC) with a non-polar column and flame ionization detection (FID). The method has remained nearly unchanged since the debut of alkenone paleothermometry over three decades ago. The conventional stationary phase used [poly(dimethylsiloxane)] is adequate for calculation of the U37K′ index for most marine samples, but performs poorly in the separation of methyl and ethyl C38 LCAs and fails to separate certain lacustrine LCAs, including structural isomers featuring differing double bond and carbonyl positions. Here, we demonstrate major improvements in the selectivity and resolution of numerous LCAs using a mid-polarity, dipole selective poly(trifluoropropylmethylsiloxane) GC stationary phase with FID. Various columns featuring this phase achieved baseline resolution of all C38 LCAs in a marine sediment and Emiliania huxleyi culture samples, improving peak resolution of C38 LCAs by>350%. Such dipole selective stationary phases also reveal novel tri-unsaturated isomeric LCAs in lacustrine samples, for which we provide the first report and preliminary characterization. We conduct a thorough evaluation of LCA retention behavior on poly(trifluoropropylmethylsiloxane) and poly(dimethylsiloxane) columns and report an optimal range of operating conditions for both phases.

Oxygen isotope analysis of California mussel shells: seasonality and human sedentism at an 8,200-year-old shell midden on Santa Rosa Island, California

To evaluate coastal settlement and land use strategies among maritime hunter-gatherers, we analyzed oxygen isotope (δ18O) data from 131 marine carbonate samples from 21 California mussel (Mytilus californianus) shells obtained from a large ∼8,200-year-old shell midden (CA-SRI-666) on California’s Santa Rosa Island. Seasonal distributions of the isotopic data were assigned using a paleo-sea surface temperature model created by comparing modern sea surface temperatures (SST) to a fully profiled ∼8,200-year-old shell. For 20 additional shells, we used two sampling strategies to compare season-of-harvest inferences and explore whether the Early Holocene site occupants were sedentary. Estimated season-of-harvest differed by 35 % between the two sampling methods, corroborating recent isotope analysis of an 8,800-year-old shell midden on San Miguel Island. Shellfish appear to have been collected year-round at CA-SRI-666 from intertidal or subtidal water temperatures similar to modern SST in the vicinity of eastern Santa Rosa Island. The isotope results are consistent with other evidence from CA-SRI-666 that suggest that the site served as a residential base for relatively sedentary maritime people.

DIAGENETIC ALTERATION OF TRIASSIC CORAL FROM THE ARAGONITE KONSERVAT-LAGERSTATTE IN ALAKIR CAY, TURKEY: IMPLICATIONS FOR GEOCHEMICAL MEASUREMENTS

Skeletons of Norian corals from the aragonite Konservat-Lagerstatte in Alakir Cay, Turkey are commonly considered to be exceptionally preserved. However, one example of Pachysolenia cylindrica shows that although aragonite is the dominant mineral phase (about 86.2%), the distribution of microscale diagenetic features in the skeleton complicate its usage as a paleoenvironmental archive. We used a density separation technique to isolate aragonite-enriched powders of skeletal material, which compared to powders of bulk skeleton (∼8.3% more calcite than aragonite-enriched powders), exhibit higher Sr/Ca (by an average of 0.27 mmol/mol), lower Mg/Ca (by an average of 1.34 mmol/mol), and lower δ18O (by an average of 0.28‰). In addition, paleo-sea surface temperatures calculated using values from bulk measurements are 1.1–3.4 °C colder (3.3–3.4 °C for Sr/Ca and 1.1–2.0 °C for δ18O) than temperatures calculated from the density-separated, aragonite-enriched powders. These data suggest that the presence of about 13.8% of secondary calcite can influence the robustness of geochemical proxies and that density separation is an effective tool for recovering the original geochemical signal from diagenetically altered samples.

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.

Analysis of trace amounts of alkenones in complex environmental samples by way of NaBH 4/NaBD 4 reduction and silylation

The U 37 K ′ index is widely used to infer paleo-sea surface temperatures from the relative abundances of sedimentary 37:3 and 37:2 methyl alkenones. However, the measurements can be difficult and error prone when the alkenones are present in only trace amount, when one is much more abundant than the other, or when co-eluting compounds are present. Silylated alkenols can be analyzed by way of gas chromatography–electron ionization mass spectrometry (GC–EIMS) with much greater sensitivity and specificity than the parent alkenones and near-baseline resolution can be obtained without the use of very long chromatographic columns and/or slow temperature programs. We show, using particulate matter from the Black Sea and the DYFAMED site in the Ligurian Sea, and surface sediments from the Black Sea and SE Alaska, that conversion of C 37 alkenones to alkenols using NaBH 4 reduction and subsequent derivatization to TMSi ethers allows U 37 K ′ values to be reliably calculated without time consuming purification steps, using the relative abundances of the diagnostic m/ z 117 ion. The response factor for m/ z 117 varies for 37:3 and 37:2 alkenols, so a specific correction is required. The reduction method is easy to use, highly sensitive and widely applicable. Ethyl alkenols give a characteristic ion at m/ z 131 and, if NaBD 4 is used, the ions at m/ z 117 and 131 shift to m/ z 118 and 132, providing further confirmation of the structures.

Variations in upper water structure during MIS 3 from the western South China Sea

Core 17954 is located in the modern summer upwelling area in western South China Sea, its sediments recorded the variations of upwelling generated by East Asia Summer Monsoon (EASM) during MIS 3. Based on the strict age model of AMS 14C dating, the paleo-Sea Surface Temperature (SST) and Salinity (SSS) are reconstructed by pairing Mg/Ca-Paleothermometer and δ18O of planktonic foraminifera Globigerinoides ruber (white s.s.). Results show that in Core 17954, the δ18O record of G.ruber has significant millennium fluctuations as the δ18O records from NGRIP icecore and Hulu Cave stalagmites, this indicates that the climate changes of western SCS contains both signals from High Latitude of Northern Hemisphere as well as EASM. In order to get more information on upwelling changes, previous records of thermocline and foraminiferal primary productivity in Core 17954 are collected, restudied and compared. Five distinct shallowing periods of thermocline (referred to as S1–S5) are identified in this study. In S1–S4, SST is lower and productivity is higher, these indicate to an enhanced upwelling and strengthened EASM during these periods. And the lower SSS, caused by increasing precipitation or fresh water input, also prove this standpoint.

Simulation of the Middle Miocene Climate Optimum

Proxy data constraining land and ocean surface paleo‐temperatures indicate that the Middle Miocene Climate Optimum (MMCO), a global warming event at ∼15 Ma, had a global annual mean surface temperature of 18.4°C, about 3°C higher than present and equivalent to the warming predicted for the next century. We apply the latest National Center for Atmospheric Research (NCAR) Community Atmosphere Model CAM3.1 and Land Model CLM3.0 coupled to a slab ocean to examine sensitivity of MMCO climate to varying ocean heat fluxes derived from paleo sea surface temperatures (SSTs) and atmospheric carbon dioxide concentrations, using detailed reconstructions of Middle Miocene boundary conditions including paleogeography, elevation, vegetation and surface temperatures. Our model suggests that to maintain MMCO warmth consistent with proxy data, the required atmospheric CO2 concentration is about 460–580 ppmv, narrowed from the most recent estimate of 300–600 ppmv.

Intensified mid-Holocene Asian monsoon recorded in corals from Kikai Island, subtropical northwestern Pacific

Coupled records of Sr/Ca and oxygen isotope ratios (δ 18O) of coral skeletons have been used to produce quantitative estimates of paleo-sea surface temperature (SST) and δ 18O of surface seawater that can in some cases be converted to sea surface salinity (SSS). Two fossil corals from Kikai Island in the subtropical northwestern Pacific, a location affected by East Asian summer and winter monsoons, were analyzed to investigate differences between mid-Holocene and present-day SST and SSS. At 6180 cal yr BP, SSTs were roughly the same as today, both in summer and winter; δ 18O seawater and SSS values were higher both in summer (+ 0.5‰, +1.1 psu) and in winter (+ 0.2‰, + 0.6 psu) than modern values. At 7010 cal yr BP, SSTs were slightly cooler both in summer and winter (−0.8 and −0.6 °C), whereas δ 18O seawater and SSS had higher values in summer (+ 0.3‰, + 0.6 psu) and in winter (+ 0.8‰, + 1.9 psu) than present-day values. These results are consistent with other marine records for the mid-Holocene of the low and midlatitudes in the northwestern Pacific. Such regional conditions indicate that the East Asian summer and winter monsoons were more intense in the mid-Holocene, which was likely a function of the mid-Holocene insolation regime.

Coral barium/calcium record of mid-Holocene upwelling activity in New Caledonia, South-West Pacific

A variety of inimical factors, i.e. sea surface temperature, nutrient input, nature of substrates, were invoked previously to explain the retardation in reef settlement in New Caledonia during the Holocene. In order to clarify the respective role played by temperature and nutrient level in the growth of fringing reefs along the southwest coast of the island, trace element ratios, i.e. Sr/Ca and Ba/Ca, were used, respectively, as a paleothermometer and as a tracer of nutrient load from modern and mid-Holocene (about 6000 calendar years old) massive Porites coral colonies. The results obtained from the modern coral record indicate that, near the outer barrier reef, upwelling activity is controlled by southeast winds blowing in summer. By contrast, along the southwest coastline, the fringing reefs are subjected to barium input mainly derived from weathering of barite-rich rocks cropping out in the adjacent mainland. Comparison between the fossil and modern coral Sr/Ca and Ba/Ca records shed light on three different aspects of the mid-Holocene climate and coral reef evolution in New Caledonia: (1) the variability of paleo-sea surface temperature (SST) seems to have been similar to the modern one, although the island probably experienced SST about 1 °C higher than at present; (2) upwelling activity appears to have been greater than at present. Barium being supplied from the open sea to coastal zones by strong-wind generated upwelling currents, in relation to cooling events; (3) the onset of reef development in the southwest areas of New Caledonia was delayed due to the stronger upwelling of nutrient-enriched waters during the early–middle Holocene. The flowing of upwelled waters over the southwest coastal areas began at 8200 calendar years BP, and ceased at the time when the outer barrier reef crest reached present-day sea level, about 5500 to 5000 calendar years ago.

The upper end of the UK′37 temperature calibration revisited

Being able to decipher paleo sea surface temperatures for warm oceanic waters is of the utmost importance to further evaluate the role of the tropics in global climate change. The UK′37 index is one of such paleothermometers, although the warm range of temperatures often poses serious analytical challenges. This paper discusses and reviews several important aspects of the UK′37 index calibration for waters warmer than 26°–27°C. Amongst them, insights into the possible existence of a limit above which the UK′37 index is not able to accurately record temperature variations are presented. Some warnings are also given on the misleading practice and expected consequences of reporting UK′37 index values of unity in the cases when abundances of the minor triunsaturated alkenone below detection limit are encountered. Future directions toward an improved knowledge of the behaviour of the UK′37 calibration for warm waters are suggested.

Carbonate dissolution and planktonic foraminiferal assemblages observed in three piston cores collected above the lysocline in the western equatorial Pacific

Planktonic foraminiferal assemblages were analyzed in three piston cores recovered from depths above the recent sedimentary lysocline (3.5 km) in the western Pacific Ocean for the purposes of (1) evaluating the effects of carbonate dissolution on fossil assemblages and (2) the impact of dissolution on paleoceanographic reconstructions for the past 300 kyr. We used the perfect test ratio (PTR) of Globorotalia menardii as an indicator of carbonate dissolution. Down-core variations of the PTR indicate that significant dissolution occurred during reglaciation steps represented by isotope stages 7–6, 5–4, and 3–2. The results of principal component analysis (PCA) based on the relative abundances of dominant species of planktonic foraminifera indicate that down-core changes in factor 1 loadings correlate with variations in PTR and with variations in the percentage of coarse fraction (>63 μm) present in the sediments. These correlations indicate that foraminiferal assemblages were apparently altered by dissolution events despite the fact that they were deposited at water depths shallower than the modern lysocline. We also estimated variations in paleo-sea surface temperature (paleo-SST) using the transfer function FP-12E. A negative correlation was found in communality as calculated from PCA and factor 1 loadings; this trend is distinct for the case of communality lower than 0.9. Thus, we conclude that estimated paleo-SST values based upon analysis of planktonic foraminifera are biased by carbonate dissolution.

The effects of varying CO 2 concentration on lipid composition and carbon isotope fractionation in Emiliania huxleyi

We have measured the stable carbon isotopic composition of bulk organic matter (POC), alkenones, sterols, fatty acids, and phytol in the coccolithophorid Emiliania huxleyi grown in dilute batch cultures over a wide range of CO 2 concentrations (1.1–53.5 μmol L −1). The carbon isotope fractionation of POC (ε POC) varied by ca. 7‰ and was positively correlated with aqueous CO 2 concentration [CO 2aq]. While this result confirms general trends observed for the same alga grown in nitrogen-limited chemostat cultures, considerable differences were obtained in absolute values of ε POC and in the slope of the relationship of ε POC with growth rate and [CO 2aq]. Also, a significantly greater offset was obtained between the δ 13C of alkenones and bulk organic matter in this study compared with previous work (5.4, cf. 3.8‰). This suggests that the magnitude of the isotope offset may depend on growth conditions. Relative to POC, individual fatty acids were depleted in 13C by 2.3‰ to 4.1‰, phytol was depleted in 13C by 1.9‰, and the major sterol 24-methylcholesta-5,22E-dien-3β-ol was depleted in 13C by 8.5‰. This large spread of δ 13C values for different lipid classes in the same alga indicates the need for caution in organic geochemical studies when assigning different sources to lipids that might have δ 13C values differing by just a few ‰. Increases in [CO 2aq] led to dramatic increases in the alkenone contents per cell and as a proportion of organic carbon, but there was no systematic effect on values of U 37 k′ used for reconstructions of paleo sea surface temperature.