Paleo-sea Surface 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.

Assessment of the nutrient diol index (NDI) as a sea surface nutrient proxy using sinking particles in the East Sea

The Nutrient Diol Index (NDI) has been proposed as a paleo sea surface nutrient proxy. However, the NDI's broad applicability needs to be further assessed by examining its temporal variability in association with modern sea surface nutrient concentrations. This study is the first report to evaluate the NDI with respect to sinking particles in the East Sea. The NDI values showed seasonal variations resembling those of the surface phosphate concentrations in the study area. However, the calibration based on the sinking particles at 1000 m water depth (NDI = 1.134 × [PO4−] + 0.115, R2 = 0.59, n = 26) deviated from the newly compiled global surface sediment dataset (NDI = 0.392 × [PO4−] + 0.023; R2 = 0.58, n = 566). This local calibration predicted the phosphate concentrations in surface sediments in the East Sea better than the global calibration. Our study suggests that the difference of the calibration slope between the East Sea trap dataset and the global surface sediment dataset might be associated with different 1,14-diol producers and their sensitivity to nutrient concentrations in the East Sea. Accordingly, the NDI should be further assessed in various oceanic settings before it is routinely used to reconstruct sea surface nutrient conditions.

Paleogeography and volcanic morphology reconstruction of a buried monogenetic volcanic field (part 2)

Over the last two decades, a growing number of studies have identified the presence of ancient volcanic systems buried and preserved within sedimentary basins. Continuous improvements in remote-sensing techniques such as seismic reflection offer an opportunity to analyze how these extinct buried volcanoes have formed and evolved. In this paper, we reveal the Maahunui Volcanic Field (MVF), a cluster of middle Miocene volcanoes and shallow intrusions currently buried by more than 1000 m of sedimentary strata of the Canterbury Basin, New Zealand. This “fossil” volcanic field is imaged by high-quality 2D seismic lines and has representative igneous rocks penetrated by the Resolution-1 borehole. These data have been used to reconstruct the original morphology of the volcanoes and the regional paleogeography in which eruptions and shallow ( 200 m) located in a neritic setting were possibly emergent at the paleosea surface, forming an archipelago of nine small extinct volcanic islands. This study demonstrates the value of detailed seismic reflection mapping and borehole analysis for improving understanding of the processes that formed, transformed, and preserved ancient volcanoes now buried in sedimentary basins.

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.

Distribution of long chain alkyl diols along a south-north transect of the northwestern Pacific region: Insights into a paleo sea surface nutrient proxy

Long chain alkyl diols (LCDs) have previously been used to develop several diol indices as a proxy for upwelling or stratification conditions and sea surface temperatures. However, there is still a lack of data on the distribution of LCDs in the Pacific Ocean, which calls into question to what extent these diol indices can serve as a proxy in the Pacific Ocean. Hence, we investigated the distribution of LCDs in suspended particulate matter (SPM) samples collected in July 2015 along a south-north transect ranging from the East Sea of Korea to the Bering Sea in the northwestern Pacific region. Our results showed that both saturated and unsaturated C28 and C30 1,14-diols were dominant in most SPM samples investigated. The principal component analysis (PCA) results of the SPM data revealed that the C28:1 and C30:1 1,14-diols were positively associated with nutrient concentrations. However, only the C28:1 1,14-diol in addition to the C28 1,14-diol was positively associated with nutrient concentrations in the global surface sediment data set previously published, while the C30:1 1,14-diol was related to sea surface temperature (SST). Based on these observations, we defined a new diol index, i.e., the nutrient diol index (NDI), which included the C28 1,14-diol as well as the C28:1 1,14-diol in the numerator and all the 1,13-, 1,14-, and 1,15-diols, excluding the C32 1,15-diol, in the denominator. The NDI was positively associated with the ocean atlas values of surface water phosphate and nitrate concentrations for the SPM and surface sediment data sets. Accordingly, the results of our study showed that the NDI has potential as a quantitative nutrient proxy for estimating sea surface phosphate (or nitrate) changes. Further studies should be undertaken in order to assess the applicability of the NDI as a paleo-nutrient proxy for confidently reconstructing surface water nutrient variations in the past, by analyzing more SPM, surface sediments, and sediment cores collected from various oceanic settings.

Effects of alkalinity and salinity at low and high light intensity on hydrogen isotope fractionation of long-chain alkenones produced by <i>Emiliania huxleyi</i>

Abstract. Over the last decade, hydrogen isotopes of long-chain alkenones have been shown to be a promising proxy for reconstructing paleo sea surface salinity due to a strong hydrogen isotope fractionation response to salinity across different environmental conditions. However, to date, the decoupling of the effects of alkalinity and salinity, parameters that co-vary in the surface ocean, on hydrogen isotope fractionation of alkenones has not been assessed. Furthermore, as the alkenone-producing haptophyte, Emiliania huxleyi, is known to grow in large blooms under high light intensities, the effect of salinity on hydrogen isotope fractionation under these high irradiances is important to constrain before using δDC37 to reconstruct paleosalinity. Batch cultures of the marine haptophyte E. huxleyi strain CCMP 1516 were grown to investigate the hydrogen isotope fractionation response to salinity at high light intensity and independently assess the effects of salinity and alkalinity under low-light conditions. Our results suggest that alkalinity does not significantly influence hydrogen isotope fractionation of alkenones, but salinity does have a strong effect. Additionally, no significant difference was observed between the fractionation responses to salinity recorded in alkenones grown under both high- and low-light conditions. Comparison with previous studies suggests that the fractionation response to salinity in culture is similar under different environmental conditions, strengthening the use of hydrogen isotope fractionation as a paleosalinity proxy.

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.

Large effect of irradiance on hydrogen isotope fractionation of alkenones in Emiliania huxleyi

The hydrogen isotopic (δD) composition of long-chain alkenones produced by certain haptophyte algae has been suggested as a potential proxy for reconstructing paleo sea surface salinity. However, environmental parameters other than salinity may also affect the δD of alkenones. We investigated the impact of the level of irradiance on hydrogen isotopic fractionation of alkenones versus growth water by cultivating two strains of the cosmopolitan haptophyte Emiliania huxleyi at different light intensities. The hydrogen isotope fractionation decreased by approximately 40‰ when irradiance was increased from 15 to 200μmolphotonsm−2s−1 above which it was relatively constant. The response is likely a direct effect of photosystem I and II activity as the relationship of the fractionation factor α versus light intensity can be described by an Eilers–Peeters photosynthesis model. This irradiance effect is in agreement with published δD data of alkenones derived from suspended particulate matter collected from different depths in the photic zone of the Gulf of California and the eastern tropical North Pacific. However, haptophyte algae tend to bloom at relatively high light intensities (>500μmolphotonsm−2s−1) occurring at the sea surface, at which hydrogen isotope fractionation is relatively constant and not affected by changes in light intensity. Alkenones accumulating in the sediment are likely mostly derived from these surface water haptophyte blooms, when the largest amount of biomass is produced. Therefore, the observed irradiance effect is unlikely to affect the applicability of the hydrogen isotopic composition of sedimentary long chain alkenones as a proxy for paleosalinity.

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.

Evidence of Holocene and historical changes of sea level in the Langkawi Islands

About eighty radiometrically dated biogenic and morphological indicators of sea level of the Langkawi Islands prove that since the maximum Mid-Holocene inundation, the paleo-sea surface descended stepwise thrice to reach its current position several hundred years ago. In presumably historical time some parts of the island group were raised between 0.5 and one meter. One of such events was related to the Aceh/Simeulue mega-earthquake of December 2004 which caused live bands of rock-clinging oysters and barnacles to shift 30 to 40 centimeters upward at Teluk Burau. GPS study also yields evidence of 9 to 11 millimeters co-seismic uplift of the northwestern sector of Peninsular Malaysia. The anomalously high sea stands in the early part of the Holocene and latest Pleistocene in northwestern Peninsular Malaysia remain the most outstanding issue in this investigation. Comparison with recently published sea-level curves of the Sunda Shelf strongly suggests that the geoid of the Strait of Malacca was 50 to 40 meters higher in the period of the LGM (Last Glacial Maximum: 21 ka to 19 ka) to the early half of the Holocene at 10 ka to 5 ka. In the early period of the Holocene, sea level was still up to 24 meters higher than over the Sunda Shelf.

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.

Alkenone distribution impacts the hydrogen isotopic composition of the C37:2 and C37:3 alkan-2-ones in Emiliania huxleyi

The hydrogen isotopic composition (δD) of long-chain alkenones derived from haptophyte algae has recently been applied as paleo sea surface salinity proxy, but this application has been complicated by reported differences in δD of individual C37 alkenones of up to 45‰. Here we show that this difference in D/H ratios is the result of the desaturation of the C37:2 alkenone to form the C37:3 alkenone and related to the relative abundance of the individual alkenones, and, therefore, the UK′37. For the purpose of reconstructing paleo sea surface salinities it is recommended to determine the δD of combined rather than individual C37 alkenones.

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 temperature and salinity of the surface water above the middle Okinawa Trough during the past 37 kyr

East China Sea (ECS) is an important climate modulator of East Asia. In the last glacial period, the global sea level, the path and strength of the Kuroshio Current experienced great changes; combined with the variable volume of fresh run-off input, they made the hydrographic situation in the ECS quite different from nowadays. Based on high-resolution alkenone-sea surface temperature (SST) and oxygen isotope composition of planktonic foraminifera Globigerinoides sacculifer we reconstructed paleo-sea surface salinity (SSS) of a long piston core DGKS9604 retrieved from the middle Okinawa Trough of the eastern ECS. The δ 18O and SST records display significant variations with global ice volume. Synchrony of the millennial-scale climate events like YD and Heinrich events of core DGKS9604 to the ice core from the northern high latitudes, and the synchroneity of deglacial warming with the Bølling–Allerød warming suggests a strong coupling of the SST variations in the marginal Pacific Ocean to the climate of the North Atlantic, most likely through the Asian monsoon atmospheric circulation. The ECS documents lowest SST (22 °C) at ~ 26 cal kyr BP and ~ 3 °C SST difference between the full glaciation (26 to 19 cal kyr BP) and mid-to-late Holocene (6 cal kyr BP–present). The overall long-term hydrographic variations in the middle Okinawa Trough are controlled by temporal and spatial variations in: (i) the intensity and position of the Kuroshio Current, (ii) intensity of the Asian summer monsoon and (iii) sea-level fluctuations coupled with ECS topography. Saline surface water dominated over the middle Okinawa Trough during early pre-glaciation (37 to 31 cal kyr BP), last deglaciation (19 to 11.6 cal kyr BP), and mid-to-late Holocene (6 cal kyr BP–present), whilst freshened surface water prevailed during the late pre-glaciation (31 to 26 cal kyr BP), full glaciation (26 to 19 cal kyr BP) and early Holocene (11.6 to 6 cal kyr BP).