Benthic δ18O Data Research Articles

An 800 kyr planktonic δ18O stack for the Western Pacific Warm Pool

Abstract. The Western Pacific Warm Pool (WPWP) exhibits different glacial–interglacial climate variability compared to high latitudes, and its sea surface temperatures are thought to respond primarily to changes in greenhouse forcing. To better characterize the orbital-scale climate response covering the WPWP, we constructed a planktonic δ18O stack (average) of 10 previously published WPWP records of the last 800 kyr, available at https://doi.org/10.5281/zenodo.10211900 (Bowman et al., 2023), using the new Bayesian alignment and stacking software BIGMACS (Lee et al., 2023b). Similarities in stack uncertainty between the WPWP planktonic δ18O stack and benthic δ18O stacks, also constructed using BIGMACS, demonstrate that the software performs similarly well when aligning regional planktonic or benthic δ18O data. A total of 65 radiocarbon dates from the upper portion of five of the WPWP cores suggest that WPWP planktonic δ18O change is nearly synchronous with global benthic δ18O during the last glacial termination. However, the WPWP planktonic δ18O stack exhibits a smaller glacial–interglacial amplitude and less spectral power at all orbital frequencies than benthic δ18O. We assert that the WPWP planktonic δ18O stack provides a useful representation of orbital-scale regional climate response and a valuable regional alignment target, particularly over the 0 to 450 ka portion of the stack.

Bayesian age models and stacks: combining age inferences from radiocarbon and benthic δ18O stratigraphic alignment

Abstract. Previously developed software packages that generate probabilistic age models for ocean sediment cores are designed to either interpolate between different age proxies at discrete depths (e.g., radiocarbon, tephra layers, or tie points) or perform a probabilistic stratigraphic alignment to a dated target (e.g., of benthic δ18O) and cannot combine age inferences from both techniques. Furthermore, many radiocarbon dating packages are not specifically designed for marine sediment cores, and the default settings may not accurately reflect the probability of sedimentation rate variability in the deep ocean, thus requiring subjective tuning of the parameter settings. Here we present a new technique for generating Bayesian age models and stacks using ocean sediment core radiocarbon and probabilistic alignment of benthic δ18O data, implemented in a software package named BIGMACS (Bayesian Inference Gaussian Process regression and Multiproxy Alignment of Continuous Signals). BIGMACS constructs multiproxy age models by combining age inferences from both radiocarbon ages and probabilistic benthic δ18O stratigraphic alignment and constrains sedimentation rates using an empirically derived prior model based on 37 14C-dated ocean sediment cores (Lin et al., 2014). BIGMACS also constructs continuous benthic δ18O stacks via a Gaussian process regression, which requires a smaller number of cores than previous stacking methods. This feature allows users to construct stacks for a region that shares a homogeneous deep-water δ18O signal, while leveraging radiocarbon dates across multiple cores. Thus, BIGMACS efficiently generates local or regional stacks with smaller uncertainties in both age and δ18O than previously available techniques. We present two example regional benthic δ18O stacks and demonstrate that the multiproxy age models produced by BIGMACS are more precise than their single-proxy counterparts.

Reflooding and repopulation of the Mediterranean Sea after the Messinian Salinity Crisis: Benthic foraminifera assemblages and stable isotopes of Spanish basins

Benthic foraminiferal, sedimentological, and stable isotope analyses performed on early Zanclean sediments from Alboran Basin ODP Site 976 and southern Spanish land-based sections in the Malaga, Nijar and Sorbas basins have enabled the reconstruction of Mediterranean environmental conditions immediately after the Messinian Salinity Crisis. The presence at the Miocene – Pliocene boundary of dark layers, often enriched in organic matter, suggests that the Zanclean reflooding has created water column stratification, and reduced bottom-water oxygen levels. Considering that such dark layers are recorded at both deep and marginal settings far away from the Gibraltar gateway/s, a Mediterranean-scale water-mass stratification must have occurred. This stratification could be the result of saline Atlantic waters sinking into a less saline Mediterranean Basin still under the influence of the Paratethys. Our early Zanclean benthic δ18O data show that the Mediterranean water budget was indeed less negative than at present, explaining the lower salinity of the basin. However, the Atlantic values of the benthic δ13C registered in the Alboran basin suggest that bottom-water renewal rates were quite high during the early Zanclean, preventing the reduction of δ13C at the seafloor as observed in the Messinian records. Zanclean benthic foraminiferal repopulation sequences show similarities with recovery from low-oxic episodes during sapropel deposition. These observations, paired with the gradual deepening of the basins, suggests that the Zanclean reflooding led to a progressive shift from stressed and unstable environments towards benthic associations typical of efficient circulation and bottom water ventilation.

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

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

A two million year record of low-latitude aridity linked to continental weathering from the Maldives

Indian-Asian monsoon has oscillated between warm/wet interglacial periods and cool/dry glacial periods with periodicities closely linked to variations in Earth’s orbital parameters. However, processes that control wet versus dry, i.e. aridity cyclical periods on the orbital time-scale in the low latitudes of the Indian-Asian continent remain poorly understood because records over millions of years are scarce. The sedimentary record from International Ocean Discovery Program (IODP) Expedition 359 provides a well-preserved, high-resolution, continuous archive of lithogenic input from the Maldives reflecting on low-latitude aridity cycles. Variability within the lithogenic component of sedimentary deposits of the Maldives results from changes in monsoon-controlled sedimentary sources. Here, we present X-ray fluorescence (XRF) core-scanning results from IODP Site U1467 for the past two million years, allowing full investigation of orbital periodicities. We specifically use the Fe/K as a terrestrial climate proxy reflecting on wet versus dry conditions in the source areas of the Indian-Asian landmass, or from further afield. The Fe/K record shows orbitally forced cycles reflecting on changes in the relative importance of aeolian (stronger winter monsoon) during glacial periods versus fluvial supply (stronger summer monsoon) during interglacial periods. For our chronology, we tuned the Fe/K cycles to precessional insolation changes, linking Fe/K maxima/minima to insolation minima/maxima with zero phase lag. Wavelet and spectral analyses of the Fe/K record show increased dominance of the 100 kyr cycles after the Mid Pleistocene Transition (MPT) at 1.25 Ma in tandem with the global ice volume benthic δ18O data (LR04 record). In contrast to the LR04 record, the Fe/K profile resolves 100-kyr-like cycles around the 130 kyr frequency band in the interval from 1.25 to 2 million years. These 100-kyr-like cycles likely form by bundling of two or three obliquity cycles, indicating that low-latitude Indian-Asian climate variability reflects on increased tilt sensitivity to regional eccentricity insolation changes (pacing tilt cycles) prior to the MPT. The implication of appearance of the 100 kyr cycles in the LR04 and the Fe/K records since the MPT suggests strengthening of a climate link between the low and high latitudes during this period of climate transition.

A continuous simulation of global ice volume over the past 1 million years with 3-D ice-sheet models

Sea-level records show large glacial-interglacial changes over the past million years, which on these time scales are related to changes of ice volume on land. During the Pleistocene, sea-level changes induced by ice volume are largely caused by the waxing and waning of the large ice sheets in the Northern Hemisphere. However, the individual contributions of ice in the Northern and Southern Hemisphere are poorly constrained. In this study, for the first time a fully coupled system of four 3-D ice-sheet models is used, simulating glaciations on Eurasia, North America, Greenland and Antarctica. The ice-sheet models use a combination of the shallow ice and shelf approximations to determine sheet, shelf and sliding velocities. The framework consists of an inverse forward modelling approach to derive a self-consistent record of temperature and ice volume from deep-sea benthic δ18O data over the past 1 million years, a proxy for ice volume and temperature. It is shown that for both eustatic sea level and sea water δ18O changes, the Eurasian and North American ice sheets are responsible for the largest part of the variability. The combined contribution of the Antarctic and Greenland ice sheets is about 10 % for sea level and about 20 % for sea water δ18O during glacial maxima. However, changes in interglacials are mainly caused by melt of the Greenland and Antarctic ice sheets, with an average time lag of 4 kyr between melt and temperature. Furthermore, we have tested the separate response to changes in temperature and sea level for each ice sheet, indicating that ice volume can be significantly influenced by changes in eustatic sea level alone. Hence, showing the importance of a simultaneous simulation of all four ice sheets. This paper describes the first complete simulation of global ice-volume variations over the late Pleistocene with the possibility to model changes above and below present-day ice volume, constrained by observations of benthic δ18O proxy data.

Mindanao Dome variability over the last 160 kyr: Episodic glacial cooling of the West Pacific Warm Pool

We present sea surface, upper thermocline, and benthic δ18O data, as well as temperature and paleoproductivity proxy data, from the International Marine Global Change Study Program (IMAGES) Core MD06‐3067 (6°31′N, 126°30′E, 1575 m water depth), located in the western equatorial Pacific Ocean within the flow path of the Mindanao Current. Our records reveal considerable glacial‐interglacial and suborbital variability in the Mindanao Dome upwelling over the last 160 kyr. Dome activity generally intensified during glacial intervals resulting in cooler thermocline waters, whereas it substantially declined during interglacials, in particular in the early Holocene and early marine oxygen isotope stage (MIS) 5e, when upwelling waters did not reach the thermocline. During MIS 3 and MIS 2, enhanced surface productivity together with remarkably low SST and low upper ocean thermal contrast provide evidence for episodic glacial upwelling to the surface, whereas transient surface warming marks periodic collapses of the Mindanao Dome upwelling during Heinrich events. We attribute the high variability during MIS 3 and MIS 2 to changes in the El Niño Southern Oscillation state that affected boreal winter monsoonal winds and upper ocean circulation. Glacial upwelling intensified when a strong cyclonic gyre became established, whereas El Niño–like conditions during Heinrich events tended to suppress the cyclonic circulation, reducing Ekman transport. Thus, our findings demonstrate that variations in the Mindanao Dome upwelling are closely linked to the position and intensity of the tropical convection and also reflect far‐field influences from the high latitudes.

Transient nature of the Earth's climate and the implications for the interpretation of benthic [formula omitted] records

From the marine benthic δ18O records it is known that the Earth's climate has experienced significant variability over the past 40million years. In general, a number of assumptions are often needed to disentangle the benthic δ18O data into its temperature and ice-volume contributions. In this study, a transient (1-D ice-sheet) model is used which overcomes these shortcomings by relating temperature to the benthic δ18O data, leading to a self-consistent and continuous record of δ18O, temperature and sea level. The contribution of land ice to benthic δ18O is examined with a set of sensitivity experiments, varying the mean δ18O of the ice for different ice sheets. It is shown that the scaling factor of sea-water δ18O (δw) to sea level is not constant over the long time scales. However, our sensitivity experiments do show that over the long time scale, the general assumed 1.0–1.1‰ per 100m of sea level is in reasonable agreement with our model results. Moreover, the sea-level (ice volume) response to temperature (ΔS/ΔT) is shown to vary through time, with the largest response found when variations in Antarctic ice volume dominates the sea-level variations during the Oligocene to Early Miocene. Furthermore, this response is quite large compared to the individual response of ice sheets during the Plio-Pleistocene, whereas the combined ΔS/ΔT in this period is comparable to that during the Oligocene. Accordingly, the transient behaviour is very important for the interpretation of data records, with respect to climate sensitivity and climate change during the past 40million years.

Early Maastrichtian carbon cycle perturbation and cooling event: Implications from the South Atlantic Ocean

Published stable isotope records in marine carbonate are characterized by a positive δ18O excursion associated with a negative δ13C shift during the early Maastrichtian. However, the cause and even the precise timing of these excursions remain uncertain. We have generated high‐resolution foraminiferal stable isotope and gray‐scale records for the latest Campanian to early Maastrichtian (∼73–68 Ma) at two Ocean Drilling Program sites, 525 (Walvis Ridge) and 690 (Weddell Sea). We demonstrate that the negative δ13C excursion is decoupled from the δ18O increase with a lag of about 600 ka. Our δ13C records (both planktic and benthic) show an amplitude for the negative excursion of 0.7‰ that falls between about 72.1 and 70.7 Ma. Our planktic δ18O records indicate an overall increase of 1.2‰ from 73 to 68 Ma at Site 690, whereas at Site 525 they record a slightly smaller increase (∼1‰) that peaks around 70.1 Ma with decreasing values thereafter. Our benthic δ18O data indicate an increase of ∼1.5‰ at Site 525 and ∼0.7‰ at Site 690 between about 71.4 and 69.9 Ma. Benthic δ18O values show different baseline values at the two sites before and after the excursion, but the larger increase at Site 525 means that the values attained at the peak of the excursion are similar at the two sites. We interpret this observation in terms of water mass changes. The excursion is interpreted to reflect a cooling of bottom waters in response to the strengthening contribution of intermediate‐ to deep‐water production in the high southern latitudes rather than increased ice volume. The associated carbon cycle perturbations that we observe are interpreted to reflect a weakening of surface water stratification and increased productivity, as supported by our gray value data.

Geomagnetic excursions and paleointensities in the Matuyama Chron at Ocean Drilling Program Sites 983 and 984 (Iceland Basin)

We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9–2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm kyr−1, respectively. For the older part of the record (>1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference δ18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic δ18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30–60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Réunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190–1215 (Cobb Mountain Subchron), 1255, 1472–1480, 1567–1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.

パプアニューギニア北東部のサンゴ礁段丘の総合研究 ヒュオン半島のサンゴ礁段丘から新たに得られた第四紀後期の海面高度と深海底コアの酸素同位体記録との調和

A major discrepancy exists between the late Quaternary sea level changes derived from raised coral reef terraces at Huon Peninsula in Papua New Guinea and from oxygen isotopes in deep sea cores. The two methods agree closely from 120 ka to 80 ka and from 20 ka to the present. However, oxygen isotopes from benthic forams indicate lower sea levels by 20 to 40 m during the later stages of the last glacial cycle, from 70 to 30 ka. New, high precision U-series age measurements and revised stratigraphic data for Huon Peninsula terraces that formed during this interval give similar sea levels to those based on planktonic and benthic δ18O data. The record of ice volume changes through the last glacial cycle now appears to be reasonably complete.

Benthic foraminiferal δ18O in the ocean's temperature‐salinity‐density field: Constraints on Ice Age thermohaline circulation

Benthic δ18O data from 95 core sites are used to infer possible temperature‐salinity (T‐S) fields of the Atlantic and Pacific oceans at the Last Glacial Maximum (LGM). A constraint of stable density stratification yields logically consistent scenarios for both T and S. The solutions are not unique but are useful as a thinking tool. Using GEOSECS data, we solve for the modem relationship between δ18Owater (δw) and salinity in the deep sea: δw (SMOW) = 1.529 * S ‐ 53.18. As a starting point, we assume that the slope of this equation applies to LGM conditions and predict δ18Ocalcite (δc) gradients in equilibrium with probable T‐S fields of LGM deep and bottom waters. Benthic foraminiferal δ18O data from the deep Pacific (2–4 km depth), and the bottom Atlantic (> 4 km depth), are 0.1–0.2‰ lower than from the deep Atlantic (2–4 km depth) at the LGM. If the modern δw‐S slope applies, Atlantic deep and bottom waters were more dense than Pacific deep waters. This assumption would imply bottom waters both fresher (ΔS >0.5) and colder (ΔT ∼3°C) than overlying deep waters, in conflict with other data, suggesting ice age deep water much colder than at present. It is also possible that the observed δc gradients are an artifact of laboratory intercalibration. If Atlantic deep and bottom water δc values were similar to deep Pacific values, this would be consistent with the hypothesis of a stronger southern ocean versus North Atlantic source for deep‐ocean ventilation at the LGM. Taking the observed gradients at face value, however, a solution could be that the LGM δw‐S slope in deep and bottom waters was higher than at present, conceivably because of a stronger contribution of salt to the deep ocean via more intense sea ice freezing. This would allow Pacific deep waters and Atlantic bottom waters to have a common source, again in the Antarctic. Both would be more dense than Atlantic deep waters, even though the deep waters were much colder than at present. To better constrain these inferences drawn from the spatial distribution of benthic δ18O, we must reduce scatter in the δ18O data with more high‐quality measurements in high sedimentation rate cores. This is especially true at bottom water sites. Also, we must intercalibrate mass spectrometers at different isotope laboratories more accurately, to insure our isotope data are compatible.