Benthic Foraminifera δ18O Research Articles

Biogenic opal δ18O in marine deep-sea sediments through the Cenozoic Era and implications for ocean cooling

Benthic foraminifera δ18O increased by 5.4 ‰ through the Cenozoic Era, the past 65 million years of Earth's history. This increase is attributed to growth of global ice sheets and global ocean cooling. On the other hand, δ18O measurements of planktonic foraminifera do not exhibit significant change. These measurements initially were interpreted to suggest cool tropical and subtropical temperatures and introduced the concept of the “cool tropics paradox”, in which global bottomwater temperatures were significantly warmer but tropical surface temperatures were not during the early Cenozoic. This lack of change in δ18O of planktonic foraminifera was explained by secondary postburial alteration of the primary planktonic isotope signal.We provide an alternative record of sea surface water characteristics of Cenozoic Era by measuring δ18O of siliceous microfossils. This scarce phase of marine silica, unaltered biogenic opal-A, composed of diatoms, radiolaria and sponge spicules, was separated and purified from deep-sea Cenozoic sediments. We measured δ18O of siliceous microfossils from a set of drilled cores: DSDP 278 from the South Pacific, DSDP 513 from the South Atlantic, and ODP 1050–1053 and DSDP 391 from the subtropical North Atlantic. The purity of the biogenic opal and its mineralogical phase was assessed with SEM, SEM/EDS, XPS, and XRD.The δ18O of biogenic opal-A fluctuates around an average value of 43.0 ‰ through the Cenozoic Era, comparable to the Southern Ocean average of 43.2 ‰ of diatoms and radiolaria from the last 30,000 years which represent the last glacial and interglacial period and suggesting cold surface temperatures of 2–7 °C, both in the subtropical and polar latitudes. Taking into consideration changes in δ18O associated with bottomwater interaction with the lower crust and changes in sea level associated with waxing and waning of ice sheets, we conclude lack of or minimal change in the surface water temperature through the Cenozoic Era. This revives the original idea of the “cool tropics” paradox with a carbonate independent δ18O record and without issues associated with diagenesis of benthic and planktonic foraminifera.

Burial History and Hydrocarbon Potential of the Harudi and Fulra Limestone of Kachchh, Western India Constrained using Carbonate Clumped Isotope Thermometry

Abstract The clumped isotope paleothermometry is used to estimate minimum burial depth of the mid Eocene Harudi and Fulra Limestone Formations of Kachchh, western India. The Nummulites and other fossil foraminiferal genera were separated for conventional (δ18O, δ13C) and clumped isotopic (Δ47) measurements. The measurements were also made on whole rock (WR) to visualize the different phases of diagenetic alteration, expected depth of burial and capacity to generate hydrocarbon. The measured δ18O and δ13C values of WR samples vary between -3.09 and -2.21‰ and -0.78 and 0.96‰ respectively, with average values of -2.88‰ and 0.16‰. The δ18O and δ13C values of benthic foraminifera vary between -3.52 and -3.20‰ (Average: -3.35±0.013) and -1.04 and 0.81‰ (Average: 0.26±0.76) respectively. The measured Δ47 for the foraminifera translates to temperature of 35 and 49°C and between 26 and 48°C for WR indicating minor diagenetic alteration. The minimum estimates for the burial depth of the studied formations are up to 2 km which is based on the temperature difference between the ecological preference temperature of benthic foraminifera and the diagenetically altered foraminifera and δ13C of WR and foraminifera, indicating that rocks have potential to generate methane.

THE DISTRIBUTION RANGE OF CALCAREOUS NANNOFOSSIL SPECIES <em>RETICULOFENESTRA PSEUDOUMBILICUS</em> IN THE MIOCENE: AN EXAMPLE OF ECOLOGICAL INFLUENCE ON EVOLUTIONARY DEVELOPMENT

Reticulofenestra pseudoumbilicus is a Neogene calcareous nannofossil species whose highest stratigraphic occurrence (Top) is a reliable biohorizon in the Pliocene, calibrated at 3.82 Ma. The species is present in the stratigraphic record from at least the Middle Miocene, within an interval around the biohorizon Top Sphenolithus heteromorphus, calibrated at 13.53 Ma, but its lower distribution range is not precisely delineated. The study of nannofossil assemblages in sediment cores from Integrated Ocean Drilling Program IODP Site U1338 (eastern equatorial Pacific) indicates a lower stratigraphic position for the evolutionary emergence (Base) of R. pseudoumbilicus, detected in the Early Miocene with an estimated age of 16.46 Ma. This age results from a new astronomically tuned chronology, which dates the deepest sediments at Site U1338 to 16.67 Ma. Base R. pseudoumbilicus is followed above by a temporary disappearance of the taxon until a re-entrance after ~3 Myr. This lengthened stratigraphic range has been confirmed by data from other locations at low and mid-latitudes in the Atlantic. The distribution range of R. pseudoumbilicus, lasting ~13 Myr during the Neogene, is thus characterized by a variable pattern of repeated occurrences and disappearances. Comparison to benthic foraminifera δ18O and δ13C records suggests a control by global climatic/environmental conditions on these events, particularly by temperature variations. The recurrent presence of R. pseudoumbilicus at stratigraphically different intervals could represent an example of iterative evolution, expressed as repeated speciation events that are in part influenced by complex external factors related to the dynamic climate and environmental evolution during the Miocene.

East Antarctic Ice Sheet variability during the middle Miocene Climate Transition captured in drill cores from the Friis Hills, Transantarctic Mountains

This study describes 16 well-dated, terrestrial glacial sedimentary cycles deposited during astronomically paced climate cycles from the termination of the Miocene Climatic Optimum (MCO) through the middle Miocene Climate Transition (MMCT) (15.1−13.8 Ma) in the Friis Hills, Transantarctic Mountains, Antarctica. Three locations were continuously cored (79% recovery) to a maximum depth of 50.48 m through a succession of interbedded till sheets and fossil-bearing, fluvio-lacustrine sediments. A composite chronostratigraphic framework is presented for the cores based on the previous mapping, a seismic refraction survey that defines basin geometry, and a new, integrated age model based on paleomagnetic stratigraphy that is constrained by radioisotopic 40Ar/39Ar numeric ages on two newly identified silicic tephra. The paleoecologic and sedimentologic characteristics of organic-rich lithologies are relatively consistent up-section, which implies that successively younger interglacial deposits during the MMCT represented broadly similar environmental and climatic conditions. During these interglacials, the Friis Hills hinterland was likely ice-free. Major disconformities in the section suggest a transition to colder climates, and after ca. 14.6 Ma, thicker, more extensive and erosive ice cover occurred across the Friis Hills during glacial episodes. Diamictites in the upper three cycles suggest that climate cooled and became drier after ca. 14.2 Ma. However, cyclical retreat of the ice and a return to warm climate conditions during interglacials continued through ca. 13.9 Ma. These direct records reflect a highly variable East Antarctic Ice Sheet margin but show that the ice margin became progressively more extensive during successive glacial intervals, which is consistent with a cooling trend toward more glacial values in the far-field benthic foraminifera δ18O proxy ice volume and temperature record. Age constraints show that glacial-interglacial variability at the terrestrial margin of the East Antarctic Ice Sheet was primarily paced by astronomical precession (∼23 k.y.) through the onset of the MMCT (15−14.7 Ma). Precession-driven cycles are modulated by short-period (∼100 k.y.) eccentricity cycles. Intervals of maximum eccentricity (high seasonality) coincide with sedimentary cycles comprising thin diamictites and relatively thick interglacial sandstone and mudstone units. Intervals of minimum eccentricity (low seasonality) coincide with sedimentary cycles comprising thick diamictites and relatively thin interglacial sedimentary deposits. Major disconformities in the Friis Hills succession that span more than ∼100 k.y. reflect episodes of expansion of erosive ice across, and well beyond, the Transantarctic Mountains and coincide with nodes in eccentricity (∼400 k.y.). These relationships suggest that during relatively warm intervals in the middle Miocene, the East Antarctic Ice Sheet expanded and contracted over 100 k.y. cycles, while its margins continued to fluctuate at higher (∼23 k.y.) frequency. After 14.5 Ma, obliquity is the dominant frequency in δ18O records, marking a period during which large regions of the Antarctic Ice Sheet grounded in marine environments.

Iceberg discharge events in the northwest Pacific and related sequence of Kamchatka glaciations over the last 190 kyr

Evidence of meltwater and ice rafted debris from the Tenji and Detroit seamounts in the northwestern corner of the North Pacific are key to understanding ice sheet dynamics across the Kamchatka Region. Marine isotopic stages 6.5–1 were established in this study for the Tenji Seamount sediment core based on accelerator mass spectrometry (AMS) 14C data, benthic foraminifera δ18O stratigraphy, and relative paleointensity excursions. Records of planktic foraminifera δ18O and ice rafted debris (IRD) in sediment cores pertaining to the last 190 kyr were used to outline several periods of surface water freshening and increased accumulation of coarse terrigenous grains induced by iceberg supply and melting in the northwest Pacific. Iceberg discharge events (IDEs) were reconstructed over the last and penultimate glacial periods in the central and eastern parts of the Okhotsk Sea and southwestern part of the Bering Sea using relevant published proxies. Similar reconstructions of IDEs were also performed for two published sediment cores from the Detroit Seamount. Synthesis of planktic foraminifera δ18O and IRD records along with other related proxies from the studied areas concordantly revealed five IDEs in the northwest Pacific and its marginal seas during the last 190 kyr: IDE 1 (42–29 ka), IDE 2 (76–57 ka), IDE 3 (118–110 ka), IDE 4 (152–141 ka), and IDE 5 (179–160 ka). Data showed that IDEs 4–1 occurred at low values of Earth obliquity and under a weak Aleutian Low, facilitating snowfall precipitation and enhancing glacier formation at Kamchatka, which is a priority area for capturing atmospheric moisture transferred into northeast Asia. Compared with the other IDEs, IDEs 2 and 5 were more pronounced and prolonged events over the last 190 kyr. In addition, IDE 2 was also characterised by a catastrophic and short IRD peak that was probably forced by the influx of lake water from the Central Kamchatka Depression. It is likely that IDEs 2 and 1 occurred during phases I (more extensive) and II (less extensive) of the published Kamchatka glaciations during the Late Pleistocene.

Unmixing deep-sea paleoclimate records: A study on bioturbation effects through convolution and deconvolution

Marine sediments are typically subject to bioturbation from benthic macrofauna, which disrupt sediments and can have a substantial impact on the amplitude and timing of climate proxy signals recorded in marine sediment cores. This study explores a computational approach to quantify and remove these post-depositional effects from marine climate proxy records. The bioturbation process is modeled as a linear time-invariant filter building upon prior work (Guinasso and Schinck, 1975). Using forward modeling we find that, given modeled mixing layer depths ≥ 5 cm, simulated centennial scale climate variability is generally not preserved even when the sedimentation rate in our model is above 15 cm/kyr. On the scale of ice ages (104-105 yr), the observed effects of bioturbation have the smallest impact, considering the event scale and typical sedimentation rates and mixed layer depth in the deep sea. For millennial scale events, the signal attenuation strongly depends on the event scale and specific bioturbation and sedimentation parameters. To account for the bioturbation effect on a given climate proxy series, a deconvolution method is proposed. We apply the approach to three individual benthic foraminifera oxygen isotope records to reconstruct δ18O across the last interglacial (Marine Isotope Stage 5e; MIS 5e), and to assess the magnitude of the deglacial shift between MIS 6 and MIS 5e. In the highest resolution records (Site MD952042, Site 1012), the deconvolution results reveal an increase in the amplitude of the recovered deglacial δ18O shift by approximately 0.1‰, with implications for reconstructed global average sea level variations and temperature estimates from benthic foraminifera δ18O. The recovered signal at Site 677 suggests high amplitude δ18O variability during MIS 5e, but exhibits larger uncertainty due to a lower sampling frequency, lower sedimentation rate, and an inferred higher noise level. While uncertainties on the deconvolution results can be large, the results provide a new view of the potential impact of bioturbation on paleoclimate reconstruction. Future work can reduce the uncertainty in the deconvolution estimates, through quantitative integration of additional constraints, provided by our knowledge of the climate and depositional systems.

A Ternary Mixing Model Approach Using Benthic Foraminifer δ13C‐δ18O Data to Reconstruct Late Pliocene Deep Atlantic Water Mass Mixing

AbstractLate Pliocene deep Atlantic δ13C data have been interpreted as evidence for enhanced Atlantic Meridional Overturning Circulation (AMOC) compared to the present, but this hypothesis is not supported by the Pliocene Model Intercomparison Project (PlioMIP). Here, we adopt a new approach to assess variability in deep ocean circulation based on paired stable carbon (δ13C) and oxygen isotopes (δ18O) of benthic foraminifera, both (semi)conservative water mass tracers. Assuming that deep Atlantic benthic δ13C‐δ18O variability is predominantly driven by mixing, we extrapolate the δ13C‐δ18O data outside the sampled range to identify the end‐members. At least three end‐members are needed to explain the spatial δ13C‐δ18O variability in the deep North Atlantic Ocean: two Northern Component Water (NCW) and one Southern Component Water (SCW) water masses. We use a ternary mixing model to quantify the mixing proportions between SCW and NCW in the deep Atlantic Ocean. Our analysis includes new data from Ocean Drilling Program Sites 959 and 662 in the eastern equatorial Atlantic and suggests that the AMOC cell was deeper during the M2 glacial than during late Pliocene interglacials. Moreover, we identify a new cold and well‐ventilated water mass that was geographically restricted to the southeast Atlantic Ocean between 3.6 and 2.7 Ma and did not contribute significantly to the δ13C‐δ18O variability of the rest of the basin. This high‐δ13C high‐δ18O water mass has led to the misconception of a reduced latitudinal δ13C gradient. Our analyses show that the late Pliocene δ13C gradient between NCW and SCW was similar to the present‐day value of 1.1‰.

Assessing the impact of diagenesis on foraminiferal geochemistry from a low latitude, shallow-water drift deposit

Due to their large heat and moisture storage capabilities, the tropics are fundamental in modulating both regional and global climate. Furthermore, their thermal response during past extreme warming periods, such as super interglacials, is not fully resolved. In this regard, we present high-resolution (analytical) foraminiferal geochemical (δ18O and Mg/Ca) records for the last 1800 kyr from the shallow (487 m) Inner Sea drift deposits of the Maldives archipelago in the equatorial Indian Ocean. Considering the diagenetic susceptibility of these proxies, in carbonate-rich environments, we assess the integrity of a suite of commonly used planktonic and benthic foraminifera geochemical datasets (Globigerinoides ruber (white), Globigerinita glutinata (with bulla), Pulleniatina obliquiloculata (with cortex) and Cibicides mabahethi) and their use for future paleoceanographic reconstructions.Using a combination of spot Secondary Ion Mass Spectrometer, Electron Probe Micro-Analyzer and Scanning Electron Microscope image data, it is evident that authigenic overgrowths are present on both the external and internal test (shell) surfaces, yet the degree down-core as well as the associated bias is shown to be variable across the investigated species and proxies. Given the elevated authigenic overgrowth Mg/Ca (∼12–22 mmol/mol) and δ18O values (closer to the benthic isotopic compositions) the whole-test planktonic G. ruber (w) geochemical records are notably impacted beyond ∼627.4 ka (24.7 mcd). Yet, considering the setting (i.e. bottom water location) for overgrowth formation, the benthic foraminifera δ18O record is markedly less impacted with only minor diagenetic bias beyond ∼790.0 ka (28.7 mcd). Even though only the top of the G. ruber (w) and C. mabahethi records (whole-test data) would be suitable for paleo-reconstructions of absolute values (i.e. sea surface temperature, salinity, seawater δ18O), the long-term cycles, while dampened, appear to be preserved. Furthermore, planktonic species with thicker-tests (i.e. P. obliquiloculata (w/c)) might be better suited, in comparison to thinner-test counter-parts (i.e. G. glutinata (w/b), G. ruber (w)), for traditional whole-test geochemical studies in shallow, carbonate-rich environments. A thicker test equates to a smaller overall bias from the authigenic overgrowth. Overall, if the diagenetic impact is constrained, as done in this study, these types of diagenetically altered geochemical records can still significantly contribute to studies relating to past tropical seawater temperatures, latitudinal scale ocean current shifts and South Asian Monsoon dynamics.

Muted cooling and drying of NW Mediterranean in response to the strongest last glacial North American ice surges

Abstract The massive North Atlantic iceberg discharges of the last glacial period, the so-called Heinrich events (HE), resulted in atmospheric and oceanic responses of the Mediterranean region that remain poorly documented and understood. This paper focuses on the climatic phases termed Heinrich stadials (HS) 4 and 5 generated by the HE 4 and 5 that occurred during a period of similar intermediate global ice volume and greenhouse gas concentrations but with different iceberg discharges and orbital boundary conditions. Our comparison of sea surface temperature and salinity changes with deep water conditions in the Gulf of Lions (planktonic and benthic foraminifera δ18O and δ13C records) and regional pollen-based temperature and precipitation reconstructions in NW Mediterranean for these Heinrich stadials reveal a paradoxical situation. A lower North American iceberg discharge during HS 5 compared to HS 4 is associated with colder and drier conditions in the NW Mediterranean borderlands. During the moderate iceberg discharge of HS 5 a relatively high salinity in the Gulf of Lions lead to stronger Western Mediterranean Deep Water formation and mixing of the surface with the deeper layers. By contrast during HS 4, we suggest that the massive North Atlantic iceberg break-up decreased the salinity of the Gulf of Lions and reduced the wind stress in the Mediterranean, leading to the stratification of the Mediterranean water column and inducing limited upward mixing of cold water, resulting in regional atmospheric warming and wetting compared to HS 5. This work highlights the potential crucial role of local processes in modulating the regional response to a global climate change related with ice-sheet instabilities.

Integrating palaeoclimate time series with rich metadata for uncertainty modelling: strategy and documentation of the PalMod 130k marine palaeoclimate data synthesis

Abstract. Palaeoclimate data hold the unique promise of providing a long-term perspective on climate change and as such can serve as an important benchmark for climate models. However, palaeoclimate data have generally been archived with insufficient standardisation and metadata to allow for transparent and consistent uncertainty assessment in an automated way. Thanks to improved computation capacity, transient palaeoclimate simulations are now possible, calling for data products containing multi-parameter time series rather than information on a single parameter for a single time slice. Efforts are underway to simulate a complete glacial–interglacial cycle using general circulation models (https://www.palmod.de/, last access: 6 May 2020), and to confront these simulations with palaeoclimate data, we have compiled a multi-parameter marine palaeoclimate data synthesis that contains time series spanning 0 to 130 000 years ago. We present the first version of the data product that focuses exclusively on time series for which a robust chronology based on benthic foraminifera δ18O and radiocarbon dating is available. The product contains 896 time series of eight palaeoclimate parameters from 143 individual sites, each associated with rich metadata, age–depth model ensembles, and information to refine and update the chronologies. This version contains 205 time series of benthic foraminifera δ18O; 169 of benthic foraminifera δ13C; 131 of seawater temperature; 174 and 119 of planktonic foraminifera δ18O and δ13C; and 44, 38 and 16 of carbonate, organic carbon and biogenic silica content, respectively. The data product is available in three formats (R, LiPD and netCDF) facilitating use across different software and operating systems and can be downloaded at https://doi.org/10.1594/PANGAEA.908831 (Jonkers et al., 2019). This data descriptor presents our data synthesis strategy and describes the contents and format of the data product in detail. It ends with a set of recommendations for data archiving.

Seawater paleotemperature and paleosalinity evolution in neritic environments of the Mediterranean margin: Insights from isotope analysis of bivalve shells

The first step of ice-sheet build-up on Antarctica at the Eocene-Oligocene boundary was followed by a phase of climate instability culminating during the Miocene with a warming event called the Middle Miocene Climate Optimum (MMCO), that ended with a marked cooling phase identified as the Middle Miocene Climate Transition (MMCT). While numerous benthic foraminifera δ18O and Mg/Ca data have been used to capture the global climate evolution during this interval of climate and ice-sheet instability, geochemical records from shallow-water carbonates still remain scarce. Yet such records are crucial to capture the diversity of regional environmental responses to global climate changes, and thus to better understand the behavior of our climate system during this critical interval. In this work, we test the potential of a multi-proxy approach (δ18O, clumped isotope (Δ47), strontium isotopes (87Sr/86Sr)) applied to bivalves recovered from the Liguro-Provencal Basin and Rhodano-Provencal basin (Northwestern of the Mediterranean Sea) to reconstruct the evolution of shallow seawater temperature and salinity in this region over the latest Oligocene to Middle Miocene interval (~10 Myrs). Our results highlight a local cooling in the northwestern Mediterranean Sea during the MMCO that contrasts with the warming observed in other regions, with seawater temperatures inferred from Δ47 analyses in the 13–18 °C range. These cool seawater temperatures recorded in the studied region during the MMCO are much cooler than those recorded in the open oceans, but are in agreement with the proliferation of bryozoan observed in the Castillon-Du-Gard area during this interval. Low bivalve δ18O values (−3.24‰ in average) are recorded during this episode are interpreted to reflect enhanced freshwater inputs, lowering local salinity and seawater δ18O. Such enhanced freshwater inputs point to a phase of enhanced hydrological cycle in the studied region, possibly linked to the global mid-Miocene warming event. Bivalve 87Sr/86Sr data mostly fall within the global seawater Sr isotope reference curve, but remain compatible with enhanced freshwater inputs during the Aquitanian and during the Langhian, as these inputs were probably not important enough to induce a large departure from the seawater curve at a regional scale.

Possible linkage between the long-eccentricity marine carbon cycle and the deep-Pacific circulation: Western equatorial Pacific benthic foraminifera evidences of the last 4 Ma

Abstract The ~400-kyr period of marine carbon isotope records, in concert with the long eccentricity cycle, is a meaningful signature of the global carbon cycle, and this period was prolonged to ~500 kyr in the last ~1.6 Ma. The cause of the elongation of the long-eccentricity carbon cycle at ~1.6 Ma has not been fully understood, partly due to the conundrum of its relationship to changes in the circulation of deep-water masses. In this study, we present the benthic foraminifera δ13C and δ18O and the relative abundances of selected benthic foraminifera, from the upper ~80 m of Site U1489 of IODP Expedition 363. First, we established an age model for our samples, covering the last 4 Ma, based on the correlation of benthic foraminifera δ18O to the LR04 global stack as well as ship-board magneto-stratigraphy. Then we compared the δ18O and δ13C between Cibicidoides and Uvigerina to reconfirm the inter-genus isotopic adjustments. The coarse fraction, benthic foraminifera δ13C and the abundances of selected benthic species/genera show detectable 400–500 kyr cycles, with minimal coarse fraction and maximal δ13C around eccentricity minima. The variations in the relative abundances of selected benthic foraminifera, especially Favocassidulina favus, and the comparisons among deep-Pacific δ13C records, reveal a shift to a more sluggish mode of Pacific deep-water circulation occurring coincidently at ~1.6 Ma. Therefore, it may be implied that the lengthening of the long-term δ13C cycle at ~1.6 Ma is linked to a deep Pacific circulation change that prolonged the cycling of carbon in the ocean.

100‐kyr Paced Climate Change in the Pliocene Warm Period, Southwest Pacific

AbstractThe mid to late Pliocene (~4.2–2.8 Ma.) represents an experiment in climate sensitivity to orbital pacing in which nearly all continental ice was confined to the Southern Hemisphere. Most studies have emphasized the dominant role of obliquity in determining changes in ice volume and temperature at this time, although most records come from the Northern Hemisphere, instead of the hemisphere where the bulk of ice resided. We present the first orbitally‐resolved, mid to late Pliocene Southern Hemisphere paired records of surface and subsurface variability from two deep ocean archives from the Southwest Pacific Ocean (Sites 594 and 1125). These records indicate dominance of low frequencies centered at ~100 kyr for this time period. Because these signatures extend coherently and synchronously from middepth water properties (δ13C, δ18O of benthic foraminifera), which have their chemistry set in the subantarctic belt, to the surface (alkenone‐derived surface temperature estimates, color reflectance, and magnetic susceptibility), we infer that the fingerprint of the ~100‐kyr cycles must have extended over a large region of the Southern Hemisphere. We propose that nonlinearities in climate response to precessional forcing—most likely through ice sheet and/or carbon cycle behavior—generated the observed low frequency. A review of published mid to late Pliocene time series suggests that the ~100‐kyr pacing may be a widespread phenomenon and that major approximately 100‐kyr excursions in Pliocene climate were an important overlay to the underlying 41‐kyr glacial‐interglacial rhythm. These results caution against uncritical use of existing Pliocene isotopic templates to construct high‐resolution age models.

Quantitative organic-walled dinoflagellate cyst stratigraphy across the Eocene-Oligocene Transition in the Gulf of Mexico: A record of climate- and sea level change during the onset of Antarctic glaciation

The Eocene – Oligocene Transition (EOT, 34–33.5 Ma) marks a major transition in Cenozoic climate evolution through the relatively rapid establishment of a continental-scale ice sheet on Antarctica. The EOT is characterized by two 200 kyr spaced shifts (termed EOT-1 and Oi-1) in the oxygen isotopic composition (δ18O) of benthic foraminifera, representing both changes in continental ice-volume and temperature. Estimates of the timing and magnitude of these changes during this critical phase in Earth’s climatic evolution are controversial. Here we present marine palynological assemblage data, in particular of organic-walled dinoflagellate cysts (dinocysts), across a classic upper Eocene to lower Oligocene neritic succession cored in Alabama, USA; the Saint Stephens Quarry (SSQ) borehole. These palynological data combined with lithological information allow the identification of three sequence boundaries across the EOT. Critically, we identify a sequence boundary at the level corresponding to the EOT-1. Integrated sea level and paleotemperature records show that EOT-1 primarily represents cooling with some minor and transient continental ice sheet expansion. Furthermore, we identify a significant hiatus, likely caused by major sea level fall at the base of Magnetochron C13n that corresponds to the Oi-1 shift. This clarifies the δ18O records from SSQ that essentially lack the expected pronounced shift to positive δ18O values so characteristic for Oi-1. Furthermore, we document originations and extinctions of potentially temperature-sensitive dinocysts associated with the EOT-1. In contrast, the Oi-1 does not stand out as period of substantial dinoflagellate turnover. The combined results illustrate that major cooling, limited and transient ice growth and major biotic change were occurring before the full-size expansion of the Antarctic cryosphere. © 2018 The Authors.

An abrupt decrease in Southern Hemisphere terrestrial temperature during the Eocene–Oligocene transition

Earth's climate transitioned from the greenhouse conditions of the Eocene to the icehouse of the Oligocene ∼34 million years ago. Global cooling was triggered by a decrease in atmospheric pCO2 and is associated with the first appearance of large, permanent ice-sheets on Antarctica. The Eocene–Oligocene transition (EOT) is marked by a large two-step increase in the δ18O of benthic foraminifera, reflecting a combination of cooler global temperatures and continental ice growth. We measured the hydrogen isotope composition of hydrated volcanic glasses deposited in Patagonia during the EOT to reconstruct stable isotope hydrology and associated changes in temperature during this critical climate transition. These data show changes in stable hydrogen isotope ratios that are consistent with Patagonian terrestrial mean annual temperature decreasing by ∼5°C during the EOT before returning to 2°C below pre-EOT values by the Early Oligocene. This is approximately synchronous with Northern Hemisphere cooling during the EOT, and indicates close coupling of temperature and atmospheric pCO2.

Coupling between Grand cycles and Events in Earth\u2019s climate during the past 115 million years

Geological sediment archives document a rich periodic series of astronomically driven climate, but record also abrupt, severe climatic changes called events, the multi-Myr boundary conditions of which have generally been ascribed to acyclic processes from Earth’s interior dynamics. These events have rarely been considered together within extended time series for potential correlation with long-term (multi-million year, Myr) cycling. Here I show a coupling between events and multi-Myr cycles in a temperature and ice-volume climatic proxy of the geological past 115 Myr. I use Cenozoic through middle Cretaceous climatic variations, as recorded in benthic foraminifera δ18O, to highlight prominent ~9 and ~36 Myr cyclicities. These cyclicities were previously attributed either to astronomical or tectonic variations. In particular, I point out that most of the well-known events during the past 115 Myr geological interval occur during extremes in the ~9 and ~36 Myr cycling. One exception is the early Cenozoic hyperthermal events including the salient Paleocene-Eocene Thermal Maximum (~56 Ma), which do not match extremes in long-period cyclicities, but to inflection point of these cycles. Specific focus on climatic events, as inferred from δ18O proxy, suggest that some “events”, marked by gradual trends within the ~9 and ~36 Myr cycle extremes, would principally be paced by long-term cycling, while “events”, recorded as abrupt δ18O changes nearby cycle extremes, would be rather induced by acyclic processes. The connection between cyclic and acyclic processes, as triggers or feedbacks, is very likely. Such link between cycling and events in Earth’s past climate provides insight into celestial dynamics governing perturbations in Earth’s surface systems, but also the potential connection between external and Earth’s interior processes.

Marine Isotope Stage 4 in Australasia: A full glacial culminating 65,000 years ago – Global connections and implications for human dispersal

Over the last four decades of palaeoclimate research, significant emphasis has been placed on the Last Glacial Maximum (LGM) spanning 26.5–19 thousand years ago (ka), a period that saw significant (∼125 m) sea-level reductions and major ice caps adorning large parts of the Northern Hemisphere. Here, we present evidence for another major glacial period spanning 71–59 ka (Marine Isotope Stage 4: MIS4) from a well-dated marine sequence offshore South Australia. The astronomically-tuned chronology of this deep-sea core is confirmed using single-grain optically stimulated luminescence dating (OSL), providing confidence in our high-resolution age model. Our approach to the study of our MD03-2607 core has been to employ many different proxies. These are: δ18O of both planktic and benthic foraminifera for stratigraphic purposes, faunal counts of planktonic foraminifera to reconstruct the position of oceanic fronts and currents, alkenone palaeothermometry, XRF core scanning to determine the presence of aeolian dust, and εNd isotope to identify fluvial discharge over the core site. We compare our new proxy findings with other archives for mainland Australia and Tasmania.Our multi-proxy palaeoclimate reconstructions are consistent with other marine, terrestrial and cryosphere archives across the Southern Hemisphere and suggest, for the first time, that MIS 4 was almost as dramatic as the LGM. During MIS4, global sea-level was reduced by ∼100 m, glaciers across Australasia were more significant compared to the LGM, and sea-surface temperatures were much reduced. These glacial conditions uniformly peaked around 65 ka. Global comparisons show major glacial conditions and vegetation shifts elsewhere during MIS4, but many are poorly dated. The significant environmental changes taking place during this glacial period were paralleled by waves of human dispersal across Eurasia and the earliest evidence of human occupation in northern Australia at 65 ka.

Detrital Sr–Nd isotopes, sediment provenances and depositional processes in the Laxmi Basin of the Arabian Sea during the last 800 ka

Abstract87Sr/86Sr ratios and εNd values of detrital particles at International Ocean Discovery Program (IODP) Site U1456 in the Laxmi Basin of the Arabian Sea were measured to trace changes in sediment provenance over glacial–interglacial cycles. Based on the correlation of planktonic foraminiferal (Globigerinoides ruber) δ18O fluctuations with the LR04 stack of benthic foraminifera δ18O values, combined with shipboard biostratigraphic and palaeomagnetic data, the studied interval spans ∼1.2 Ma. Over the past 800 ka, 87Sr/86Sr values ranged from 0.711 to 0.726 while εNd values ranged between −12.5 and −7.3 in the detrital particles. By comparing 87Sr/86Sr ratios and εNd values of the possible sources of river sediments with our data, we found that sediments in the Laxmi Basin were influenced to various degrees by proportions of at least three sediment sources (i.e. Tapi River, Narmada River and Indus River). The Indus River might be a more important contributor to glacial sediments. Although 87Sr/86Sr ratios and εNd values varied quasi-cyclically, this pattern did not correspond precisely to the glacial–interglacial cycles. In particular, low-magnetic-susceptibility (low-MS) intervals coinciding with pelagic carbonates were characterized by low 87Sr/86Sr ratios and high εNd values, whereas high-MS intervals matching turbidite deposits showed high 87Sr/86Sr ratios and low εNd values. Thus, this study reveals that differences in the depositional processes between glacial and interglacial periods, governed by changes in sea level and monsoon activity, are an important factor in deciding 87Sr/86Sr ratios and εNd values of the detrital fraction in the Indus Fan of the Arabian Sea.

Distribution and constraining factors of planktonic and benthic foraminifers in bottom sediments of the southern South China Sea

A total of 93 surface sediment samples were collected to analyze the environmental factors constraining the foraminiferal distribution pattern in the southern South China Sea (SCS). Species distributions and stable isotopic compositions were combined to reveal the controlling factors, such as depth, salinity, substrate, runoff, currents, and cold seep activities. Water depth is the dominant factor controlling both assemblage composition and δ18O of benthic foraminiferal tests. The 1000 m isobath separates the sites into two clusters (Cluster A and B), which are dominated by deep-water species and shallow-water species, respectively. The sites in the deep-water zone (Cluster A) are characterized by lower absolute abundances, species richness and Shannon Index values (a measure of diversity), and higher proportions of planktonic foraminifers compared with the sites in the shallow-water zone (Cluster B). Increasing proportions of agglutinated tests with depth and rapidly decreasing proportions of planktonic foraminifera in the Nansha Trough Basin provide evidence of calcium dissolution occurring at a depth corresponding with previous reports. Oxygen stable isotopes (δ18OB) of benthic foraminifera become more positive with depth only up to 1000 m and remain constant beyond. Differences in the proportion of agglutinated and porcelaneous tests in the shallow-water zone suggest that terrestrial runoff from nearby river systems (Mekong River and northern Borneo rivers) and seasonal surface currents (SCS Southern Cyclonic Gyre and SCS Southern Anticyclonic Gyre) jointly influence the distribution patterns of foraminifera in the study area. Enrichment of planktonic δ18O is a response to cold waters brought by the SCS southern cyclonic gyre during winter. An upwelling current (Winter Natuna Off-Shelf Current) containing higher amounts of organic matter/nutrients contributes to the depleted δ13C of planktonic foraminifera and to the abnormal abundance of foraminifera at the sites within its area of influence. The dominance of the foraminifer Melonis barleeanus at sites belonging to Subcluster A1 and the stable isotope compositions of benthic foraminifera (δ18O > 0, δ13C < 0) across the sites suggest the influence of active cold seeps in the southern SCS. This research highlights the complexity of environmental variables that interact to influence the foraminiferal assemblages and geochemistry in the southern South China Sea, which could serve as a model for paleoenvironmental and palaeoceanographic reconstructions.

Dynamics of benthic marine communities across the Early-Middle Pleistocene boundary in the Mediterranean region (Valle di Manche, Southern Italy): Biotic and stratigraphic implications

The Valle di Manche (VdM) section (Calabria, Southern Italy) offers the opportunity to investigate the ostracod turnover along a continuous shelf succession straddling the Early-Middle Pleistocene boundary, and compare it against other paleoenvironmental (i.e., mollusks) and paleoclimatic (Uvigerina peregrina δ18O) records. High-resolution (ca. 1 sample/m) ostracod fauna quantitative data, coupled with gradient analysis (Detrended Correspondence Analysis and non-Metric Multi-Dimensional Scaling), document a strong relationship between changes in faunal composition and lithofacies vertical stacking patterns. The comparison between the mollusk- and ostracod-derived ordination data demonstrates that the meio- and macro-faunal turnover is guided by a common complex gradient: bathymetry. The integrated ostracod-mollusk gradient analysis also provides a trend in water depths along the section, highlighting to what extent such multivariate approach can improve the paleoenvironmental and sequence stratigraphic interpretation of ancient shallow marine successions. When plotted stratigraphically, ordination major axis sample scores reveal two increasing-decreasing patterns in water depth that fit well with the T-R cycles previously identified. Paleobathymetric estimates combined with the vertical distribution of key ostracod groups (i.e., epiphytic taxa on sandy substrates vs. deep-sea mud related taxa) allow refining depositional trends, stratal stacking patterns and position of previously not well resolved sequence stratigraphic surfaces within each T-R cycle (e.g., Transgressive Surface-TS). Indeed, two rapid increases in water depth values mark the TSs that separate shallowing-upward, progradational (RST) from deepening upward, retrogradational (TST) stratal stacking patterns of shelf deposits. The TSs, which underline fine-grained successions dominated by deep-sea mud-lover taxa, are invariably constrained to the inception of interglacials Marine Isotope Stages (MIS) 21 and 19, identified within the VdM section by benthic foraminifera δ18O values. Within both the VdM T-R cycles, the deepest conditions (ca. 140 m of water depth) are invariably identified within the Neopycnodonte unit, slightly above the lightest δ18O intervals. The overlying decreasing bathymetric trend, coupled with shifts in ostracod ecological groups, allows to identify in the bryozoans lithofacies the stillstand+falling of the relative sea-level, also tracked by a progressively heavier δ18O record. More stable paleobathymetric conditions (around 40–45 m of water depth) characterize the overlying silt-sand deposits dominated by epiphytic species and showing the heaviest δ18O values.