Search

International audience; Mio-Pliocene sedimentary archives of the East Asian summer monsoon (EASM) in NE Tibet record a monotonic response to orbital forcing, dominated by eccentricity. By contrast, Pleistocene archives display a more stochastic response that varies regionally and temporally. When and why this response changed is poorly understood. Here, we present a new high-resolution Rb/Sr ratio data set of EASM intensity from the Sanmenxia Basin, North China, that spans the Plio-Pleistocene transition. Our results indicate decreased monsoonal rainfall in the late Pliocene, dated at 2.75–2.6 Ma, associated with an intensified response to obliquity and enhanced climate stochasticity. This transition is attributed to the increase of Northern Hemisphere ice volume. Quaternary monsoons display a sensitivity unique to the modern icehouse with large bipolar ice sheets, while pre-Quaternary monsoons were solely impacted by Antarctic ice sheet dynamics on orbital time-scales.

Widespread marine anoxia triggered by the runoff and recycling of nutrients was a key phenomenon associated with the Frasnian–Famennian (FF) mass extinction. However, the relative importance of global-scale processes versus local influences on site-specific environmental change remains poorly understood. Here, nitrogen-isotope (δ15N) trends are combined with organic-biomarker, phosphorus, and Rock-Eval data in FF sites from the USA (H-32 core, Iowa), Poland (Kowala Quarry), and Belgium (Sinsin). Up-to-date cyclostratigraphic age models for all three sites allow the nature and timing of changes to be precisely compared across the globe. Negative δ15N excursions across the FF interval from the H-32 core and Kowala correlate with geochemical evidence for euxinic, phosphorus-rich, water columns, and possible cyanobacterial activity, suggestive of increased diazotrophic N fixation, potentially coupled with ammonium assimilation at the latter site. By contrast, previously studied sites from Western Canada and South China document enhanced water-column denitrification around the onset of the Upper Kellwasser (UKW) Event, re-emphasizing the geographical heterogeneity in environmental perturbations at that time. Moreover, environmental degradation began >100 kyr earlier in Poland, coeval with a major increase in bioavailable phosphorus supply, than in Iowa, where no such influx is recorded. These regional differences in both the timing and nature of marine perturbations during the FF interval likely resulted from the variable influx of terrigenous nutrients to different marine basins at that time, highlighting the importance of local processes such as terrestrial runoff in driving environmental degradation during times of climate cooling such as the UKW Event.

International audience; The geologic time scale for the Cenozoic Era has been notably improved over the last decades by virtue of integrated stratigraphy, combining high-resolution astrochronologies, biostratigraphy and magnetostratigraphy with high-precision radioisotopic dates. However, the middle Eocene remains a weak link. The so-called “Eocene time scale gap” reflects the scarcity of suitable study sections with clear astronomically-forced variations in carbonate content, primarily because large parts of the oceans were starved of carbonate during the Eocene greenhouse. International Ocean Discovery Program (IODP) Expedition 369 cored a carbonate-rich sedimentary sequence of Eocene age in the Mentelle Basin (Site U1514, offshore southwest Australia). The sequence consists of nannofossil chalk and exhibits rhythmic clay content variability. Here, we show that IODP Site U1514 allows for the extraction of an astronomical signal and the construction of an Eocene astrochronology, using 3-cm resolution X-Ray fluorescence (XRF) core scans. The XRF-derived ratio between calcium and iron content (Ca/Fe) tracks the lithologic variability and serves as the basis for our U1514 astrochronology. We present a 16 million-year-long (40-56 Ma) nearly continuous history of Eocene sedimentation with variations paced by eccentricity and obliquity. We supplement the high-resolution XRF data with low-resolution bulk carbon and oxygen isotopes, recording the long-term cooling trend from the Paleocene-Eocene Thermal Maximum (PETM – ca. 56 Ma) into the middle Eocene (ca. 40 Ma). Our early Eocene astrochronology corroborates existing chronologies based on deep-sea sites and Italian land sections. For the middle Eocene, the sedimentological record at U1514 provides a single-site geochemical backbone and thus offers a further step towards a fully integrated Cenozoic geologic time scale at orbital resolution.

An unusually short glaciation interrupted the warm Pliocene around 3.3 Ma (Marine Isotope Stage (MIS) M2). Different hypotheses exist to explain why this glaciation event was so pronounced, and why the global climate system returned to warm Pliocene conditions relatively quickly afterwards. One of these proposed mechanisms is a reduced equator-to-pole heat transfer, in response to a tectonically reduced Indonesian Throughflow (ITF). The ITF is a critical part of the global thermohaline ocean circulation, transporting heat from the Indo-Pacific Warm Pool to the Indian Ocean. When ITF connectivity is reduced, the water and heat supply for the Leeuwin Current, flowing poleward along Australia's west coast, is also diminished. To assess the possible relationship between mid-Pliocene glaciations and latitudinal heat transport through the Indonesian Throughflow, we constructed a multi-proxy orbital-scale record for the 3.7–2.8 Ma interval from International Ocean Discovery Program (IODP) Site U1463, off northwest Australia. The comparison of the Site U1463 record with paleoclimate records from nearby Site 763 and West Pacific Warm Pool Site 806 allows for a detailed regional reconstruction of Pliocene paleoceanography and thus for testing the proposed hypothesis. An astronomically-paced decrease in potassium content characterizes the late Pliocene interval of U1463. This record documents the increasing aridity of northwest Australia, periodically alleviated by reinforced summer monsoon precipitation under summer insolation maxima. The δ 18 O record of the planktonic foraminifer Globigerinoides sacculifer correlates exceptionally well with the sea surface temperature (SST) record from Site 806 in the West Pacific Warm Pool, even during MIS M2. Hence, Site U1463 preserves an uninterrupted ITF signal even during Pliocene glaciations. However, the U1463 δ 18 O G.sacculifer record exhibits a 0.5‰ offset with the nearby Site 763A record around MIS M2. This implies that Site 763A, about 500 km west of U1463, more closely tracks Indian Ocean SST records across MIS M2. The U1463 data reveal that heat-transport through the Indonesian Throughflow did not shut down completely during MIS M2, but rather its intensity decreased prior to and during MIS M2, causing Site 763A to temporarily reflect an Indian Ocean, rather than an ITF signal. We conclude that ITF variability significantly influenced latitudinal heat transport by means of the Leeuwin Current and hence contributed to the relative intensity of MIS M2. We propose the ITF valve between the Pacific and Indian Ocean as a positive feedback mechanism, in which an initial sea level lowering reduces ITF heat transport, in turn amplifying global cooling by advancing the thermal isolation of Antarctica.

North Atlantic Deep Water (NADW) currently redistributes heat and salt between Earth's ocean basins, and plays a vital role in the ocean-atmosphere CO2 exchange. Despite its crucial role in today's climate system, vigorous debate remains as to when deep-water formation in the North Atlantic started. Here, we present datasets from carbonate-rich middle Eocene sediments from the Newfoundland Ridge, revealing a unique archive of paleoceanographic change from the progressively cooling climate of the middle Eocene. Well-defined lithologic alternations between calcareous ooze and clay-rich intervals occur at the ∼41-kyr beat of axial obliquity. Hence, we identify obliquity as the driver of middle Eocene (43.5–46 Ma) Northern Component Water (NCW, the predecessor of modern NADW) variability. High-resolution benthic foraminiferal δ 18 O and δ 13 C suggest that obliquity minima correspond to cold, nutrient-depleted, western North Atlantic deep waters. We thus link stronger NCW formation with obliquity minima. In contrast, during obliquity maxima, Deep Western Boundary Currents were weaker and warmer, while abyssal nutrients were more abundant. These aspects reflect a more sluggish NCW formation. This obliquity-paced paleoceanographic regime is in excellent agreement with results from an Earth system model, in which obliquity minima configurations enhance NCW formation.

During International Ocean Discovery Program (IODP) expeditions, shipboard-generated data provide the first insights into the cored sequences. The natural gamma radiation (NGR) of the recovered material, for example, is routinely measured on the ocean drilling research vessel DV JOIDES Resolution. At present, only total NGR counts are readily available as shipboard data, although full NGR spectra (counts as a function of gamma-ray energy level) are produced and archived. These spectra contain unexploited information, as one can estimate the sedimentary contents of potassium (K), thorium (Th), and uranium (U) from the characteristic gamma-ray energies of isotopes in the ^(40)K, ^(232)Th, and ^(238)U radioactive decay series. Dunlea et al. [2013] quantified K, Th and U contents in sediment from the South Pacific Gyre by integrating counts over specific energy levels of the NGR spectrum. However, the algorithm used in their study is unavailable to the wider scientific community due to commercial proprietary reasons. Here, we present a new MATLAB algorithm for the quantification of NGR spectra that is transparent and accessible to future NGR users. We demonstrate the algorithm's performance by comparing its results to shore-based inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma-emission spectrometry (ICP-ES), and quantitative wavelength-dispersive X-ray fluorescence (XRF) analyses. Samples for these comparisons come from eleven sites (U1341, U1343, U1366-U1369, U1414, U1428-U1430, U1463) cored in two oceans during five expeditions. In short, our algorithm rapidly produces detailed high-quality information on sediment properties during IODP expeditions at no extra cost.

Around the early–middle Eocene boundary, the first occurrence of contourite drift sediments and widespread deep ocean erosion indicate changes in the North Atlantic ocean circulation. Interestingly, these changes coincide with the first steps of Cenozoic cooling from the Paleogene greenhouse climate towards the modern icehouse. The cause for this ocean circulation reorganization is poorly understood since modern water mass tracers may have worked fundamentally different in the past and the paleoceanographic proxy record is limited in both time and space. As a result, it is challenging to reliably reconstruct the climatic and tectonic boundary conditions e.g. atmospheric greenhouse gas concentration and the depth and geometry of developing and closing passages between ocean basins. In this study, we attempt to identify thresholds in tectonic gateway passages and atmospheric CO2 concentration, using the fully coupled Earth System Model COSMOS. Indeed, the simulation of Earth's past climates can unravel the physical processes driving deep-water formation in a greenhouse world. Specifically, we use COSMOS to evaluate the impact of changes in the North Atlantic gateways at the early–middle Eocene boundary on the North Atlantic Deep Western Boundary Currents under low obliquity configuration. We find that Northern Component Waters start to form when the Greenland Scotland Ridge reaches a threshold depth of deeper than 200 m, while the Arctic Ocean is still shut off from the North Atlantic. In this scenario, the relatively deep Greenland Scotland Ridge allows for sufficient inflow of warm, salty Atlantic surface waters into the Nordic Seas to initiate convection during winter cooling. Opening the seaway towards the Arctic leads to a cessation of Northern Component Water formation as it allows for inflow of brackish surface waters into the northern Nordic Seas, hindering Northern Component Water formation.

Cyclostratigraphic analyses rely on techniques that trace astronomical components in paleoclimate signals. These techniques have demonstrated their value but rely on certain assumptions on the presence and quality of the astronomical imprint. Here we explore a new conceptual approach to time series analysis. Specifically, we evaluate the potential of spectral moments to characterize the full spectral characteristics of a record and thus not only the frequency ranges of interpreted astronomical components. Mathematically speaking, moments are unique quantities describing a specific set of points. In the case of spectral moments, we apply the concept of moments on the distribution of spectral power in a signal's periodogram. We present four case studies that illustrate the advantages and disadvantages of the spectral moment approach in gaining insight in the (astronomical) features of a particular data record. We discuss the effects of outliers in a series, variable sedimentation rate, and changing climate dynamics on the spectral moments of a power spectrum. At the same time, we carry out a sedimentation rate reconstruction based on the spectral moment approach and compare that reconstruction to results obtained through classic approaches. Compared to classic approaches, the spectral moments approach is robust and requires less strict assumptions to obtain similar first‐order information. Yet such assumptions are still necessary to achieve more precise sedimentation rate reconstructions. In summary, we show that the spectral moments approach is suitable to obtain first‐order insights in variable components embedded in a depth series.

International audience; Until now, the middle Eocene has remained a poorly constrained interval of efforts to produce an astrochronological timescale for the entire Cenozoic. This has given rise to a so-called "Eocene astronomical timescale gap" (Vandenberghe et al., 2012). A high-resolution astrochronological calibration for this interval has proven to be difficult to realize, mainly because carbonate-rich deep-marine sequences of this age are scarce. In this paper, we present records from middle Eocene carbonate-rich sequences from the North Atlantic Southeast Newfoundland Ridge (IODP Exp. 342, Sites U1408 and U1410), of which the cyclical sedimentary patterns allow for an orbital calibration of the geologic timescale between ∼38 and ∼48 Ma. These carbonate-rich cyclic sediments at Sites U1408 and U1410 were deposited as drift deposits and exhibit prominent lithological alternations (couplets) between greenish nannofossil-rich clay and white nannofossil ooze. The principal lithological couplet is driven by the obliquity of Earth's axial tilt, and the intensity of their expression is modulated by a cyclicity of about 173 kyr. This cyclicity corresponds to the interference of secular frequencies s3 and s6 (related to the precession of nodes of the Earth and Saturn, respectively). This 173-kyr obliquity amplitude modulation cycle is exceptionally well recorded in the XRF (X-ray fluorescence)-derived Ca/Fe ratio. In this work, we first demonstrate the stability of the (s3-s6) cycles using the latest astronomical solutions. Results show that this orbital component is stable back to at least 50 Ma, and can thus serve as a powerful geochronometer in the mid-Eocene portion of the Cenozoic timescale. We then exploit this potential by calibrating the geochronology of the recovered middle Eocene timescale between magnetic polarity Chrons C18n.1n and C21n. Comparison with previous timescales shows similarities, but also notable differences in durations of certain magnetic polarity chrons. We present a revision of previous astronomical timescales from the Equatorial and South Atlantic, to overcome the differences between different mid-Eocene astrochronologies. Using our new records from the North Atlantic, combined with existing records from the South Atlantic (ODP Site 1263 and Hole 702B) and Equatorial Atlantic (ODP Site 1260), we revise the durations of magnetic polarity Chrons C18n.1n to C21n, thereby arriving at a robust and self-consistent closure of the middle Eocene astronomical timescale gap.

Geophysical research letters 44(13), 6914-6925(2017). doi:10.1002/2017GL072977, Published by Wiley, Hoboken, NJ

Recent advances in radiometric dating result in significant improvements in the geo- logical timescale and provide better insight into the timing of various processes and evolutions within the Earth's system. However, no radiometric ages are contained within the Givetian. Con- sequently, the absolute ages of the Givetian Stage boundaries, as well as the stage's duration, remain poorly constrained. As an alternative, the analysis of sedimentary cycles allows for the esti- mation of the duration of this stage. We examined the high-resolution magnetic susceptibility signals of four Givetian outcrops in the Givet area for a possible astronomical imprint, to fully understand the rates of evolutionary and environmental change. All four sections are firmly corre- lated and wavelet analyses of the magnetic susceptibility signals reveal the imprint of astronomical eccentricity forcing. The highly stable 405 kyr cycles constrain the duration of the Givetian Stage at 4.35+ 0.45 Myr, which is in good agreement with the International Chronostratigraphic Chart (5.0 Myr). The studied sections also exhibit an imprint of obliquity, suggesting a climatic telecon- nection between low and high latitudes. The corresponding microfacies curves demonstrate similar astronomical imprint, and thereby indicate that the observed 10 5 year-scale cyclicity is the result of climatic and environmental change.

Rhythmical alternations between limestone and shales or marls characterize the famous Kowala section, Holy Cross Mountains, Poland. Two intervals of this section were studied for evidence of orbital cyclostratigraphy. The oldest interval spans the Frasnian–Famennian boundary, deposited under one of the hottest greenhouse climates of the Phanerozoic. The youngest interval encompasses the Devonian– Carboniferous (D–C) boundary, a pivotal moment in Earth’s climatic history that saw a transition from greenhouse to icehouse. For the Frasnian–Famennian sequence, lithological variations are consistent with 405-kyr and 100-kyr eccentricity forcing and a cyclostratigraphic floating time-scale is presented. The interpretation of observed lithological rhythms as eccentricity cycles is confirmed by amplitude modulation patterns in agreement with astronomical theory and by the recognition of precession cycles in high-resolution stable isotope records. The resulting relative time-scale suggests that � 800 kyr separate the Lower and Upper Kellwasser Events (LKE and UKE, respectively), two periods of anoxia that culminated in massive biodiversity loss at the end of the Frasnian. Th/U and pyrite framboid analyses indicate that during the UKE, oxygen levels remained low for 400 kyr and d 13 C org measure

Low-field magnetic susceptibility (?in) measurements are quick and sensitive enabling the creation of high-resolution records; making ?in an attractive correlation tool and a proxy for paleoclimate and paleoenvironments. In geologically young material – foremost in Cenozoic sediments – ?in belongs to the geoscientist's toolkit. However, ?in is a convolved signal and may reflect other processes than the often implicitly inferred depositional conditions. Diagenesis, remagnetization and low-grade metamorphism, can potentially obscure the original, depositional, ?in signal. This aspect is particularly important when interpreting ?in records from Paleozoic rocks. Here, we review data obtained from a large sample collection of Middle to Upper Devonian sections in Belgium. Comparison of ?in trends with paleoenvironmental indicators (facies) and with detrital input proxies (Zr, Th, Ti, Al) allowed to assess the persistence of depositional trends. Furthermore, the ?in signal was deconvolved into its dominant mineralogical contributions with the help of magnetic-property analysis. The main results are pointing to a magnetic signal dominated by fine-grained magnetite, of which paleomagnetic analysis indicated a formation during Carboniferous remagnetization. This prompts a potentially strong influence of post-depositional processes and it complicates the interpretation of ?in records in terms of depositional environments. However, in most of the sections, there is a relatively good relationship between ?in trends and facies evolution and between ?in and geochemical proxies for detrital inputs. This indicates that the newly formed magnetite grains would at least partly remain where they are formed, and this allows a relative preservation of the original signal, despite the strong influence of diagenesis. Two sections show a stronger impact of diagenesis, where for about half of these sections, the primary, depositional information is lost. The Eifelian–Givetian Monts de Baileux section was selected for time-series analysis of the ?in series. The Average Spectral Misfit (ASM) method is applied to explicitly evaluate the null hypothesis of no orbital signal and in this section, there is 99.05% chance that the MS signal is reflecting an orbital imprint.

This study provides a Miocene-to-recent history of Tasman Leakage (TL), driving surface-to-intermediate waters from the Pacific into the Indian Ocean. TL, in addition to Indonesian ThroughFlow (ITF), constitutes an important part of the Southern Hemisphere Supergyre. Here, we employ deep-sea benthic delta C-13 timeseries from the southwestern Pacific and eastern Indian Oceans to identify the history of Tasman Leakage. The delta C-13 results combined with sedimentary evidence show that an inter-ocean connection south of Australia existed from 7 Ma onward. A southward shift in Westerlies combined with a northward movement of Australia created the oceanic corridor necessary for Tasman Leakage (between Australia and the sub-Antarctic Front) at this time. Furthermore, changes in the northern limb of the Supergyre (ITF) are evident in the sedimentary record on Broken Ridge from similar to 3 to 2 Ma when Banda Sea intermediate waters started originating from the North Pacific.

International audience; Mechanisms that drive cyclicity in marine sediment deposits during hothouse climate periods in response to Earth's orbit variations remain debated. Orbital cycles fingerprint in the oceanographic records results from the combined effect of terrestrial (e.g., Weathering derived nutrient supply, freshwater discharge) and oceanic (e.g., productivity, oxygenation) processes, whose respective contribution remains to be clarified. Here we investigate the effect of extreme orbital configurations on the oxygenation state of the ocean using marine biogeochemistry simulations with the IPSL-CM5A2 Earth System Model under Cenomanian-Turonian boundary conditions. Our simulations show that small changes in ocean ventilation triggered by orbitally induced variations in deep water formation have a strong impact on the spatial distribution of dissolved oxygen. This phenomena is amplified in enclosed and already poorly oxygenated basins, such as the proto-Atlantic ocean, where up to 50% of the water volume become anoxic for some of the configurations.