Significant changes in atmospheric CO
over glacial-interglacial cycles have mainly been attributed to the Southern Ocean through physical and biological processes. However, little is known about the ...contribution of global biosphere productivity, associated with important CO
fluxes. Here we present the first high resolution record of Δ
O of O
in the Antarctic EPICA Dome C ice core over Termination V and Marine Isotopic Stage (MIS) 11 and reconstruct the global oxygen biosphere productivity over the last 445 ka. Our data show that compared to the younger terminations, biosphere productivity at the end of Termination V is 10 to 30 % higher. Comparisons with local palaeo observations suggest that strong terrestrial productivity in a context of low eccentricity might explain this pattern. We propose that higher biosphere productivity could have maintained low atmospheric CO
at the beginning of MIS 11, thus highlighting its control on the global climate during Termination V.
Insolation is the engine of monsoon and Walker circulations over the tropical Indian Ocean. Here, we present Holocene coccolith‐related net primary productivity (NPP) signals from two sediment cores ...retrieved in the wind‐driven coastal upwelling systems off southern India and southern Sumatra. Upwelling‐induced NPP is enhanced during summer and autumn and is a powerful tool to reconstruct atmospheric features at a seasonal scale. Our records indicate that during summer and autumn, westerly winds off southern India strengthened from the early‐Holocene (EH) to late‐Holocene (LH), while southeasterly winds off southern Sumatra strengthened from the EH to mid‐Holocene (MH) and weakened from the MH to LH. Comparisons with previous paleoclimate records and simulations, allow us to confirm such wind patterns at a regional scale and identify distinct atmospheric features associated to insolation before and after the MH. From the EH to MH, as the insolation in the Northern Hemisphere weakens during summer and strengthens during autumn, the equatorial Indian Ocean is characterized by more vigorous Walker and monsoon circulations in summer and autumn, respectively. From the MH to LH, as the insolation weakens in the Northern Hemisphere during summer and over the equator during autumn, the equatorial Indian Ocean is influenced by a general reinforcement of the Walker circulation during both seasons, a feature that we relate to a modern negative IOD‐like mode. The changes in wind result in increasing precipitation over Indonesia and India from EH to MH and over Indonesia from MH to LH as India is getting dryer.
Plain Language Summary
The aim of this study is to examine how monsoon and Walker circulations from the tropical Indian Ocean responded to changing insolation over the Holocene. Understanding their dynamics under varying boundary conditions is important for predicting future climate, as they shape precipitation of a region that sustains billions of people. First, we focus on reconstructing changes in seasonal surface wind intensity, based on net coccolith‐related primary productivity signal from two sediment cores retrieved in the coastal upwelling systems off southern India and off southern Sumatra. When these upwellings occur during summer and autumn, nutrient‐rich deep waters surface and enhance the primary productivity. Second, we use paleoclimate records and simulations to provide broad‐scale atmospheric changes and test our assumptions. Our results show that westerly winds off southern India strengthened from early to late Holocene, while southeasterly winds off southern Sumatra strengthened from early to mid‐Holocene and weakened from mid to late Holocene, thus implying distinct wind and precipitation dynamics patterns before and after mid‐Holocene. These features appear to result in weaker and stronger precipitation over Africa and Indonesia from early‐ to mid‐Holocene, respectively, and in a precipitation maximum over India during the mid‐Holocene.
Key Points
Stronger zonal Walker circulation during summer from early‐ to late‐Holocene
Stronger meridional monsoon circulation during autumn from early‐ to mid‐Holocene
Stronger (weaker) precipitation over Indonesia (Africa) from early‐ to mid‐Holocene and precipitation maximum over India in mid‐Holocene
The Arabian Sea (AS) is one of the most productive oceanic regions in the world due to several monsoon‐related processes that can increase nutrients contents in the euphotic zone. Previous studies of ...the imprint of oceanic primary productivity (PP) in AS sediments yielded diverse results depending on the studied area and the chosen proxies, with unprecise paleoclimatic interpretations. Here, we provide multi‐decennial PP and coastal upwelling dynamic records off northern Oman over the last 23 Kyr, based on the analysis of coccoliths from sediment core MD00‐2354. Our results have been compared with previous paleoenvironmental records as well as new modeling data to get precise paleoclimatic interpretations. We document higher PP and weaker coastal upwelling during the Last Glacial Maximum relative to the Holocene, and significant millennial‐scale variations over the last deglaciation corresponding to the fluctuations of the Atlantic Meridional Overturning Circulation strength. Higher PP and weaker upwelling are found during the cold stadials, while lower PP and stronger upwelling during the warm interstadial. We propose that the increases of PP were driven by increased bioavailable nutrient content in surface waters under both stronger winter monsoon conditions that strengthened the convective mixing, and higher aeolian inputs. Over the Holocene, stronger upwelling and slightly lower PP are found during the Early‐Mid Holocene, when higher summer insolation triggered stronger summer monsoon. At that time, the lower PP was probably the result of restricted advection of eutrophic summer upwelling seawater under negative wind stress curls and less aeolian inputs.
Plain Language Summary
Ocean primary productivity (PP) is the production rate of organic carbon from inorganic carbon by ocean phytoplankton through photosynthesis, in which nutrients and sunlight are necessary. In tropical oceans, this rate is usually limited by the availability of nutrients in the sunlit ocean, which is controlled by physical processes such as seawater upward motion, ocean mixing, and dust storms. All these processes play a significant role in the northwestern Arabian Sea (AS) today. It is surrounded by arid lands that provide the highest amount of airborne material in the world and it is influenced by the Indian monsoon system that drives seawater upward motion and mixing in summer and winter, respectively. However, not much is known about their impact on PP in the past. This work aims to reconstruct changes of PP in the northwestern AS over the past twenty thousand years and understand by which process(es) they are impacted, based on a model‐data comparison approach. We have found that PP was higher during cold periods than during warm ones. The changes were mainly controlled by both, winter monsoon through changes in convective mixing, and dust storms through changes in nutrient‐enrich aeolian inputs.
Key Points
Past primary productivity (PP) and upwelling in the NW Arabian Sea are reconstructed by coccolith assemblages from a marine sediment core
PP was relatively high during the cold periods compared to the warm periods of the last 23 Kyr
PP changes were mainly controlled by winter monsoon through changes in convective mixing and by aeolian inputs
Several synergistic mechanisms were likely involved in the last deglacial atmospheric pCO
rise. Leading hypotheses invoke a release of deep-ocean carbon through enhanced convection in the Southern ...Ocean (SO) and concomitant decreased efficiency of the global soft-tissue pump (STP). However, the temporal evolution of both the STP and the carbonate counter pump (CCP) remains unclear, thus preventing the evaluation of their contributions to the pCO
rise. Here we present sedimentary coccolith records combined with export production reconstructions from the Subantarctic Pacific to document the leverage the SO biological carbon pump (BCP) has imposed on deglacial pCO
. Our data suggest a weakening of BCP during the phases of carbon outgassing, due in part to an increased CCP along with higher surface ocean fertility and elevated CO
. We propose that reduced BCP efficiency combined with enhanced SO ventilation played a major role in propelling the Earth out of the last ice age.
A high resolution δ18O record obtained on benthic foraminifera (Cibicides wuellerstorfi) combined with clay mineralogy, major element concentrations and Sr–Nd isotopes have been investigated for the ...ODP Site 659 located off North Africa in order to reconstruct African monsoon intensity between 6.2 and 4.9Myr. Mineralogical and geochemical results indicate that sedimentation on the Cape Verde plateau results from a mixture of sediments deriving from the neighbouring Saharan arid region and the wet African equatorial zone (Sahelian Band), characterised by a low and a high state of weathering, respectively. Past variations in terrigenous inputs from these areas allow us to track past extension and displacement of the West African Monsoon–ITCZ system. Maximal summer insolation induced a northward migration of the inter-tropical convergence zone (ITCZ), wetter conditions in the Sahel, and a diminishing of wind erosion over the South Sahara and Sahel. In addition, there was an increase in river input to the Cape Verde plateau of sediments derived from southern sedimentary sources, which are characterised by high kaolinite contents and CIA values and low K/Al and Ti/Al ratios. On the other hand, minimal summer insolation induced a southward movement of the ITCZ, drier climate in the Sahel and stronger easterly winds resulting in an increase in dust transport from the Sahara to the Cape Verde Plateau. Dust particles are characterised by high illite and smectite content and by a low chemical state of weathering (low CIA values and high K/Al and Ti/Al ratios). Finally, our results provide new clues regarding the re-flooding of the Mediterranean Sea at the end of the MSC (5.33Myr). This event was associated with enhanced aridity in the Sahara, implying a reorganisation of the atmospheric circulation and a southward migration of the ITCZ.
•New mineralogical and geochemical proxies to trace the displacement of the ITCZ•Evidence of the migration of the ITCZ over North Africa during the MSC•Evidence of enhanced aridity in the Sahara at the end of the Messinian Salinity Crisis
The Valanginian stage (Early Cretaceous) includes an episode of significant environmental changes, which are well defined by a positive δ
C excursion. This globally recorded excursion indicates ...important perturbations in the carbon cycle, which has tentatively been associated with a pulse in volcanic activity and the formation of the Paraná-Etendeka large igneous province (LIP). Uncertainties in existing age models preclude, however, its positive identification as a trigger of Valanginian environmental changes. Here we report that in Valanginian sediments recovered from a drill core in Wąwał (Polish Basin, Poland), and from outcrops in the Breggia Gorge (Lombardian Basin, southern Switzerland), and Orpierre and Angles (Vocontian Basin, SE France), intervals at or near the onset of the positive δ
C excursion are significantly enriched in mercury (Hg). The persistence of the Hg anomaly in Hg/TOC, Hg/phyllosilicate, and Hg/Fe ratios shows that organic-matter scavenging and/or adsorbtion onto clay minerals or hydrous iron oxides only played a limited role. Volcanic outgassing was most probably the primary source of the Hg enrichments, which demonstrate that an important magmatic pulse triggered the Valanginian environmental perturbations.
Mineralogical and geochemical analyses conducted on cores located on the active channel‐levee system of the northern Bengal Fan are used to establish changes in the weathering pattern and the ...sediment transport of the Himalayan system, and evaluate the effect of Indian summer monsoon rainfall during the Holocene. Our data indicate that during the Holocene, sediments from the northern Bengal Fan originate mainly from the G‐B river system without any significant changes in the relative contribution of these rivers. From 9.8 to around 6 ka, relatively low smectite/(illite+chlorite) ratios and relatively high K/Si* ratios indicate high physical denudation rates of the Himalayan highlands together with a rapid transfer of the detrital material to the Bengal Fan. The period between 9.2 and 7 ka is associated to lower values of K/Si* and corresponds to the maximum of Indian monsoon rainfall which indicates a more important chemical weathering material that rapidly transits by the G‐B river system without a long storage in the Indo‐Gangetic plain. From 6.0 ka to present day, higher smectite/(illite+chlorite) ratio and lower K/Si* ratio document a gradual increase of sediments originated from the Indo‐Gangetic plain, characterized by higher degree of chemical weathering. During the last 2.5 ka, the drastic increase in the smectite/(illite+chlorite) ratio could be associated to enhanced alteration of the plain soils due to anthropogenic activity. The comparison of mineralogical and geochemical data with previous reconstructions of the Indian monsoon dynamic indicates a rapid response of erosion and sediment transfer of the G‐B river system to changes of monsoon rainfall intensity.
Key Points
Sedimentary sources and weathering state of detrital material derived from the G‐B river system over the last ∼9.8 ka
Increasing input of detrital material from the highlands (reliefs) during the early‐middle Holocene
Rapid response of erosion and sediment transfer of the G‐B river system to changes in the dynamic of the summer monsoon rainfall
While numerous studies have highlighted the central role of Southern Ocean (SO) dynamics in modulating rapid increases in atmospheric CO2 concentrations during deglaciations, fewer studies have yet ...focused on the impact of the Biological Carbon Pump - and more specifically the Carbonate Counter Pump (CCP) - in contributing to increase the CO2 concentration in oceanic surface waters and thus, in the atmosphere. Here, we present micropaleontological (coccolith, planktonic foraminifera) and geochemical (CaCO3, CaXRF, δ13CN. pachyderma) constraints from sediment core MD04-2718 retrieved in the Polar Front Zone of the Indian Ocean covering the time interval spanning Marine Isotope Stage (MIS) 12 to MIS 10 (440,000–360,000 years). We compare our results with published records from the SO to reconstruct past changes in CCP and upwelling dynamics and understand their leverage on the ocean-atmosphere portioning of CO2. We demonstrate that the sharp increase in atmospheric pCO2 during Termination V was likely associated with enhanced deep-water ventilation in the SO, that promoted the release of previously sequestered CO2 to the ocean surface as the westerly wind belt and the frontal system migrated southwards. Enhanced CCP is observed later, during MIS 11, and is likely the consequence of higher sea surface temperature and higher nutrient availability due to the reinvigoration of SO upwelling leading to increased coccolith (and to a lesser degree, planktonic foraminifera) production and export. The low eccentricity signal recorded during MIS 11 might have additionally strengthened the CCP, exerting a specific control on Gephyrocapsa morphotypes. In addition to the strong global biological productivity and higher carbon storage on land, these synergistic mechanisms may have permitted to shape the distinctive 30 ka-long pCO2 plateau characteristic of MIS 11.
•Multiproxy approach to reconstruct Carbonate Counter Pump strength and upwelling dynamics.•Increase in pCO2 during Termination V coincides with enhanced upwelling.•Strong Carbonate Counter Pump during MIS 11 in the Indian Southern Ocean.•Enhanced upwelling and Carbonate Counter Pump helped shape the pCO2 plateau.
High-resolution clay mineral and δ18Ocarb analyses have been performed on three sections of Valanginian age (Early Cretaceous), from northwestern (~20–30°N) and southern (~53°S) Tethyan realms. The ...data have been integrated in a large set of published mineralogical (clays), and geochemical (δ18Ocarb, δ18Obivalve,δ18Olenticulina, δ18Obelemnite, and TEX86) data from 17 sections, situated between ~45°N and ~53°S. Based on this data set, we provide new insights into Valanginian climate conditions from a wide range of palaeolatitudes. We highlight climate trends within specific areas and identify main climate belts. In the Proto-North Atlantic realm (~15–17°N) large amounts of smectite together with the occurrence of palygorskite testify for a semi-arid climate belt throughout the Valanginian. Significant variations in kaolinite content in the northwestern Tethyan realm (~20 to 30°N) suggest that this realm has been more sensitive to changes in hydrological cycling and subsequent erosion and runoff processes during the early to early–late Valanginian transition interval. This time interval, that is coincident with the pronounced positive and globally recognized carbon isotope excursion (CIE; the Weissert episode), documents the wet conditions of a humid subtropical climate belt. Temperature ranges derived from δ18O records suggest climate warming by about 3–5±0.9°C in the northern hemisphere, during the Weissert episode. This is followed by a global cooling of about ca. 1–5±0.9°C in both hemispheres, during the late Valanginian.
We postulate that these patterns were due to interplays between tectonic and orbital factors, which have controlled the distribution of regional palaeoclimate belts during the Valanginian. The semi-arid belt expressed in the Proto North Atlantic realm has probably been induced by its specific palaeogeographic configuration and the existence of isolated basins, which were each characterized by strong and continuous thermohaline circulation. An eccentricity paced monsoon-like system might have played a significant role in maintaining the subtropical belt around the northwestern Tethyan realm, up to the mid latitudes (~35–45°N). Superimposed on these regional processes, a widespread magmatic pulse in Paraná-Etendeka volcanic activity might have stimulated greenhouse conditions, impacting marine biota and favouring the development of a dense vegetation cover on the continent. An increase in sedimentary burial of 13C depleted organic carbon on the continents may have contributed to the progressive global positive carbon isotope excursion. In the medium term, it has also probably led to a significant decrease in atmospheric CO2, as testified by the global cooling recorded during the late Valanginian in both hemispheres and the probable concomitant extension of polar ice caps.
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