International Ocean Discovery Program Site Research Articles

New Insights Into the Surface‐Ocean Dynamics of the Northeastern Atlantic Ocean Across the Marine Isotope Stage 7

AbstractThe upper water‐column dynamics and surface productivity variability in the Northeastern Atlantic Ocean across the Marine Isotope Stage (MIS) 7 interglacial complex is not well understood. Here, we present high‐resolution planktic foraminiferal proxies combined with Artificial Neural Network based‐sea‐surface temperature (SST) and ice‐rafted detritus records from International Ocean Discovery Program Site U1385, SW Iberian Margin for the intervals representing MIS 8 deglaciation, MIS 7 interglacial complex and MIS 6 glacial inception. The long‐term SST pattern is modulated by insolation and precession parameters across the MIS 7 interglacial complex and is superimposed by the millennial‐scale variability (stadials at ∼250, ∼243, ∼230, ∼221, ∼203, ∼196 and ∼192 ka). The regional SST records indicate high temperature gradient (∼6°C) between the sub‐polar North Atlantic Ocean and the SW Iberian Margin during MIS 7d which enhanced the moisture transport from mid‐to‐high latitudes. Further, low obliquity with low insolation induced cooling at high latitudes and promoted the expansion of ice‐sheets during MIS 7d. Comparison of our faunal proxies with the published marine and terrestrial records from SW Europe and western Mediterranean Sea suggested a weakening and southward shift of Azores High (AH) pressure system, similar to the present‐day (−) NAO‐like atmospheric configuration during the early phases of MIS 7e, MIS 7c and MIS 7a, resulting reduced surface productivity of SW Iberian Margin. During the late phases of MIS 7e, MIS 7c and MIS 7a, the AH pressure system strengthened and shifted northward causing the intensification of the trade winds, a scenario similar to present‐day (+) NAO‐like atmospheric configuration, which resulted in high surface productivity of SW Iberian Margin.

Variations in weathering conditions related to sea level changes during the Albian-Santonian interval in the Western Australian margin as evidenced by clay minerals and Nd/Sr isotopes

The Albian-Santonian interval (∼113–83 Ma) is a key interval for Cretaceous climatic and environmental changes. This interval is associated to a rise in sea level until the Late Cenomanian (∼ 94 Ma) and to a progressive increase in oceanic temperatures, leading to consider the Late Cenomanian-Santonian interval (∼ 94–83 Ma) as the warmest period of the last 200 Myr. While the Albian-Santonian interval has been well studied in the Northern Hemisphere, the climatic and environmental variations and their consequences on weathering conditions are less documented of the Southern Hemisphere, especially in mid and high latitudes. To better understanding the evolution of weathering conditions, associated to continental climate and sea level changes during this key period, an integrated study, based on a coupled mineralogical and geochemical approach, was carried out on the clay-sized (< 2 μm) fraction from sediments of International Ocean Discovery Program Site U1513 (Mentelle Basin, South-western Australia) and Oceanic Drilling Program Site 763 (Carnarvon Basin, North-western Australia). To determinate variations in weathering conditions, the mineral assemblages of the clay fraction was determined by X-ray diffraction and observed for selected samples by electron microscopy. To identify sources of clay minerals, the concentrations of major and selected trace elements, including rare earth elements, together with strontium and neodymium isotopic measurements were performed on the clay-sized fraction. The X-ray diffraction analyses reveal that clay fraction is dominated by R0-type illite/smectite mixed-layers (smectites), followed by variable proportions of illites, kaolinites for both sites and only on Site 763 of palygorskites. These clay minerals are associated with opal and clinoptilolites. Electron microscopy observations have highlighted the dominance of detrital fleecy smectites but also the occurrence of recrystallized lathed smectites and volcanogenic folded smectites for Site U1513, and authigenesis of clay minerals, associated to palygorskites especially on Turonian to Santonian deposits for Site 763. This authigenesis, confirmed by negative cerium anomaly, is also carried by other minerals in the clay-sized fraction (e.g., clinoptilolites, barite). These different authigenic minerals record a seawater-derived isotopic signature. Therefore, isotopic compositions of the clay-sized fraction reflect a mix between a continental contribution, carried by detrital clay minerals and marine one, supported by authigenic minerals, which complicates the identification of sediment sources. However, despite this, our study demonstrates the major influence of sea level variations on mineralogical variations from Albian to Cenomanian in Western margin of Australia. The decrease in kaolinite proportions at Site U1513 from the Albian to the end of the Cenomanian, is probably related to the increase of sea level, which cause a decrease in detrital inputs in the Mentelle Basin associated with a decrease of weathering and drainage conditions. The progressive sea level rise has also enhanced the effect of the differential settling process of clay minerals by the deposition of coarse clays (e.g., kaolinites and illites) on the continental shelf. Kaolinite-bearing clay deposits, derived from the weathering of the Western Australian Craton during the Albian-Cenomanian times, have been progressively substituted from the Turonian by a carbonate sedimentation, for which the terrigenous component consists only of rare detrital and volcanogenic smectites. In Site 763, the occurrence in kaolinite content during the Coniacian would indicate a decrease of differential settling process related to the weathering of the north part of the Western Australia Craton.

Diagenetic dolomite in planktonic foraminifera on the Australian Northwest Shelf

ABSTRACTPlanktonic foraminiferal shells are widely used to investigate past oceanographic and climatic variations via their trace elements and stable isotopes. However, these geochemical methods may be compromised by the presence of diagenetic high‐Mg calcite. In this study, dolomite crystals are observed in planktonic foraminifera from International Ocean Discovery Program Site U1464 sediments on the Australian Northwest shelf, a shallow marine environment (&lt;300 m). This study investigates the formation of the dolomite using new scanning electron microscopy, X‐ray diffraction, stable isotope (carbon and oxygen), Mg/Ca and rare earth element data, combined with shipboard geochemistry and available literature. This study finds that foraminiferal dolomite formation was probably associated with early diagenesis, microbial and ageing processes. Two potential diagenetic processes (seafloor diagenesis and post‐depositional diagenesis) are proposed, and both indicate that the favourable redox condition of dolomite formation is low oxygen, which may activate methanogenic catalysis to facilitate dolomite formation. It is postulated that high‐Mg calcite may be the precursor of foraminiferal dolomite in this case, and that dissolution–precipitation may be the ageing process of dolomite formation. In contrast to deeper, more normal pelagic environments, this study shows that the marine environment can facilitate dolomite precipitation by higher fluxes of organic matter to the seafloor.

North Atlantic Temperature Change Across the Eocene‐Oligocene Transition From Clumped Isotopes

AbstractThe Eocene‐Oligocene transition (EOT) (∼34 Ma) is marked by the rapid development of a semi‐permanent Antarctic ice‐sheet, as indicated by ice‐rafted debris and a 1–1.5‰ increase in deep sea δ18O. Proxy reconstructions indicate a drop in atmospheric CO2 and global cooling. How these changes affected surface ocean temperatures in the North Atlantic and ocean water stratification remains poorly constrained. In this study, we apply clumped‐isotope thermometry to well‐preserved planktonic foraminifera, that are associated with lower mixed‐layer to subthermocline dwelling depths from the drift sediments at international ocean discovery program Site 1411, Newfoundland, across four intervals bracketing the EOT. The thermocline/lower mixed‐layer dwelling foraminifera record a cooling of 1.9 ± 3.5 K (mean ± 95% CI) across the EOT. While the cooling amplitude is similar to previous sea surface temperature (SST) reconstructions, absolute temperatures (Eocene 20.0 ± 2.9°C, Oligocene 18.0 ± 2.2°C) appear colder than previous organic proxy reconstructions for the northernmost Atlantic extrapolated to this location. We discuss seasonal bias, recording depth, and appropriate consideration of paleolatitudes, all of which complicate the comparison between SST reconstructions and model output. Our subthermocline dwelling foraminifera record a larger cooling across the EOT (Eocene 19.0 ± 3.5°C, Oligocene 13.0 ± 3.2°C, cooling of 5.5 ± 4.6 K) than foraminifera from the thermocline/lower mixed‐layer, consistent with global cooling and an increase in ocean stratification which may be related to the onset or intensification of the Atlantic meridional overturning circulation.

Late Miocene to Early Pliocene benthic foraminifera from the Tasman Sea (International Ocean Discovery Program Site U1506)

Abstract. Modern and fossil benthic foraminifera have been widely documented from New Zealand, but detailed studies of material collected from drilling expeditions in the Tasman Sea are scarcer. This study aims to provide an updated taxonomic study for the Late Miocene–Early Pliocene benthic foraminifera in the Tasman Sea, with a specific focus on the paleoceanographic phenomenon known as the Biogenic Bloom. To achieve these goals, we analysed 66 samples from International Ocean Discovery Program (IODP) Site U1506 located in the Tasman Sea and identified a total of 98 taxa. Benthic foraminifera exhibit good preservation, allowing for accurate taxonomic identification. The resulting dataset serves as a reliable and precise framework for the identification and classification of the common deep-water benthic foraminifera in the region. The paleobathymetric analysis based on depth-dependent species indicates deposition at lower bathyal depths. Additionally, the quantitative analysis of the benthic foraminiferal assemblages allowed us to explore their response to the Biogenic Bloom at Site U1506. The paleoenvironmental analysis, focused on the Early Pliocene part of the Biogenic Bloom, points to high-productivity conditions driven by phytoplankton blooms and intensified vertical mixing of the ocean waters. These results provide valuable insights into the paleoceanographic events in the Tasman Sea, particularly the Biogenic Bloom, highlighting the significance of benthic foraminifera as reliable proxies for deciphering paleoenvironmental conditions. The taxonomic identifications and paleoenvironmental interpretations presented herein will aid in future paleoceanographic studies and facilitate comparisons with other deep-sea regions.

Evaluation of geomagnetic relative palaeointensity as a chronostratigraphic tool in the Southern Ocean: Refined Plio-/Pleistocene chronology of IODP Site U1533 (Amundsen Sea, West Antarctica)

International Ocean Discovery Program (IODP) Expedition 379 to the Amundsen Sea margin of West Antarctica recovered drill cores at two sites spanning the Latest Miocene–Holocene interval with the aim of reconstructing past West Antarctic Ice Sheet dynamics. The recovered Plio-/Pleistocene sediment sequences offer an opportunity to apply and test different dating approaches in an Antarctic deep-sea drift setting, where the records are nearly continuous and unaffected by scouring of icebergs or grounded ice. Here, through palaeomagnetic analysis of continuous u-channel samples and application of X-ray fluorescence (XRF) scanning, we revise the IODP Exp. 379 Site U1533 age model for the uppermost Pliocene and Pleistocene composite interval (0.0–2.9 Ma). We first refine the magnetostratigraphic age model with high-resolution u-channel analysis and interpreted directional data. Consistent with shipboard results, all major geomagnetic polarity chrons and subchrons are identified in the Pleistocene section. The new high-resolution u-channel dataset also allows us to identify a geomagnetic polarity excursion at ∼884 ka (interpreted as the Kamikatsura excursion) and another excursion at ∼2734 ka with confidence (potentially the Porcupine excursion). Based on the improved polarity stratigraphy, we then develop two new highly resolved age models for Site U1533 using: (i) barium enrichment cycles identified in XRF scanning data, and (ii) geomagnetic relative palaeointensity (RPI). In our first age model, we correlate cyclic variations in sedimentary barium enrichment, inferred to represent changes in export productivity, to glacial‒interglacial cycles of the Lisiecki and Raymo (2005) benthic foraminiferal oxygen isotope (δ18O) stack (LR04). Nearly all Pleistocene Marine Isotope Stages (MIS) are interpreted to be present in the barium enrichment record of Site U1533, assuming simultaneous changes in Antarctic sea-ice extent/local export productivity and global oxygen isotope stratigraphy. We then construct the second, independent age model using the Plio-/Pleistocene RPI record developed for Site U1533, which represents the longest (nearly) continuous RPI record currently available for the Antarctic margin. Comparison of the two, independently derived age models shows a variable offset, on average ± 12 kyr, with the RPI-based ages consistently older than the barium-based ages in the interval from 1.9 to 2.9 Ma and then consistently younger from 0.0 to 1.9 Ma. We interpret these offsets to result from a combination of lock-in depth effects in the younger interval (due to the relatively low sedimentation rates at this site, ∼2 cm/kyr), temporal offsets between global δ18O changes in the deep ocean and productivity response on the Antarctic margin, and/or systematic miscorrelation in the construction of the two age models. Finally, we construct a hybrid age model for the Pleistocene section of Site U1533 by combining a mixture of RPI- and barium-based age tie points that are deemed to be robust. The Site U1533 RPI record is then used, together with other Southern Ocean RPI records, to construct an Antarctic RPI stack (designated as ‘ANT-1600’) for the interval 0.0–1.6 Ma. Although sedimentation rates at two-thirds of the sites selected for the stack are lower than 10 cm/kyr, the new ANT-1600 stack is strongly coherent with the SINT-2000 RPI stack (Valet et al., 2005) on time scales of ∼20–200 kyr, allowing for its use as a regional RPI reference curve in future studies. Overall, we demonstrate that RPI at Antarctic margin/Southern Ocean sites provides a viable and valuable independent dating method for application to Plio-/Pleistocene Antarctic sediments.

Biochronology and evolution of Pulleniatina (planktonic foraminifera)

Abstract. Pulleniatina is an extant genus of planktonic foraminifera that evolved in the late Miocene. The bottom and top occurrences of its six constituent morphospecies (P. primalis, P. praespectablis, P. spectabilis, P. praecursor, P. obliquiloculata, P. finalis) provide a series of more or less useful constraints for correlating tropical and subtropical deep-sea deposits, as do some prominent changes in its dominant coiling direction and a substantial gap in its record in the Atlantic Ocean. Biostratigraphic information about these events has accumulated over many decades since the development of systematic deep-sea drilling in the 1960s, during which time the geochronological framework has evolved substantially, as have taxonomic concepts. Here we present new data on the biochronology of Pulleniatina from International Ocean Discovery Program Site U1488, which has a record of its entire evolutionary history from the centre of its geographic range in the Western Pacific Warm Pool. We then present and compare revised calibrations of 183 published Pulleniatina bioevents worldwide, with stated sampling errors as far as they are known, using a consistent methodology and in the context of an updated evolutionary model for the genus. We comment on the reliability of the various bioevents; their likely level of diachrony; and the processes of evolution, dispersal, and extinction that produced them.

Global Cooling‐Driven Summer Monsoon Weakening in South China Across the Eocene‐Oligocene Transition

AbstractRecent research has suggested that East Asia has experienced a prevailing monsoon climate since the Eocene. However, there is little knowledge about the development of the East Asian monsoon system before the Miocene, particularly in southern China, due to a lack of well‐dated continuous sediment records. Here, we present new magnetic proxy records from International Ocean Discovery Program Site U1501 in the northern South China Sea. We conducted rock magnetic experiments, made scanning electron microscopy observations and performed diffuse reflectance spectrum analysis on the late Eocene‐early Oligocene core sediments. The magnetic signal of the sediments is dominated by detrital magnetite and titanomagnetite formed during the silicate weathering and erosion processes, which were used to infer the evolution of summer monsoon precipitation in southern China. Our results along with geochemical and clay mineral data from Site U1501 strongly indicate that the East Asian summer monsoon generally weakened across the Eocene‐Oligocene transition. This change was linked to coeval global cooling rather than tectonic processes. Prior to the Eocene‐Oligocene transition, the summer monsoon intensified, likely due to the latest Eocene warming event caused by the enhancement of the Atlantic Meridional Overturning Circulation, which may have preconditioned the Earth system for the greenhouse‐to‐icehouse transition.

Coring tools have an effect on lithification and physical properties of marine carbonate sediments

Abstract. The International Ocean Discovery Program (IODP) JOIDES Resolution Science Operator typically uses an advanced piston corer (APC) in soft ooze and sediments and an extended core barrel (XCB) in firm sediments. The coring tool exchange typically occurs around the same depth in adjacent holes of the same site. However, during IODP Expedition 356, the coring tool switch occurred at different depths: IODP Sites U1463 and U1464 are marked by a stratigraphic interval (&gt; 25 m thick) that was XCB cored in one hole and APC cored in other holes. Shipboard scientists remarked that APC-cored sediments were unlithified or partially lithified, while XCB-cored sediments were fully lithified. This difference in sedimentological description of the same formation seems to be an effect of coring technique. To provide further insight, we assessed the physical properties (bulk density, porosity, and P-wave velocity), downhole wireline logging data, scanning electron microscope (SEM) images, and micro-computed tomography (µCT) scans of those intervals. We find systematic differences between the different coring techniques. XCB cores are characterized by systematically lower bulk density, higher porosity, and higher P-wave velocity than APC cores. Downhole logging data suggest that the original P-wave velocity of the formation is better preserved in XCB cores, despite the typical “biscuit-and-gravy” core disturbance (i.e. well-preserved core fragments surrounded by squelched core material). In conjunction with SEM and µCT images, we conclude that the APC tool destroyed early lithification by breaking cements between individual grains. Moreover, µCT images reveal denser packing and smaller pore volumes in the APC cores. These sedimentary changes likely occur when the APC pressure wave passes through the sediment. The destruction of grain-to-grain cements provides an explanation for the significantly lower P-wave velocities in APC cores. Interestingly, the gravy sections in XCB drilled cores mimic the destruction of early lithification and reduction of pore volume. We conclude that APC remains the tool of choice for recovering soft sediments, especially for paleoclimate purposes. However, for the study of lithification, XCB biscuits provide a more representative image of the formation. For the study of early diagenesis, further studies are required to ascertain the preservation of key sedimentary features using existing and new drilling tools.

Rapid reorganization of the Pearl River network driven by spreading of the South China Sea at around 32 Ma

Provenance analyses on sediments from depositional sinks have provided valuable information about the birth of the Pearl River, a key event in the landscape history of South China. However, uncertainty remains due to the difficulty in clearly identifying the source regions by geochemical or geochronological approaches, giving rise to disputes on the driving mechanisms underlying river network evolution. Here we combined pollen and grain-size analyses of terrigenous sediments from International Ocean Discovery Program site U1501 to further constrain the timing and pattern of the Pearl River network evolution during the late Eocene and Oligocene. The results reveal a sharp, synchronous transition at ∼32 Ma in both proxies. The sudden occurrence of the rainforest coniferous pollen Dacrydium and increased abundances of the alpine coniferous pollen Abies/Picea indicate a new mountainous source region stretching across the tropical-subtropical ecotone after 31.5 Ma. The simultaneous input of a distinct coarse fraction of siliciclastic components implied that the terrigenous sediments were transported by a large, regional-scale river, in addition to the preceding small rivers draining local uplifts proximal to the site. The formation of a proto-Pearl River originating from the Cathaysian mountains and flowing through the rift basins of the continental margin is thus proposed. Moreover, when integrated with co-located calcareous nannofossil records indicative of a synchronous deepening process of the sea basin, our results link this early river network reorganization event to the spreading of the South China Sea rather than to the uplift of the Tibetan Plateau.

Upper Cretaceous (Turonian–Santonian) Haplophragmoides from IODP site U1512, Great Australian Bight

Results from the International Ocean Discovery Program (IODP) Site U1512 in the Great Australian Bight offer insights into climate immediately after the peak of the Cretaceous hothouse. Lower Turonian to Santonian deposits yield a unique high-resolution micropaleontological record of the Australo-Antarctic Gulf that is dominated by agglutinated foraminifera. The complex paleoenvironment at Site U1512 illustrates a constantly changing marine setting characterized by the interplay of runoff from the vast Ceduna River system to the north and varying degrees of influence of the Indian Ocean.A common element in the agglutinated foraminiferal assemblage is the genus Haplophragmoides, which represents between 20 and 40% of the total benthic foraminiferal assemblage, and its occurrence seems particularly common in intervals with higher terrestrial influx. Like many agglutinated foraminiferal assemblages, the agglutinated taxa at Site U1512 suffer from preservational issues. In addition to effects during early diagenesis, like the degradation of organic cement, the delicate planispiral taxa especially experienced extensive deformation, which can affect the correct identification of taxonomically relevant features. Among the six species of Haplophragmoides that represent most of this genus at Site U1512, we identify and describe three new species: H. petaliformis n.sp., H. tenellulus n.sp., and H. antarcticus n.sp.

Petrogenesis of high-alumina basalts: Implications for magmatic processes associated with the opening of the South China Sea

Results of International Ocean Discovery Program (IODP) Expeditions on the northern continental margin of the South China Sea (SCS) have nullified the early notion that the SCS is a magma-poor margin. However, there are continuing debates on how the rapid transition from continent to ocean is supported by geochemical and petrologic data, and whether such a process was related to plate subduction or mantle plume activities. Here we present the results of bulk-rock and plagioclase phenocryst geochemistry of the basalts generated during the early spreading stage (initial and steady ocean stages) of SCS basin extension, which were drilled at IODP Sites U1500 and U1503, respectively. We combined these findings with published data from the late spreading stage of the SCS basin, which provides an excellent opportunity to examine the magmatic processes associated with the evolution of the SCS basin. Our results revealed that the SCS basalts generally exhibit higher Al contents than global Mid-Ocean Ridge Basalts, particularly at lower MgO contents, which is similar to the characteristics of modern arc basalts. Nevertheless, their origins are site-specific and complex. For instance, some of the Site U1503 basalts originated through fractional crystallization of water-rich parental magmas with the lowest degree of melting, while the basalts from the other sites, including Site U1500, were primarily generated by the accumulation of plagioclase. More specifically, the water-rich magmas from Site U1503 displayed strong subduction signals in terms of trace elements, and their correlations with Al2O3 content suggest that they are the product of partial melting of the mantle wedge, closely related to the northward subduction of proto-SCS. This subduction had a significant effect on triggering the opening of the SCS, rather than a mantle plume. Moreover, the Site U1500 basalts exhibit a significant increase in plagioclase An values with the increase of host magma Al contents, indicating that the plagioclase floatation mechanism cannot account for the origin of their plagioclase accumulation. Therefore, we propose that the high abundance of plagioclase in Site U1500 basalts requires rapid ascent of magma, supporting a rapid rifting and strong magmatism during the initial opening of the SCS.

Globigerinoides rublobatus – a new species of Pleistocene planktonic foraminifera

Abstract. We describe Globigerinoides rublobatus n. sp., a new morphospecies of fossil planktonic foraminifera, from the Pleistocene sediments (∼810 ka) of the Indian Ocean and Pacific Ocean. We use image analysis and morphometry of 860 specimens from International Ocean Discovery Program Site U1483 in the tropical Indian Ocean to document morphological variability in the new morphospecies and related taxa, and we also report it from Pacific Ocean Site U1486 for the first time. The new morphospecies combines characteristics typical of Globigerinoides conglobatus (Brady, 1879) and Globigerinoides ruber (d'Orbigny, 1839), with which it co-occurs, but is distinct from both. Morphometric data indicate that G. rublobatus n. sp. is closer to G. conglobatus, potentially signalling an evolutionary affinity. We find that Globigerinoides rublobatus n. sp. occurs as two variants, a pigmented (pink) form and a non-pigmented (white) form. The non-pigmented forms are on average ∼50 % larger than the pigmented forms. This is so far only the third instance of fossil planktonic foraminifera known to exhibit this pink pigmentation. We regard the pink and white forms as variants of a single morphospecies and suggest the pink form may represent a later evolutionary adaptation.

Rapid Formation of the Ellice and Osbourn Basins and Ontong Java Nui Breakup Kinematics

AbstractBreakup of the proposed greater Ontong Java Nui large igneous province during the Cretaceous Normal Superchron can be constrained by the opening of the Ellice Basin (EB) separating the Ontong Java and Manihiki Plateaus and the Osbourn Basin separating the Manihiki and Hikurangi Plateaus. Dating of recovered dredged samples using plagioclase 40Ar/39Ar and zircon U/Pb geochronology methods indicates that spreading was well underway in the EB by 118 Ma with full spreading rates up to 3X faster than any observed today of 30–45 cm/yr and spreading likely continued until 112‐108 Ma. Ellice Basin samples show diverse geochemical affinities ranging from mid‐ocean ridge basalt (MORB) or Ontong Java‐like to more enriched OIB‐like. Pb and Nd isotopes from six samples contain varying influences from Pacific MORB and possibly Ontong Java. The geochemistry shows a lack of a clear mantle plume influence despite EB's close temporal and spatial relationship to Ontong Java, while some data resemble the Louisville Seamounts. This compositional diversity complements morphological differences among dredge sites and shows that both in situ MORB and younger overprinted features related to the nearby Tuvalu Seamounts were sampled. 40Ar/39Ar geochronology confirms the age of International Ocean Discovery Program Site U1365 near the Osbourn Trough (OT) to be 102.60 ± 0.26 Ma (2σ, n = 18). This age constrains the timing of a spreading reorientation event observed in the OB to coincide with a global plate reorganization event around 105 Ma and estimates the cessation of spreading at the OT to 96 Ma.

Obliquity-driven subtropical forcing of the thermocline after 240 ka in the southern sector of the Western Pacific Warm Pool

Changes in vertical geochemical gradients within the Western Pacific Warm Pool (WPWP) are important indicators of the region's upper ocean response to climate changes and to the oceanographic coupling between the thermocline and surface mixed layer. Here, we reconstruct temperature and δ18Osw at International Ocean Discovery Program Site U1486 (1332 m water depth, 2°22′S, 144°36′E) located in the Bismarck Sea in the southern sector of the WPWP. A 670-kyr record of Δδ18O between the surface-dwelling foraminifera Globigerinoides ruber sensu stricto and the thermocline-dwelling foraminifera Pulleniatina obliquiloculata and Globorotalia tumida (when combined with Mg/Ca-based temperature and δ18Osw estimates) suggests long-term thermocline shoaling and a progressively increasing vertical salinity gradient commencing near 240 ka. Through a detailed comparison to other Pacific records, it becomes clear this is not solely a local phenomenon, as we identify widespread cooling of the thermocline in the low-latitude Pacific after ∼240 ka. After examining our temperature reconstructions alongside new δ18Osw and constant flux proxy-derived focusing factor records, we potentially also validate previous models which find obliquity-induced strengthening of low-latitude Pacific currents. We extend this to support periods of increased transport of high-salinity thermocline water masses through the South Pacific low-latitude western boundary current system. These results indicate greater variability in thermocline circulation given amplified obliquity and strengthen previous evidence that variability in the WPWP thermocline is independent from the drivers of WPWP surface variability.

Cretaceous southern high latitude benthic foraminiferal assemblages during OAE 2 at IODP Site U1516, Mentelle Basin, Indian Ocean

At Site U1516 (Mentelle Basin, southeast Indian Ocean, offshore western Australia), the International Ocean Discovery Program (IODP) Expedition 369 recovered an almost complete pelagic record of the Upper Cretaceous, including the Oceanic Anoxic Event 2 (OAE 2). To better understand paleoenvironmental changes across OAE 2, 32 samples were analysed for benthic foraminiferal abundance data that represent one of the few benthic foraminiferal datasets spanning the OAE 2 in the southern high latitudes.The OAE 2 interval at Site U1516 is characterized by an interval of low CaCO3 content that contains a prominent positive Carbon Isotope Excursion (CIE). The record of the OAE 2 can be subdivided in pre OAE 2, pre max-CIE, low CaCO3, and post low CaCO3 intervals. Through the Cenomanian–Turonian boundary, we document an extreme decline in benthic foraminifera during OAE 2, that is followed by a profound repopulation event in the post low CaCO3 interval.Benthic foraminiferal assemblages indicate an outer neritic to upper bathyal depositional environment. During the pre OAE 2 and pre max-CIE intervals, calcareous deep-water gavelinellids, lingulogavelinellids and gyroidinids are dominant. In the low-carbonate interval, the microfossil record documents a substantial increase in radiolaria and foraminifera are almost absent as only three out of nine samples contain benthic foraminifera. Changes in benthic foraminiferal assemblage composition are documented in the initial low CaCO3 interval, underlying the maximum CIE and associated interferences. Comparison of the pre- and post-CIE benthic foraminiferal assemblages highlights a distinct repopulation event during the post max- CIE interval mainly represented by the conspicuous increase in abundance of agglutinated taxa and Conorboides. Compared to other southern high latitude records, the dataset collected at Site U1516 represents one of the most complete benthic foraminiferal records across the OAE 2 that registers the Late Cretaceous environmental changes in the Southern Hemisphere.

Late Neogene nannofossil assemblages as tracers of ocean circulation and paleoproductivity over the NW Australian shelf

Abstract. Late Miocene to Pliocene sediments from the NW Australian shelf, drilled by the International Ocean Discovery Program (IODP) Expedition 356, provide unique records of paleoclimatic variations under warmer-than-present conditions. During the period from 6 to 3.5 Ma, the area was dominated by warm, tropical waters supplied by an intensified, uninterrupted Indonesian throughflow and was characterised by prevailing humid conditions, including increased precipitation and river runoff. Despite the available information regarding the general paleoclimatic conditions, little is known about the concurrent regional ocean circulation patterns and the relative strength of seasonally flowing boundary currents, such as the Leeuwin Current. In this study, we investigate two astronomically tuned calcareous nannofossil time series from IODP Sites U1463 and U1464 to track long-term changes in ocean circulation and water column stratification, which influences the availability of nutrients in the upper photic zone and is considered to be a primary control on the (paleo)productivity of marine phytoplankton. By documenting shifts in the dominant species within the nannofossil assemblages and comparing these to paleotemperature gradients between the NW Australian shelf and the eastern Indian Ocean, we identify a significant change in the ecological and oceanographic regime that occurred across the Miocene–Pliocene boundary (5.4–5.2 Ma), which can be attributed to an overall intensification of the upper water column mixing over the shelf. Significant changes in nannofossil abundance and species composition that reflect broader-scale processes and evolutionary events, such as the termination of the late Miocene to early Pliocene biogenic bloom in the eastern Indian Ocean (4.6–4.4 Ma) and the extinction of Sphenolithus spp. (∼3.54 Ma), occurred long after this regional regime shift.

The Late Miocene‐Early Pliocene Biogenic Bloom: An Integrated Study in the Tasman Sea

AbstractThe Late Miocene‐Early Pliocene Biogenic Bloom (∼9–3.5 Ma) was a paleoceanographic phenomenon defined by anomalously high accumulations of biological components at multiple open ocean sites, especially in certain regions of the Indian, and Pacific oceans. Its temporal and spatial extent with available information leaves fundamental questions about driving forces and responses unanswered. In this work, we focus on the middle part of the Biogenic Bloom (7.4–4.5 Ma) at International Ocean Discovery Program Site U1506 in the Tasman Sea, where we provide an integrated age model based on orbital tuning of the Natural Gamma Radiation, benthic foraminiferal oxygen isotopes, and calcareous nannofossil biostratigraphy. Benthic foraminiferal assemblages suggest changes in deep water oxygen concentration and seafloor nutrient supply during generally high export productivity conditions. From 7.4 to 6.7 Ma, seafloor conditions were characterized by episodic nutrient supply, perhaps related to seasonal phytoplankton blooms. From 6.7 to 4.5 Ma, the regime shifted to a more stable interval characterized by eutrophic and dysoxic conditions. Combined with seismic data, a regional change in paleoceanography is inferred at around 6.7 Ma, from stronger and well‐oxygenated bottom currents to weaker, oxygen‐depleted bottom currents. Our results support the hypothesis that the Biogenic Bloom was a complex, multiphase phenomenon driven by changes in ocean currents, rather than a single uniform period of sustained sea surface water productivity. Highly resolved studies are thus fundamental to its understanding and the disentanglement of local, regional, and global imprints.

East Asian summer monsoon evolution recorded by the middle Miocene pelagic reddish clay, South China Sea

The East Asian summer monsoon (EASM) is essential for the livelihoods of billions of people in East and Southeast Asia. Its present features and evolution history of the Quaternary have been well studied, yet how did EASM operate in much older geological times, especially before the late Miocene, is relatively less constrained. Scientific drillings conducted in the South China Sea (SCS) offer excellent opportunities to resolve this issue to a certain extent considering the EASM has been prevailing in this area since at least the earliest Miocene. In this study, mineralogical and stratigraphic studies were carried out on the middle Miocene pelagic reddish clay recovered at International Ocean Discovery Program Site U1433 using clay mineralogical and natural gamma radiation data. Our results show that the smectite content and illite crystallinity of the deep SCS clay were direct consequences of the EASM intensity varying in precession rhythm and modulated by eccentricity cycles. A high-resolution temporal framework constructed with onboard age controls and cyclostratigraphic analyses demonstrates that the pelagic reddish clay from Site U1433 was accumulated from 17.17 to 12.69 Ma. During this period, deep-sea sedimentation of the SCS was controlled by the EASM and influenced by the sea-level change. Importantly, clay mineral is proved to be an efficient terrigenous standard of astronomical tuning and stratigraphic correlation for pelagic sediments. It is also verified that the EASM intensified during the Miocene Climatic Optimum and the following Middle Miocene Climate Transition, and induced higher humidity and seasonality, respectively. Then, the EASM gradually weakened since the expansion of the East Antarctic Ice Sheet.

Planktonic foraminifera response to orbital and millennial-scale climate variability at the southern Iberian Margin (IODP Site U1385) during Marine Isotope Stages 20 and 19

The orbital configuration of Marine Isotope Stage (MIS) 19 provides a good analogue to our present interglacial, and it is marked by millennial-scale climate variability. To evaluate how orbital and millennial-scale climate changes interact during this interglacial we have conducted a high-resolution study of planktonic foraminifera assemblages at International Ocean Discovery Program (IODP) Site U1385, SW Iberian Margin, encompassing late MIS 20 and MIS 19. Estimates of sea surface temperature (SST), using transfer-functions, are compared with geochemical, XRF elemental data, biomarker and pollen records from the same site. These results are then compared with SST and ice-rafted debris records from North Atlantic IODP Site U1314 to reconstruct basin-wide palaeoceanographic conditions. During MIS 20, the large size of the ice sheets imparted a strong cooling in the subpolar gyre whereas the Iberian Margin remained under the influence of the subtropical gyre with the development of a high meridional thermal gradient in the North Atlantic. During MIS 20 terminal stadial (794–789 ka) and MIS 19b-a stadial events, the meltwater discharges related to the instability of ice sheets affected both the North Atlantic and the Iberian Margin leading to a weaker thermal gradient. Our data indicate that Termination IX (TIX, ∼795–788 ka) was punctuated by short-term warming/cooling phases in the ocean coupled with warming/cooling events on land. A short-term cooling episode, recognized in the ocean and on land, occurred on Termination IX and preceded the beginning of MIS 19c. During MIS 19c, the planktonic foraminifera SST indicates warm and almost stable conditions in the sea decoupling the cool and dry millennial-scale events on land inferred by pollen assemblages. Starting from 780 ka towards MIS 18, the planktonic foraminifera assemblages and SSTs indicate a cooling trend along the Iberian Margin related to a progressive southward migration of the polar front.