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Paleoclimate proxy records from regions sensitive to humidity/aridity extremes provide crucial insights into the natural forcing mechanisms underlying long‐term climate variability in broader regions. One such area is Northwest Australia, where the Australian monsoon impacts its northernmost fringes, which are bordered by the Great Sandy Desert inland. Marine sediments from the Australian Northwest Shelf record fluvial run‐off and eolian dust input during the wet and dry seasons, respectively. The location is therefore ideal for investigating long‐term variability in the Australian monsoon and Northwest Australian dust flux over orbital timescales. However, there are few continuous, high‐resolution paleoclimate records from this region spanning the early Pleistocene, when strong ice‐climate feedbacks of the late Pleistocene did not yet dominate global climate. Here, we present geochemical and environmental magnetic proxy records that reveal %CaCO3 and dust‐flux variability between 2.9 and 1.6 million years (Myr) ago from International Ocean Discovery Program Expedition 356 Site U1464 on the Australian Northwest Shelf. We establish a new, orbitally‐tuned chronology for Site U1464, primarily based on ∼400 thousand year (kyr) eccentricity cyclicity in %CaCO3, and observe strong obliquity variability (41 and 54 kyr periodicities) but almost no precession signal in the dust‐flux records. We propose that the 41 kyr cycle in Northwest Australian dust flux could be a linear response to East Asian winter monsoon intensity and/or the summer inter‐tropical insolation gradient (SITIG), whereas the 54 kyr cyclicity might be a non‐linear response to obliquity amplitude modulation via the SITIG effect on the cross‐equatorial atmospheric circulations. Key Points: This study establishes a new orbitally‐constrained chronology for International Ocean Discovery Program Site U1464Australian Northwest shelf sediments preserve ∼400 kyr eccentricity cycles in %CaCO3 and ∼41 kyr obliquity cycles in dust proxy recordsObliquity in Northwest Australian dust flux may be related to the East Asian winter monsoon and summer inter‐tropical insolation gradient [ABSTRACT FROM AUTHOR]

ABSTRACT Planktonic 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 (<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. [ABSTRACT FROM AUTHOR]

Global ice volume (sea level) and deep‐sea temperature are key measures of Earth's climatic state. We synthesize evidence for multi‐centennial to millennial ice‐volume and deep‐sea temperature variations over the past 40 million years, which encompass the early glaciation of Antarctica at ∼34 million years ago (Ma), the end of the Middle Miocene Climate Optimum, and the descent into bipolar glaciation from ∼3.4 Ma. We compare different sea‐level and deep‐water temperature reconstructions to build a resource for validating long‐term numerical model‐based approaches. We present: (a) a new template synthesis of ice‐volume and deep‐sea temperature variations for the past 5.3 million years; (b) an extended template for the interval between 5.3 and 40 Ma; and (c) a discussion of uncertainties and limitations. We highlight key issues associated with glacial state changes in the geological record from 40 Ma to present that require attention in further research. These include offsets between calibration‐sensitive versus thermodynamically guided deep‐sea paleothermometry proxy measurements; a conundrum related to the magnitudes of sea‐level and deep‐sea temperature change at the Eocene‐Oligocene transition at 34 Ma; a discrepancy in deep‐sea temperature levels during the Middle Miocene; and a hitherto unquantified non‐linear reduction of glacial deep‐sea temperatures through the past 3.4 million years toward a near‐freezing deep‐sea temperature asymptote, while sea level stepped down in a more uniform manner. Uncertainties in proxy‐based reconstructions hinder further distinction of "reality" among reconstructions. It seems more promising to further narrow this using three‐dimensional ice‐sheet models with realistic ice‐climate‐ocean‐topography‐lithosphere coupling, as computational capacities improve. Plain Language Summary: Global ice volume (hence, sea level) and deep‐sea temperature are important measures of Earth's climatic state. To better understand Earth's climate cycles in response to its orbitally driven insolation cycles, we evaluate and synthesize evidence for ice‐volume (sea‐level) and deep‐sea temperature variations at multi‐centennial to millennial resolution throughout the last 40 million years. These last 40 million years encompass the major build‐up of Antarctic glaciation from about 34 million years ago, and development of extensive Northern Hemisphere ice sheets from about 3.4 million years ago. We present a new template synthesis of ice‐volume (sea‐level) and deep‐sea temperature for the past 5.3 million years, with extension through the interval between 5.3 and 40 Ma with wider uncertainties. We also highlight a number of remaining questions about major climate transitions, including the early glaciation history of Antarctica, the end of the so‐called Middle Miocene Climate Optimum from about ∼14.5 Ma, and the descent over the past several million years into conditions with extensive ice‐age maxima in both hemispheres. Key Points: New synthesis of sea level and deep‐sea temperature over 40 million years, at millennial resolution, across seven different methodsDiscrepancies between Eocene‐Oligocene and Middle Miocene sea‐level reconstructions are highlighted for focussed future researchPlio‐Pleistocene glacial deep‐sea temperature asymptoted to a freezing limit by ∼1.25–0.9 Ma; glacial sea‐level minima decreased ∼linearly [ABSTRACT FROM AUTHOR]

Periodic and marked redox changes in eastern Mediterranean marine sediments drive environmental and diagenetic changes to which magnetic minerals are sensitive. Magnetic property changes, therefore, provide useful indications of paleoceanographic conditions during and after periods of organic‐rich sediment (sapropel) deposition. Magnetic properties of eastern Mediterranean sediments at room temperature have been studied for decades; however, few studies have considered low‐temperature magnetic properties. Here, we investigate the low‐temperature (10–300 K) magnetic properties of different eastern Mediterranean sediment types combined with room temperature (∼300 K) magnetic properties, transmission electron microscopy, and calibrated X‐ray fluorescence elemental data to illustrate the valuable information that can be obtained from low‐temperature magnetic analysis of sediments. Our low‐temperature magnetic results suggest that magnetite magnetofossils and superparamagnetic (SP) particles occur widely in eastern Mediterranean sediments. SP particle contents are highest in diagenetically reduced intervals associated with sapropels. In contrast, magnetite magnetofossils are most abundant in oxidation fronts at the tops of sapropels, where strong redox gradients formed, but are also widespread throughout other sedimentary intervals that have not been subjected to extensive reductive diagenesis. Moreover, the surfaces of magnetite particles are maghemitized (i.e., partially oxidized) in oxidation fronts at the tops of sapropels, and in other oxic sediment intervals. Our results demonstrate the value of LT magnetic measurements for quantifying diverse sedimentary magnetic signals of interest in environmental magnetism when studying paleoceanographic and paleoenvironmental processes. Plain Language Summary: Magnetic minerals are sensitive to environmental changes. Investigating their variations through sedimentary sequences can provide information about ancient environmental and climatic changes. Room temperature magnetic properties are widely measured in environmental magnetism, while low‐temperature (LT) magnetic properties that are measured between room temperature and ∼10 K are studied much less, even though they can provide useful information about the presence of particular magnetic minerals and their particle size distributions. We present extensive LT analyses of Mediterranean marine sediments, together with room temperature magnetic, transmission electron microscope, and calibrated X‐ray fluorescence elemental analyses to discuss redox changes of magnetic minerals within organic‐rich sediment intervals to quantify these diverse magnetic particle types. Our LT results suggest that the fossilized remains of magnetotactic bacteria and tiny superparamagnetic particles occur widely in eastern Mediterranean sediments, and that the surfaces of magnetite particles are widely oxidized and provide a measure of sedimentary oxidation variations. This work demonstrates that LT magnetic measurements can be used to quantify diverse sedimentary magnetic signals, and will be widely applicable in paleoenvironmental magnetic research. Key Points: Low‐temperature magnetic properties of eastern Mediterranean sediments enable quantification of magnetic particle typesMagnetofossils and superparamagnetic particles exist widely in eastern Mediterranean sedimentsMaghemitization is widespread in oxic sediment intervals and provides a measure of oxidation variations [ABSTRACT FROM AUTHOR]

Eolian mineral dust is an active agent in the global climate system. It affects planetary albedo and can influence marine biological productivity and ocean‐atmosphere carbon dynamics. This makes the understanding of the global dust cycle crucial for constraining the dust/climate relationship, which requires long‐term dust emission records for all major dust sources. Despite their importance, the sources of atmospheric dust deposited in the Southern Ocean remain poorly constrained. Eolian dust in the Pacific sector of the Southern Ocean is generally assumed to originate from Australia, with minor contributions from New Zealand. Here, we present a high‐resolution elemental record of terrestrial inputs for the past ∼410 kyr from marine sediment core PS75/100‐4 recovered from east of South Island, New Zealand. Sediment grain size is slightly finer than that of loess deposits from South Island, New Zealand, and is coarser than that of marine sediments in the Tasman Sea to the west of New Zealand, which indicates that the dust originated mainly from New Zealand and not only from Australia. Core PS75/100‐4 records lithogenic mass accumulation rates ranging from ∼0.01 to 0.69 g/cm2/kyr (∼0.20 g/cm2/kyr average), with variations over a factor of ∼3–4 over glacial versus interglacial timescales for the past 410 kyr. Our geochemical data correlate well with the Southern Ocean and Antarctic eolian dust records and suggest a westerly wind‐supplied dust signal from New Zealand. Our findings, therefore, suggest that New Zealand should be considered an important long‐term regional dust source in global dust cycle models. Key Points: We present an SW Pacific Ocean lithogenic flux record for the last 410 ka that covaries with the Antarctic and sub‐Antarctic dust recordsWesterly wind‐transported material from New Zealand is the most likely dust source over glacial‐interglacial cyclesSilt‐sized sediment was well sorted by bottom currents, which were more energetic in interglacials than glacials [ABSTRACT FROM AUTHOR]

Magnetic parameters are used extensively to interpret magnetic mineral assemblage variations in environmental studies. Conventional room temperature measurements of bulk magnetic parameters, like the anhysteretic remanent magnetization (ARM) and the ratio of the susceptibility of ARM to magnetic susceptibility (χ), can reflect, respectively, magnetic mineral concentration and/or particle size variations in sediments, although they are not necessarily well suited for identifying magnetic components within individual magnetic mineral assemblages. More advanced techniques, such as first‐order reversal curve (FORC) diagrams and low‐temperature (LT) magnetic measurements, can enable detailed discrimination of magnetic assemblages. Here, we integrate conventional bulk magnetic measurements alongside FORC diagrams, LT measurements, and X‐ray fluorescence core‐scan data, transmission electron microscope observations, and principal component analysis of FORC diagrams to identify and quantify magnetic mineral assemblages in eastern Mediterranean sediments. The studied sediments were selected because they contain complexly varying mixtures of detrital, biogenic, and diagenetically altered magnetic mineral assemblages that were deposited under varying oxic (organic‐poor marls) to anoxic (organic‐rich sapropels) conditions. Conventional bulk magnetic parameters provide continuous records of environmental magnetic variations, while more time‐consuming LT and FORC measurements on selected samples provide direct ground‐truthing of mineral magnetic assemblages that enables calculation of magnetization contributions of different end members. Thus, a combination of conventional bulk parameters and advanced magnetic techniques can provide detailed records from which the meaning of environmental magnetic signals can be unlocked. Key Points: We present an assessment of bulk and component‐specific magnetic measurements for sediments deposited under variable redox conditionsBulk magnetic parameters provide valuable delineation of continuous environmental variationsComponent‐specific techniques enable quantification of individual magnetic mineral assemblages to understand environmental variations [ABSTRACT FROM AUTHOR]

Organic‐rich sapropel layers punctuate the eastern Mediterranean sedimentary sequence, recording deep‐sea anoxic events. The timing of sapropel deposition coincides with precession minima, which are associated with the northward migration of the monsoon rain belt over North Africa. The resultant increase in monsoon precipitation over the Sahara caused an increase in low‐δ18O freshwater runoff into eastern Mediterranean surface waters, which is reflected by negative δ18O anomalies in the records of planktic foraminiferal calcite. However, despite extensive research on sapropels, the magnitude of monsoon intensification and freshwater runoff, along with its influence on δ18O, remains elusive. Here, we present a quantification of African monsoon freshwater runoff into the eastern Mediterranean for the period of deposition of last interglacial sapropel S5 (~128.3–121.5 ka). Our method uses a box model of the Mediterranean Sea, which represents different water masses, and has been calibrated using δ18O from planktic foraminiferal species of different depth and seasonal habitats. The model was constrained with existing records of sea level and sea surface temperature then inverted to deconvolve the δ18O signal of the surface‐dwelling foraminiferal species Globigerinoides ruber (w) and calculate the freshwater runoff volume. Our calculated African monsoon runoff suggests large increases in freshwater discharge to the eastern Mediterranean (up to ~8.8 times the modern pre‐Aswan Nile discharge). Rapid onset of S5 deposition following the estimated increase in runoff strongly suggests a preconditioning of the eastern Mediterranean for sapropel deposition. Our study also provides insight into the stratification and warming of eastern Mediterranean surface waters during the S5 interval. Key Points: We present a quantitative time series estimate of North African monsoon intensification for the interval of sapropel S5 depositionSea level and sea surface temperature records used to deconvolve eastern Mediterranean planktic foraminiferal stable oxygen isotope signalPreconditioning of the eastern Mediterranean for sapropel deposition suggested by rapid onset of S5 following increase in African runoff [ABSTRACT FROM AUTHOR]