By studying deep-sea drilled records from the North Atlantic Ocean, several magnetic instabilities of short duration, such as the Iceland Basin (188 ka), the Bj & ouml;rn (1,255 ka) and the Gardar (1,460 ka) excursions, were discovered. These records have contributed to our understanding of Earth's magnetic field and are the foundation of the Geomagnetic Instability Time Scale (GITS) in the Quaternary. Here, we present the magnetostratigraphy from Sites U1555 (0 to similar to 2.7 Ma) and U1563 (0 to similar to 5.2 Ma) drilled during the International Ocean Discovery Program Expedition 395C on the eastern side of the modern Mid-Atlantic Ridge (similar to 60 degrees N, 20-30 degrees W). Shipboard paleomagnetic and microfossil data provided a preliminary age model, extending the regional record to 3.4 Ma. The Virtual Geomagnetic Pole latitudes from archive halves, corroborated with data from discrete samples, were used to build a high-resolution magnetostratigraphy, which contained the expected Brunhes and Matuyama Chrons and their respective Subchrons. We also identified most of the magnetic events reported in the GITS, including the less well-documented ones, such as Osaka, Kamitzukara, Huckleberry Ridge, Reunion, Gardar, Halawa and L4 events. The high-resolution magnetostratigraphy from Sites U1555 and U1563 is compared with two previous legacy sites and contributes toward an increasingly robust GITS, expanding its use as a correlation and dating tool.

Drift sediments are characterized by a rapid accumulation rate (≥10 cm/kyr), which can provide high-resolution records of the geomagnetic field behavior. Previous ocean drilling in the North Atlantic (e.g., ODP Leg 162 and IODP Exp 306) discovered magnetic instabilities of short duration, such as the Iceland Basin Excursion (188 ka). These records have contributed to our understanding of Earth’s magnetic field variations in the geological past and the foundation of the Geomagnetic Instability Time Scale (GITS) in the Quaternary (0-2.58 Ma). Between 2020 and 2023, IODP Expeditions 384, 395C, and 395 drilled six sites: five along a transect on the eastern side of the modern Mid-Atlantic Ridge (between 20-30°W at a latitude of ~60°N) and one on the western side, off the coast of Greenland. The expedition aims to investigate the formation of V-shaped ridges and V-shaped troughs in the Reykjanes Ridge and the evolution of deepwater currents. Shipboard paleomagnetic and microfossil data provided a preliminary age model for all sites, extending the regional record to 11 Ma. During the Expeditions(s), archive half sections were subjected to alternating field stepwise demagnetization cleaning of natural remanent magnetization. Shipboard paleomagnetic data were processed to remove edge effect, core disturbance, and are integrated with data from discrete samples. All directional data were interpreted, isolating the primary magnetization through the principal component analysis. The discrete samples confirm all polarity changes recognized in the shipboard inclination data. Here, we present the preliminary results from two of the sites drilled during the Expeditions 395C, U1555 (~2.7 Ma) and U1563 (~5.2 Ma), where most of the magnetic events (including excursions and reversals) reported in the Geomagnetic Instability Time Scale of the Quaternary are recognized. These high-resolution records provide the potential to extend the GITS further in the past, expanding the potential of using magnetic instabilities as a dating tool and help to refine our understanding of how the Earth’s magnetic field changed in the past.

North Atlantic Deep Water (NADW), the return flow component of the Atlantic Meridional Overturning Circulation (AMOC), is a major inter-hemispheric ocean water mass with strong climate effects but the evolution of its source components on million-year timescales is poorly known. Today, two major NADW components that flow southward over volcanic ridges to the east and west of Iceland are associated with distinct contourite drift systems that are forming off the coast of Greenland and on the eastern flank of the Reykjanes (mid-Atlantic) Ridge. Here we provide direct records of the early history of this drift sedimentation based on cores collected during International Ocean Discovery Programme (IODP) Expeditions 395C and 395. We find rapid acceleration of drift deposition linked to the eastern component of NADW, known as Iceland-Scotland Overflow Water at 3.6 million years ago (Ma). In contrast, the Denmark Strait Overflow Water feeding the western Eirik Drift has been persistent since the Late Miocene. These observations constrain the long-term evolution of the two NADW components, revealing their contrasting independent histories and allowing their links with climatic events such as Northern Hemisphere cooling at 3.6 Ma, to be assessed.

Electron backscatter diffraction (EBSD) analysis enables a unique perspective of the internal microstructure of foraminiferal calcite. Specifically, EBSD provides crystallographic data from within the test, highlighting the highly organised “mesocrystal” structure of crystallographically aligned domains throughout the test, formed by sequential deposits of microgranular calcite. We compared EBSD maps across the test walls of both poorly preserved and well-preserved specimens of the planktonic foraminifera species Globigerinoides ruber and Morozovella crater. The EBSD maps, paired with information about intra-test distributions of Mg/Ca ratios, allowed us to examine the effects of different diagenetic processes on the foraminifera test. In poorly preserved specimens EBSD data show extensive reorganisation of the biogenic crystal microstructure, indicating differing phases of dissolution, re-precipitation and overgrowth. The specimens with the greatest degree of microstructural reorganisation also show an absence of higher concentration magnesium bands, which are typical features of well-preserved specimens. These findings provide important insights into the extent of post-depositional changes, in both microstructure and geochemical signals that must be considered when utilising foraminifera to generate proxy archive data.

The release of over 4500 Gt (gigatonnes) of carbon at the Paleocene–Eocene boundary provides the closest geological analog to modern anthropogenic CO2 emissions. The cause(s) of and responses to the resulting Paleocene–Eocene Thermal Maximum (PETM) and attendant carbon isotopic excursion (CIE) remain enigmatic and intriguing despite over 30 years of intense study. CIE records from the deep sea are generally thin due to its short duration and slow sedimentation rates, and they are truncated due to corrosive bottom waters dissolving carbonate sediments. In contrast, PETM coastal plain sections along the US mid-Atlantic margin are thick, generally having an expanded record of the CIE. Drilling here presents an opportunity to study the PETM onset to a level of detail that could transform our understanding of this important event. Previous drilling in this region provided important insights, but existing cores are either depleted or contain stratigraphic gaps. New core material is needed for well-resolved marine climate records. To plan new drilling, members of the international scientific community attended a multi-staged, hybrid scientific drilling workshop in 2022 designed to maximize not only scientifically and demographically diverse participation but also to protect participants' health and safety during the global pandemic and to reduce our carbon footprint. The resulting plan identified 10 sites for drill holes that would penetrate the Cretaceous–Paleogene (K–Pg) boundary, targeting the pre-onset excursion (POE), the CIE onset, the rapidly deposited Marlboro Clay that records a very thick CIE body, and other Eocene hyperthermals. The workshop participants developed several primary scientific objectives related to investigating the nature and the cause(s) of the CIE onset as well as the biotic effects of the PETM on the paleoshelf. Additional objectives focus on the evidence for widespread wildfires and changes in the hydrological cycle, shelf morphology, and sea level during the PETM as well as the desire to study both underlying K–Pg sediments and overlying post-Eocene records of extreme hyperthermal climate events. All objectives address our overarching research question: what was the Earth system response to a rapid carbon cycle perturbation?

Planktonic foraminifera, single-celled protists, are extensively used in biostratigraphy and biochronology thanks to their abundance in marine sediments, widespread distribution and extensive fossil record in the Cretaceous and Cenozoic. We present an overview of planktonic foraminiferal taxonomy, and evolutionary trends that are relevant to the practice and application of biostratigraphy. Additionally, we present the key features in planktonic foraminiferal paleoecology and paleogeography that characterise Cretaceous-Cenozoic species diversity and geographic distributions. The types of bioevents and biozone definitions, morphologic terminology and species identification in evolutionary lineages are illustrated through examples. Planktonic foraminifera are crucial in the development of geological timescales and their value and reliability in chronostratigraphic correlations is demonstrated by the identification of 14 primary and secondary boundary events applied in the definition of the Global Stratotype Section and Points (GSSP). The presentation of the methodologies used in biochronological studies is aimed to describe the significance of planktonic foraminifera for correlation and integration with magnetostratigraphy, chemostratigraphy and orbital tuning. Calibration difficulties such as diachroneity of species occurrences are discussed, and examples of astronomical calibrations are provided for the Late Cretaceous and Late Miocene.

Abstract. The ratio of the trace element Mg over Ca (Mg/Ca) and the oxygen isotopic composition (δ18O) of foraminiferal calcite are widely employed for reconstructing past ocean temperatures, although geochemical signals are also influenced by several other factors that vary temporally and spatially. Here, we analyze a global dataset of Mg/Ca and δ18O data of 59 middle Miocene to Holocene species of planktonic foraminifera from a wide range of depth habitats, many of which have never been analyzed before for Mg/Ca. We investigate the extent to which Mg/Ca and δ18O covary through time and space, and identify several sources of mismatch between the two proxies. Once the data are adjusted for long term non-thermal factors, Mg/Ca and δ18O are overall positively correlated in a way consistent with temperature being the dominant controller of both through space and time and across many different species, including deep-dwellers. However, we identify several species with systematic offsets in Mg/Ca values, to which multispecies calibrations should be applied with caution. We can track the appearance of such offsets through ancestor-descendent species over the last 15 million years and propose that the emergence of these offsets may be the geochemical expression of evolutionary innovations. We find virtually all of the Mg/Ca and δ18O-derived temperatures from the commonly used genera Globigerinoides and Trilobatus are within uncertainty of each other, highlighting the utility of these species for paleoceanographic reconstructions. Our results highlight the potential of leveraging information from species lineages to improve sea surface temperature reconstruction from planktonic foraminifera over the Cenozoic.

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.

The Mg/Ca ratio of foraminiferal calcite is a widely used empirical proxy for ocean temperature. Foraminiferal Mg/Ca-temperature relationships are based on extant species and are species-specific, introducing uncertainty when applying them to the fossil tests of extinct groups. Many modern species show remarkable heterogeneity in their intra-test Mg distributions, typically due to the presence of high Mg bands, which have a biological origin. Importantly, banding patterns differ between species, which could affect Mg/Ca-temperature relationships. Few studies have looked at intra-test variability in Mg/Ca ratios in extinct species of foraminifera, despite the obvious implications for paleothermometry. We used electron probe microanalysis (EPMA) to investigate intra-test Mg distributions in the fossil tests of two species of planktonic foraminifera from the extinct muricate mixed-layer-dwelling genus Morozovella, commonly used in Paleogene sea surface temperature reconstructions. Both M. aragonensis and M. crater show striking Mg banding patterns with multiple high and low Mg/Ca band pairs throughout the test wall in all chambers. The intra-test Mg variability in M. aragonensis and M. crater is similar to that in modern species widely used in paleoclimate reconstructions and banding patterns are consistent with published growth models for modern forms, albeit with subtle differences. The presence of Mg bands supports the application of Mg/Ca-palaeothermometry in extinct Morozovella species as well as the utility of EPMA for examining preservation of foraminifera tests in paleoclimatological studies. However, we emphasize the importance of rigorous assessments of inter-and intra-test Mg variability when using microanalytical techniques for foraminiferal Mg/Ca paleothermometry.

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.

The purpose of this application, under Articles 23.9.3 and 81.2.2 of the Code, is to conserve the usage of the genus-group name Trilobatus Spezzaferri, Kucera, Pearson, Wade, Rappo, Poole, Morard & Stalder, 2015 for a genus of planktonic foraminifera from the global ocean and fossil record by partial suppression of its senior objective synonym Trilobigerina Popescu, 1987. Partial suppression of the little-used senior name will allow conservation of the younger name that is in very widespread use.

Paleontological reconstructions of plankton community structure during warm periods of the Cenozoic (last 66 million years) reveal that deep-dwelling ‘twilight zone’ (200–1000 m) plankton were less abundant and diverse, and lived much closer to the surface, than in colder, more recent climates. We suggest that this is a consequence of temperature’s role in controlling the rate that sinking organic matter is broken down and metabolized by bacteria, a process that occurs faster at warmer temperatures. In a warmer ocean, a smaller fraction of organic matter reaches the ocean interior, affecting food supply and dissolved oxygen availability at depth. Using an Earth system model that has been evaluated against paleo observations, we illustrate how anthropogenic warming may impact future carbon cycling and twilight zone ecology. Our findings suggest that significant changes are already underway, and without strong emissions mitigation, widespread ecological disruption in the twilight zone is likely by 2100, with effects spanning millennia thereafter.

<p>The drawdown of CO<sub>2</sub> via the temperature-dependent weathering of silicate minerals is thought to be one of the key processes acting to maintain Earth’s climate within narrow bounds over geologic time. However, the climatic responsiveness of weathering on multi-million-year timescales is, to our knowledge, yet to be demonstrated. If other factors dominate climate regulation on geologic timsecales, previously unexplored factors may be important in driving long-term carbon cycle changes. Here, we present the first continuous Cenozoic record of the concentration of calcium in seawater ([Ca<sup>2+</sup><sub>sw</sub>]). Our record is based on the Na/Ca of exceptionally well-preserved foraminiferal calcite, a methodology which leverages the extremely long seawater Na<sup>+</sup> residence time (>40 Myr) to interpret such changes predominantly in terms of [Ca<sup>2+</sup><sub>sw</sub>] fluctuation. We show that a 12 mM decrease in [Ca<sup>2+</sup><sub>sw</sub>] occurred over the last ~50 Ma, with a close correspondence to the timing of atmospheric CO<sub>2</sub> changes, potentially implying a common driver. Using a carbon cycle box model, we demonstrate that, if the relationship between silicate weathering is shallower than commonly assumed, then this change in [Ca<sup>2+</sup><sub>sw</sub>] can mechanistically explain the majority of the Cenozoic CO<sub>2</sub> decrease, via the effect that Ca<sup>2+</sup> has on CaCO<sub>3</sub> burial rates. Given the recently identified major change in the global sea floor spreading rate, this finding shifts the key driver of long-term climate from the terrestrial to marine realm. Conversely, if there is a steep relationship between silicate weathering and climate, the climatic responsiveness of weathering is such that the system would rebalance before [Ca<sup>2+</sup><sub>sw</sub>] can drive a major CO<sub>2</sub> change. Our results therefore highlight the need to determine whether silicate weathering is responsive to climate change on geologic timescales before the long-term drivers of CO<sub>2</sub> can be determined.</p>

The fossil record of marine microplankton provides insights into the evolutionary drivers which led to the origin of modern deep-water plankton, one of the largest components of ocean biomass. We use global abundance and biogeographic data, combined with depth habitat reconstructions, to determine the environmental mechanisms behind speciation in two groups of pelagic microfossils over the past 15 Myr. We compare our microfossil datasets with water column profiles simulated in an Earth system model. We show that deep-living planktonic foraminiferal (zooplankton) and calcareous nannofossil (mixotroph phytoplankton) species were virtually absent globally during the peak of the middle Miocene warmth. The evolution of deep-dwelling planktonic foraminifera started from subpolar–mid-latitude species, during late Miocene cooling, via allopatry. Deep-dwelling species subsequently spread towards lower latitudes and further diversified via depth sympatry, establishing modern communities stratified hundreds of metres down the water column. Similarly, sub-euphotic zone specialist calcareous nannofossils become a major component of tropical and sub-tropical assemblages during the latest Miocene to early Pliocene. Our model simulations suggest that increased organic matter and oxygen availability for planktonic foraminifera, and increased nutrients and light penetration for nannoplankton, favoured the evolution of new deep-water niches. These conditions resulted from global cooling and the associated increase in the efficiency of the biological pump over the last 15 Myr.

During the Miocene (23.0–5.3 Ma) North Africa experienced both humid and arid intervals, but the underlying cause of these transitions is unknown. Earth's climate was characterized by a unipolar icehouse with a dynamic Antarctic ice sheet, which may have influenced regional hydrology through atmospheric teleconnections. However, the Miocene also witnessed the restriction of the Mesopotamian Seaway, which may have had significant climatic impacts. The Maltese il‐Blata section (Central Mediterranean) comprises Late Oligocene to Early Miocene marine deposits previously used to constrain the timing of the Mesopotamian Seaway restriction using the εNd tracer. The location of this section also makes it sensitive to climatic changes in the North African region, and biogeochemical changes in the central Mediterranean. Here, we present lithological and geochemical records of the il‐Blata section. We find a marked shift in lithology and an increase in sedimentation rate coeval with the Early Miocene (∼19–20 Ma) restriction of the Mesopotamian Seaway. Concomitant changes in bulk sediment CaCO 3 , Sr/Ca, K/Al, Ti/Al, Zr/Al, and Si/Ti support a major humid climate transition and associated intensification of river systems over western North Africa. We propose that these changes in North African hydroclimate reflect either a tipping point effect in a gradually warming global climate, or are the result of the initial restriction of the Mesopotamian Seaway, perhaps through consequent changes in Atlantic Meridional Overturning Circulation and the West African Monsoon. We also suggest the restriction of the Mesopotamian Seaway inhibited phosphorite deposition at low latitudes.

Muricate planktonic foraminifera comprise an extinct clade that was diverse and abundant in the Paleogene oceans and are widely used in palaeoclimate research as geochemical proxy carriers for the upper oceans. Their characteristic wall texture has surface projections called “muricae” formed by upward deflection and mounding of successive layers of the test wall. The group is generally considered to have lacked “true spines”: that is, acicular calcite crystals embedded in and projecting from the test surface such as occur in many modern and some Paleogene groups. Here we present evidence from polished sections, surface wall scanning electron microscope images and test dissections, showing that radially orientated crystalline spine-like structures occur in the centre of muricae in various species of Acarinina and Morozovella and projected from the test wall in life. Their morphology and placement in the wall suggest that they evolved independently of true spines. Nevertheless, they may have served a similar range of functions as spines in modern species, including aiding buoyancy and predation and especially harbouring algal photosymbionts, the function for which we suggest they probably first evolved. Our observations strengthen the analogy between Paleogene mixed-layer-dwelling planktonic foraminifera and their modern spinose counterparts.

Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long recognised as a key parameter in the production and export of organic matter at the ocean surface, its role in the ocean interior is much less frequently accounted for. There, bacteria (primarily) transform sinking particulate organic matter (POM) into its dissolved constituents and consume dissolved oxygen (and/or other electron acceptors such as sulfate). The nutrients and carbon thereby released then become available for transport back to the surface, influencing biological productivity and atmospheric pCO2, respectively. Given the substantial changes in ocean temperature occurring in the past, as well as in light of current anthropogenic warming, appropriately accounting for the role of temperature in marine carbon cycling may be critical to correctly projecting changes in ocean deoxygenation and the strength of feedbacks on atmospheric pCO2. Here we extend and calibrate a temperature-dependent representation of marine carbon cycling in the cGENIE.muffin Earth system model, intended for both past and future climate applications. In this, we combine a temperature-dependent remineralisation scheme for sinking organic matter with a biological export production scheme that also includes a dependence on ambient seawater temperature. Via a parameter ensemble, we jointly calibrate the two parameterisations by statistically contrasting model-projected fields of nutrients, oxygen, and the stable carbon isotopic signature (δ13C) of dissolved inorganic carbon in the ocean with modern observations. We additionally explore the role of temperature in the creation and recycling of dissolved organic matter (DOM) and hence its impact on global carbon cycle dynamics. We find that for the present day, the temperature-dependent version shows a fit to the data that is as good as or better than the existing tuned non-temperature-dependent version of the cGENIE.muffin. The main impact of accounting for temperature-dependent remineralisation of POM is in driving higher rates of remineralisation in warmer waters, in turn driving a more rapid return of nutrients to the surface and thereby stimulating organic matter production. As a result, more POM is exported below 80 m but on average reaches shallower depths in middle- and low-latitude warmer waters compared to the standard model. Conversely, at higher latitudes, colder water temperature reduces the rate of nutrient resupply to the surface and POM reaches greater depth on average as a result of slower subsurface rates of remineralisation. Further adding temperature-dependent DOM processes changes this overall picture only a little, with a slight weakening of export production at higher latitudes. As an illustrative application of the new model configuration and calibration, we take the example of historical warming and briefly assess the implications for global carbon cycling of accounting for a more complete set of temperature-dependent processes in the ocean. We find that between the pre-industrial era (ca. 1700) and the present (year 2010), in response to a simulated air temperature increase of 0.9 ∘C and an associated projected mean ocean warming of 0.12 ∘C (0.6 ∘C in surface waters and 0.02 ∘C in deep waters), a reduction in particulate organic carbon (POC) export at 80 m of just 0.3 % occurs (or 0.7 % including a temperature-dependent DOM response). However, due to this increased recycling nearer the surface, the efficiency of the transfer of carbon away from the surface (at 80 m) to the deep ocean (at 1040 m) is reduced by 5 %. In contrast, with no assumed temperature-dependent processes impacting production or remineralisation of either POM or DOM, global POC export at 80 m falls by 2.9 % between the pre-industrial era and the present day as a consequence of ocean stratification and reduced nutrient resupply to the surface. Our analysis suggests that increased temperature-dependent nutrient recycling in the upper ocean has offset much of the stratification-induced restriction in its physical transport.