Description: Strontium, neodymium and lead isotopic signatures were measured on the carbonate-free lithogenic fraction of a suite of terrestrial and marine sediments from the North American Southwest. We present radiogenic isotope data from 1) surface sediments from nine different playa lakes in the Mojave Desert, 2) Late Quaternary silt mantles which drape the bedrock of Anacapa and San Clemente Islands offshore California and 3) fluvially-derived and hemipelagic sedimentary horizons in Late Quaternary age marine sediments from Ocean Drilling Project sites 893 and 1015 in the California Borderland Basins. We also present a compilation of measurements of bedrock Sr, Nd and Pb isotopic composition across the North American Southwest from the published literature. These datasets allow us to identify the isotopic signature of aeolian material exported from the North American Southwest and explore the contribution of dust to the California Borderland Basins.

Description: This dataset consists of terrigenous grain-size data from Integrated Ocean Drilling Program (IODP) Sites 342-U1406 and 342-U1411 and lithogenic isotope (Hf, Nd, Pb/Pb) data from IODP Site 342-U1411. These two sites are in the Newfoundland ridges contourite drift complex and were cored during IODP Expedition 342 (Paleogene Newfoundland Drifts) in summer 2012. The grain-size data consists of 137 samples from Site 342-U1406 and 192 samples from Site 342-U1411 and was measured with a Micromeritics SediGraph 5120 particle size analyzer. The Hf and Nd data consists of 30 samples from Site 342-U1411 and were measured using a ThermoFisher Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS). The Pb/Pb data consists of 23 samples from Site 342-U1411 were also measured using a ThermoFisher Neptune MC-ICPMS. All data were generated between the years 2014 and 2019. The timespan of the data set is from ~35 Ma to 26 Ma (9 Myr total duration). Sample resolution ranges and is up to one sample per 10-15 kyr during the Eocene-Oligocene Transition (EOT; 33.65-34.44 Ma) and intervals immediately prior (35-34.44 Ma) and immediately after (33.65-32.5 Ma). Sortable silt metrics (abundance and mean) were calculated from the grain-size data and used to reconstruct bottom-current and paleocirculation history. The lower-resolution lithogenic isotope data is used to infer sediment source (provenance) changes across the EOT.

Description: Pelagic sediments from the subtropical South Atlantic Ocean contain geographically extensive Oligocene ooze and chalk layers that consist almost entirely of the calcareous nannofossil Braarudosphaera. Poor recovery and the lack of precise dating of these horizons in previous studies has limited our understanding of the exact number of acmes, their timing and durations, and the causes of their recurrence. Here we present a high-resolution, astronomically tuned stratigraphy of Braarudosphaera oozes (29.5-27.9 Ma) from Ocean Drilling Program Site 1264 in the subtropical southeastern Atlantic Ocean. We identify seven acme events in the Braarudosphaera abundance record. The longest lasting acme event corresponds to a strong minimum in the ~2.4-My eccentricity cycle, and four acme events coincide with ~110-ky and 405-ky eccentricity maxima. We propose that eccentricity-modulated precession forcing of the freshwater budget of the South Atlantic Ocean resulted in the episodic formation of a shallow pycnocline and hyperstratification of the upper water column. We speculate that stratified surface water conditions may have served as a virtual sea floor, which facilitated the widespread Braarudosphaera acmes. This explanation reconciles the contrasting distribution patterns of Braarudosphaera in the modern ocean, limited largely to shallow water coastal settings, compared to their relatively brief and expanded oceanic distribution in the past.

Description: Fine-grained magnetic particles in deep-sea sediments often statistically align with the ambient magnetic field during (and shortly after) deposition and can therefore record geomagnetic reversals. Correlation of these reversals to a geomagnetic polarity time scale is an important geochronological tool that facilitates precise stratigraphic correlation and dating of geological records globally. Sediments often carry a remanence strong enough for confident identification of polarity reversals, but in some cases a low signal-to-noise ratio prevents the construction of a reliable and robust magnetostratigraphy. Here we implement a data-filtering protocol, which can be integrated with the UPmag software package, to automatically reduce the maximum angular deviation and statistically mask noisy data and outliers deemed unsuitable for magnetostratigraphic interpretation. This protocol thus extracts a clearer signal from weakly magnetized sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 342 Site U1406 (Newfoundland margin, northwest Atlantic Ocean). The resulting magnetostratigraphy, in combination with shipboard and shore-based biostratigraphy, provides an age model for the study interval from IODP Site U1406 between Chrons C6Ar and C9n (~21–27 Ma). We identify rarely observed geomagnetic directional changes within Chrons C6Br, C7r, and C7Ar, and perhaps within Subchron C8n.1n. Our magnetostratigraphy dates three intervals of unusual stratigraphic behavior within the sediment drifts at IODP Site U1406 on the Newfoundland margin. These lithostratigraphic changes are broadly concurrent with the coldest climatic phases of the middle Oligocene to early Miocene and we hypothesize that they reflect changes in bottom water circulation.

Description: During the Mid-Pleistocene Transition (MPT; 1,200-800 kya), Earth's orbitally paced ice age cycles intensified, lengthened from ~40,000 (~40 ky) to ~100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ~43 to ~75 µatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.

Description: The origin and carbon content of the deep water mass that fills the North Atlantic Basin, either Antarctic Bottom Water (AABW) or North Atlantic Deep Water (NADW) is suggested to influence the partitioning of CO2 between the ocean and atmosphere on glacial-interglacial timescales. Fluctuations in the strength of Atlantic meridional overturning circulation (AMOC) have also been shown to play a key role in global and regional climate change on timescales from annual to millennial. The North Atlantic is an important and well-studied ocean basin but many proxy records tracing ocean circulation in this region over the last glacial cycle are challenging to interpret. Here we present new B/Ca-[CO32-] and boron isotope-pH data from sites spanning the North Atlantic Ocean from 2200 to 3900 m and covering the last 130 kyr from both sides of the Mid-Atlantic Ridge. These data allow us to explore the potential of the boron-based proxies as tracers of ocean water masses to ultimately identify the changing nature of Atlantic circulation over the last 130 kyr. This possibility arises because the B/Ca and boron isotope proxies are directly and quantitatively linked to the ocean carbonate system acting as semi-conservative tracers in the modern ocean. Yet the utility of this approach has yet to be demonstrated on glacial-interglacial timescales when various processes may alter the state of the deep ocean carbonate system. We demonstrate that the deep (∼3400 m) North Atlantic Ocean exhibits considerable variability in terms of its carbonate chemistry through the entirety of the last glacial cycle. Our new data confirm that the last interglacial marine isotope stage (MIS) 5e has a similar deep-water geometry to the Holocene, in terms of the carbonate system. In combination with benthic foraminiferal δ13C and a consideration of the [CO32-] of contemporaneous southern sourced water, we infer that AABW influences the eastern abyssal North Atlantic throughout the whole of the last glacial (MIS2 through 4) whereas, only in the coldest stages (MIS2 and MIS4) of the last glacial cycle was AABW an important contributor to our deep sites in both North Atlantic basins. Taken together, our carbonate system depth profiles reveal a pattern of changing stratification within the North Atlantic that bears strong similarities to the atmospheric CO2 record, evidencing the important role played by ocean water mass geometry and the deep ocean carbonate system in driving changes in atmospheric CO2 on these timescales.

Description: The North Atlantic Current (NAC) transports warm salty water to high northern latitudes, with important repercussions for ocean circulation and global climate. A southward displacement of the NAC and Subarctic Front, which separate subpolar and subtropical water masses, is widely suggested for the last glacial maximum (LGM) and may have acted as a positive feedback in glacial expansion at this time. However, the role of the NAC during the intensification of northern hemisphere glaciation (iNHG) ~3.5 to 2.5 Ma, is less clear. Here, we present new records from IODP Site U1313 (41°N) spanning ~2.8-2.4 Ma to trace the influence of Subarctic Front waters above this mid-latitude site. We reconstruct surface and permanent pycnocline temperatures and seawater δ18O using paired Mg/Ca-δ18O measurements on the planktic foraminifers Globigerinoides ruber and Globorotalia crassaformis, and determine abundances of the subpolar foraminifer Neogloboquadrina atlantica. We find that the first significant glacial incursions of Subarctic Front surface waters above Site U1313 did not occur until ~2.6 Ma. At no time during our study interval was (sub)surface reorganisation in the mid-latitude North Atlantic analogous to the LGM. Our findings suggest that LGM-like processes sensu stricto cannot be invoked to explain interglacial-glacial cycle amplification during iNHG. They also imply that increased glacial productivity at Site U1313 during iNHG was not only driven by southward deflections of the Subarctic Front. We suggest nutrient injection from cold-core eddies and enhanced glacial dust delivery may have played additional roles in increasing export productivity in the mid-latitude North Atlantic from 2.7 Ma.

Description: The boron isotope composition (d11B) of planktic foraminiferal calcite, which reflects seawater pH, is a well-established proxy for reconstructing palaeo-atmospheric CO2 and seawater carbonate chemistry. However, to translate d11B measurements determined in calcareous fossils into pH we need to know the boron isotope composition of the parent seawater (d11Bsw). While a number of d11Bsw reconstructions exist, the discrepancies between them reveals uncertainties and deficiencies that need to be addressed. Here we present a new d11Bsw record based on the d11B difference between planktic and benthic foraminifera and an estimate of the pH gradient between surface and deep water. We then calculate d11Bsw two different ways. One variant of our method assumes that the pH gradient between surface and deep has remained the same as today over the past 23 Ma; the other uses the d13C gradient between surface and deep to represent change in the pH gradient through time. The results of these two methods of calculating d11Bsw are broadly consistency with each other, however, based on extensive carbon cycle modelling using CYCLOPS and GENIE we favour the d13C gradient method. In our favoured d11Bsw reconstruction, d11Bsw is around 2 per mil lower than today at ~37.5 per mil during the early and middle Miocene and increases to the modern value (39.61 per mil) by ~5 Ma. A similar pattern of change is evident in the seawater composition of three other stable isotope systems, Mg, Li and Ca. Concurrent shifts in the seawater isotopic composition of all four of these elements during the late Miocene, suggest a common forcing mechanism. We hypothesise the most likely cause of these shifts is a change in the isotopic composition of the riverine input, potentially driven by an increase in secondary mineral formation since ~15 Ma.

Description: A deep-sea temperature record for the past 50 million years has been produced from the magnesium/calcium ratio (Mg/Ca) in benthic foraminiferal calcite. The record is strikingly similar in form to the corresponding benthic oxygen isotope (δ18O) record and defines an overall cooling of about 12°C in the deep oceans with four main cooling periods. Used in conjunction with the benthic δ18O record, the magnesium temperature record indicates that the first major accumulation of Antarctic ice occurred rapidly in the earliest Oligocene (34 million years ago) and was not accompanied by a decrease in deep-sea temperatures.

Description: Paired benthic foraminiferal trace metal and stable isotope records have been constructed from equatorial Pacific Ocean Drilling Program Site 1218. The records include the two largest abrupt (〈1 Myr) increases in the Cenozoic benthic oxygen isotope record: Oi‐1 in the earliest Oligocene (∼34 Ma) and Mi‐1 in the earliest Miocene (∼23 Ma). The paired Mg/Ca and oxygen isotope records are used to calculate seawater δ18O (δw). Calculated δw suggests that a large Antarctic ice sheet formed during Oi‐1 and subsequently fluctuated throughout the Oligocene on both short (〈0.5 Myr) and long (2–3 Myr) timescales, between about 50 and 100% of its maximum earliest Oligocene size. The magnitudes of these fluctuations are consistent with estimates of sea level derived from sequence stratigraphy. The transient expansion of the Antarctic ice sheet at Mi‐1 is marked in the benthic δ18O record by two positive excursions between 23.7 and 22.9 Ma, each with a duration of 200–300 kyr. Bottom water temperatures decreased by ∼2°C over the 150 kyr immediately prior to both rapid δ18O excursions. However, the onset of each of these phases of ice growth is synchronous, within the resolution of the records, with the onset of a 2°C warming over ∼150 kyr. We suggest that the warming during these glacial expansions reflect increased greenhouse forcing prompted by a sudden decrease in global chemical weathering rates as Antarctic basement silicate rocks became blanketed by an ice sheet. This represents a negative feedback process that might have operated during major abrupt growth phases of the Antarctic ice sheet.