Jerry F. McManus

The Iberian Margin provides an exceptional archive of climate variability due to its high sedimentation rates. However, uncertainties persist regarding the provenance of terrigenous detrital sediments on the Iberian Margin, and the climate‐driven changes in their source areas and transport processes. By integrating clay mineralogy with grain‐size data and end‐member modeling for IODP Site U1588 sediments, we characterize terrigenous detrital inputs to the southwestern Iberian Margin and reconstruct their orbital‐scale variations over the last ∼250 kyr. Three sources of sediment supply are identified. (I) Fine‐grained Iberian riverine inputs: rich in smectite (61%) and poor in kaolinite (7%) and chlorite (7%); (II‐A) Saharan dust: rich in illite (49%) with low kaolinite and smectite, consistent with the silt‐sized mode; (II‐B) Sahelian dust: richest in kaolinite (22%) with high smectite, reflecting additional aeolian source; and (III) Mediterranean Outflow Water‐transported particles: rich in chlorite (22%) with intermediate levels of other clay minerals, co‐varying with the sortable‐silt fraction. Our end‐member modeling results indicate North African dust as a major source for Iberian Margin sediments, as supported by spatiotemporal variations in the illite/kaolinite (I/K) ratio. Illite mineralogy further reveals a marked contrast across the glacial‐interglacial cycles. Relative to the last glacial cycle (∼130–0 ka), the penultimate cycle (∼250–130 ka) shows higher illite contents and I/K ratios, weaker chemical weathering, and pronounced precessional cyclicity. This contrast suggests a long‐term contraction of North African dust sources, expressed as reduced Saharan input during the last glacial cycle, and likely caused by a northward displacement of the Intertropical Convergence Zone.

The Quaternary Period (the past 2.58 million years) was characterized by the waxing and waning of large ice sheets in the Northern Hemisphere. Using sediment sequences from the Iberian Margin, we have demonstrated that expansion of Northern Hemisphere ice sheets around 2.7 million years ago was accompanied by the emergence of millennial climate variability (MCV) during glacial periods. The onset of MCV at ~2.7 million years ago was heralded by isolated precursor events, followed by multiple millennial climate oscillations at ~2.5 million years ago. These events coincided with deposition of ice-rafted detritus in the North Atlantic, suggesting a role for marine-terminating ice sheets. Once established, MCV became an intrinsic feature of glacial climates of the Quaternary. Our findings underscore the profound impact Northern Hemisphere glaciation had on climate variability across multiple timescales.

Data report: X-ray fluorescence core scanning and revised composite section of Pliocene–Pleistocene sediments from IODP Expedition 397 Site U1587

The flux of nutrients from continents to the oceans sustains oceanic primary productivity and is a fundamental component of the carbon cycle. In most regions of the world's oceans primary productivity is limited by the supply of nutrients. In particular, iron can become limiting in the open‐ocean due to its low solubility. Glaciated continents have been suggested as an underappreciated source of iron to the high‐latitude oceans. Yet, uncertainty remains regarding the magnitude and spatial variability of glacially derived nutrient fluxes, and the extent to which these nutrients impact open‐ocean ecosystems. To quantify lithogenic fluxes at the West Greenland margin, we measured ²³² Th and ²³⁰ Th in seawater and core‐top sediments across the shelf and slope. Our results highlight a negative correlation between low‐salinity waters and dissolved and particulate ²³² Th, suggesting a glacial source for this lithogenic isotope. We calculated dissolved ²³² Th fluxes 5–24 μg m ⁻² yr ⁻¹ (100–500 m depth), and sedimentary ²³² Th fluxes 105–711 μg m ⁻² yr ⁻¹ , higher than typical open‐ocean settings and similar to margin sites influenced by large inputs from aeolian dust and rivers. A sampling transect shows that dissolved ²³² Th fluxes increase toward Greenland, confirming that lithogenic inputs are sourced laterally from the margin. Using our ²³² Th fluxes, we estimate an elevated supply of dissolved Fe which extends over the continental slope toward the open ocean. This Fe flux is large enough to support much of the local primary productivity, highlighting the importance of lithogenic fluxes in supporting the marine ecosystem in high‐latitude oceans.

Deglaciations and glacial inceptions are the two equally important transitional periods that bridge the glacial and interglacial climate states, yet our understanding of deglaciations far exceeds that of glacial inceptions. Substantial variations in deep ocean circulation accompanied the last deglaciation, and model simulations recently suggested that a weakening of the Atlantic Meridional Overturning Circulation (AMOC) also occurred at the last glacial inception (LGI; 113-119 thousand years ago), yet evidence of such a change remains inconclusive. Here, we report three Pa/Th records from the western and central North Atlantic that display an abrupt weakening of the AMOC at the LGI. The magnitude of the reconstructed AMOC weakening approaches but never reaches the level of disruptions associated with the Heinrich ice discharge events. Our results may highlight a unique period of orbitally forced abrupt circulation changes and the importance of ocean processes in setting atmospheric CO₂ changes in motion.

Quantitative records of bottom water oxygen (BWO) are critical for understanding deep ocean change through time. Because of the stoichiometric relationship between oxygen and carbon, BWO records provide insight into the physical and biogeochemical processes that control the air‐sea partitioning of both gases with important implications for climate over Quaternary glacial‐interglacial cycles. Here, we present new geochemical data sets from Ocean Discovery Program Site 1240 in the eastern equatorial Pacific to constrain paleoproductivity (Ba _xs flux) and BWO using a multiproxy approach (aU, Mn/Al, Δδ ¹³ C, and U/Ba). This combination of approaches allows us to quantitatively identify changes in BWO and to parse local and basin‐wide contributions to the signal. We find that upwelling, not dust input, is responsible for driving productivity changes at the site. Changes in local carbon export are not the primary driver of changes in BWO, which instead reflect basin‐wide changes driven by processes in the Southern Ocean. Our BWO results provide direct evidence for the role of orbital precession and obliquity in driving deep sea respired carbon and oxygen concentrations. We find variations in BWO on the order of ∼50 μmol/kg that occur with ∼23 kyr periodicity during the substages of Marine Isotope Stage 5, and variations of ∼100 μmol/kg on glacial‐interglacial timescales. These findings have important implications for the role of insolation in driving deep ocean respired oxygen and carbon concentrations, and point to physical and biogeochemical changes in the Southern Ocean as key drivers of planetary‐scale carbon change.

Abundant proxy records suggest a profound reorganization of the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM, ~21,000 y ago), with the North Atlantic Deep Water (NADW) shoaling significantly relative to the present-day (PD) and forming Glacial North Atlantic Intermediate Water (GNAIW). However, almost all previous observational and modeling studies have focused on the zonal mean two-dimensional AMOC feature, while recent progress in the understanding of modern AMOC reveals a more complicated three-dimensional structure, with NADW penetrating from the subpolar North Atlantic to lower latitude through different pathways. Here, combining ²³¹ Pa/ ²³⁰ Th reconstructions and model simulations, we uncover a significant change in the three-dimensional structure of the glacial AMOC. Specifically, the mid-latitude eastern pathway (EP), located east of the Mid-Atlantic Ridge and transporting about half of the PD NADW from the subpolar gyre to the subtropical gyre, experienced substantial intensification during the LGM. A greater portion of the GNAIW was transported in the eastern basin during the LGM compared to NADW at the PD, resulting in opposite ²³¹ Pa/ ²³⁰ Th changes between eastern and western basins during the LGM. Furthermore, in contrast to the wind-steering mechanism of EP at PD, the intensified LGM EP was caused primarily by the rim current forced by the basin-scale open-ocean convection over the subpolar North Atlantic. Our results underscore the importance of accounting for three-dimensional oceanographic changes to achieve more accurate reconstructions of past AMOC.