Richard D. Norris

Shark populations on coral reefs have declined globally, but few baselines exist to quantify natural variability before human impact. Using fossil dermal denticles preserved in reef sediments, we reconstructed shark communities across the Isthmus of Panama before substantial exploitation [~7 to 3 thousand years ago (ka)] and recently (past century). We found differences in shark baselines and responses to fishing between the oceans on either side of the Isthmus. Reefs in the Pacific supported an ~20-fold higher shark abundance than Caribbean reefs before human impact. Caribbean shark populations—particularly pelagic species—have declined by 75% since the mid-Holocene, whereas their Pacific counterparts persisted over millennia despite higher fishing pressure. These contrasting trajectories are consistent with differences in oceanic productivity that dictate regional carrying capacities, highlighting the importance of incorporating oceanographic context into marine conservation targets.

The Indian‐Atlantic Ocean Gateway modulates interocean exchange and influences large‐scale ocean circulation, including the Atlantic Meridional Overturning Circulation. Site U1479 (Cape Basin; IODP Expedition 361) lies at the Benguela‐Agulhas transition and records interactions among the Indian, Atlantic, and Southern Ocean basins during the Mid‐Pleistocene Transition (MPT). We use diatom abundance, ecological groupings, and sediment geochemistry to evaluate the roles of sea‐level change, frontal migrations, and water‐mass variability on diatom deposition in the Cape Basin over the MPT. Diatom and geochemical records reveal four diatom phases (DP1‐4) in biogenic silica deposition across the MPT. DP1 (1050–917 ka) shows glacial‐interglacial cyclicity, with productivity linked to frontal positions. DP2 (917–740 ka) exhibits sustained diatom productivity during a broader interval of circulation and nutrient reorganization in the early MPT, likely driven by upwelling of Antarctic Intermediate Water and Subantarctic Mode Water via an expanded Benguela system. Principal Component Analysis indicates that productivity gradients and sea‐level–modulated coastal proximity are the primary controls on assemblage variability. DP3 (760–620 ka) records declining productivity as fronts migrated southward. DP4 (<620 ka) is characterized by sediments devoid of diatoms, reflecting silica retention south of the Polar Front and increased influence of the Agulhas Current. Elemental ratios record terrigenous inputs, hydroclimate variability, and shifts between calcareous and siliceous production. Glacial lowstands exposed the Agulhas Bank, reducing Agulhas influence and enhancing Benguela upwelling. Together, sea‐level changes, frontal migrations, and current reorganizations regulated the delivery of Southern Ocean‐sourced nutrients and silica availability, thereby controlling diatom production and burial across the MPT.

Mismatches between ecological time and geologic time complicate our ability to effectively compare contemporary ecosystem changes with those of the past. We amassed sub-decadal paleoecological data from the California Current System (CCS) to examine how benthic ecosystem structure evolved over the past 34,000 years in the Santa Barbara Basin across the glacial-to-interglacial transition. Our results indicate that even deep-sea ecosystems of the anthropocene sensu lato – the past few centuries characterized by outsized human impacts – are distinct compared to any point in the last 34,000 years. Novel ecosystem states and heightened variability began in the early 1800s AD, suggesting that colonial-era human land-use drove the emergence of a novel ecosystem state more than a century before global climate warming began in earnest.

The Indian-Atlantic Ocean Gateway significantly modulates thermohaline circulation and impacts the Atlantic Meridional Overturning Circulation (AMOC). Site U1479 in the Cape Basin, recovered during International Ocean Discovery Program Expedition 361, lies at the transition between the Benguela and Agulhas currents, recording interactions between the Indian, Atlantic, and Southern oceans during major climate transitions. We use absolute diatom abundance (ADA), diatom assemblage groups, and high-resolution biogenic silica and sediment geochemistry to evaluate sea-level change, ocean frontal migrations, and water-mass variability in the southeastern Cape Basin from 1,050 to 440 ka. Diatom reconstructions reveal a three-step shift in abundance, assemblage, and preservation across the Mid-Pleistocene Transition (MPT). From Marine Isotope Stage 23 to 12, the record captures evolving southern sourced water transport and upwelling, alongside a southward expansion of the Benguela Upwelling Zone. The data are consistent with paleoclimate evidence for glacial equatorward displacements of Southern Ocean fronts by roughly 5 to 7° latitude during the MPT, altering Agulhas Leakage, with significant implications for the AMOC. Elemental geochemistry, together with ADA and assemblages, also suggests periods when the exposure of the Agulhas Bank likely influenced interactions between the Agulhas and Benguela current systems, enhancing productivity in the southern Cape Basin. Overall, these records highlight a coupled system in which sea level, frontal position, and ocean current dynamics jointly control nutrient delivery and ecosystem structure. Siliceous microfossil evidence of changes in circulation and primary productivity in the Cape Basin significantly refines the timing and mechanisms of Southern Ocean reorganization during the MPT.

The cycling of calcium carbonate (CaCO ₃ ) in the ocean is closely linked to seawater alkalinity and the regulation of atmospheric CO ₂ . In the modern pelagic ocean, almost all CaCO ₃ is produced by three groups of calcifying planktonic organisms: coccolithophores, foraminifers, and shelled pteropods. In this Review, we examine the differences in functional traits that define each group’s distinctive role in the global carbon cycle and their sensitivity to climate change and ocean acidification. This synthesis reveals that a single representation of CaCO ₃ in climate models is unlikely to accurately reflect system dynamics or their impacts on biogeochemical cycling under climate change. We argue that understanding past and future CaCO ₃ cycle requires a better delineation of the traits that make up the diversity of calcifying plankton groups.

Mesopelagic fish are integral to ocean food webs and play an important role in carbon transport through their vertical migration behavior. Ocean deoxygenation caused by anthropogenic warming is expected to pose severe threats to mesopelagic fauna by enhancing physical stress and changing predator-prey relationships. In agreement with this expectation, our fish otolith record in a Mediterranean sediment core shows near absence of mesopelagic species during Sapropel deposition between ~7 and ~10 thousand years ago, concurrent with high surface productivity and low oxygenation of mid-depth waters. Instead, the otolith record is dominated by fish species adapted to epipelagic habitats, including European anchovy (Engraulis encrasicolus) and silvery lightfish (Maurolicus muelleri). Subsequent reoxygenation starting ~7 thousand years ago is accompanied by a three-fold increase in total otolith abundance. The large majority of these are mesopelagic lanternfish (Myctophidae) that dominate the otolith assemblage from the middle-Holocene to the present. Our findings corroborate expectations that future expansion of midwater deoxygenation could severely deplete mesopelagic fish communities over the coming centuries, with major impacts on marine fisheries, marine conservation, ocean food web structure, carbon storage and other marine ecosystem services.

Long-term records that span the past several centuries and capture within-population variation are critical for distinguishing ephemeral ecosystem changes from regime shifts. Using an approximately 2 kyr record of reproductive life history from the central Santa Barbara Basin, we examined population trends in reproductive mode and accumulation rate (i.e. reproductive output) across four species in the biserial benthic foraminiferan genus Bolivina. Bolivina populations were consistently dominated by asexually produced individuals until the mid-nineteenth century, after which they exhibit an increase in variance and a decrease in the mean proportion of asexually produced individuals. At the same time, they underwent an order-of-magnitude decline in accumulation rate. The magnitude and persistence of these changes suggest that the nineteenth and twentieth centuries represent a life-history regime shift. The compounding effects of anthropogenic impacts and long-term trends in the California Current System (such as heightened deoxygenation and altered sedimentation regimes) may have pushed the Santa Barbara Basin towards increased investment in sexual reproduction.

The Paleocene‐Eocene Thermal Maximum (PETM, ∼56 million years ago) is among the best‐studied climatic warming events in Earth history and is often compared to projected anthropogenic climate change. The PETM is characterized by a rapid negative carbon isotope excursion and global temperature increase of 4–5°C, accompanied by changes in spatial patterns of evaporation and precipitation in the global hydrologic cycle. Recent climate model reconstructions suggest a regionally complex and non‐linear response of one important aspect of global hydrology: enhanced moisture flux from the low‐latitude ocean. In this study, we use the elemental and stable isotope geochemistry of surface‐dwelling planktic foraminifera from a low‐latitude Atlantic deep‐sea sedimentary record (IODP Site 1258) to quantify changes in sea‐surface temperature (SST) and salinity. Foraminiferal Mg/Ca and δ¹⁸O values are interpreted with a Bayesian forward proxy system model to reconstruct how SST and salinity changed over the PETM at this site. These temperature and salinity reconstructions are then compared to recent climate model simulations of Eocene warming. Our reconstructions indicate °C of warming, in excellent agreement with estimates from other tropical locations and modeled PETM warmth. The regional change in salinity is not as straightforward, demonstrating a slight decrease at extreme pCO₂ forcing (a reversal of the modeled sense of change under moderate pCO₂ forcing) in both model and proxy reconstructions. The cause of this non‐linear response is unclear but may relate to increased South American continental runoff or shifts in the Inter‐Tropical Convergence Zone.

Multiple abrupt warming events (“hyperthermals”) punctuated the Early Eocene and were associated with deep-sea temperature increases of 2 to 4 °C, seafloor carbonate dissolution, and negative carbon isotope (δ ¹³ C) excursions. Whether hyperthermals were associated with changes in the global ocean overturning circulation is important for understanding their driving mechanisms and feedbacks and for gaining insight into the circulation’s sensitivity to climatic warming. Here, we present high-resolution benthic foraminiferal stable isotope records (δ ¹³ C and δ ¹⁸ O) throughout the Early Eocene Climate Optimum (~53.26 to 49.14 Ma) from the deep equatorial and North Atlantic. Combined with existing records from the South Atlantic and Pacific, these indicate consistently amplified δ ¹³ C excursion sizes during hyperthermals in the deep equatorial Atlantic. We compare these observations with results from an intermediate complexity Earth system model to demonstrate that this spatial pattern of δ ¹³ C excursion size is a predictable consequence of global warming-induced changes in ocean overturning circulation. In our model, transient warming drives the weakening of Southern Ocean-sourced overturning circulation, strengthens Atlantic meridional water mass aging gradients, and amplifies the magnitude of negative δ ¹³ C excursions in the equatorial to North Atlantic. Based on model-data consistency, we conclude that Eocene hyperthermals coincided with repeated weakening of the global overturning circulation. Not accounting for ocean circulation impacts on δ ¹³ C excursions will lead to incorrect estimates of the magnitude of carbon release driving hyperthermals. Our finding of weakening overturning in response to past transient climatic warming is consistent with predictions of declining Atlantic Ocean overturning strength in our warm future.