Simone Moretti

The Early Eocene Climatic Optimum (53 to 49 million years ago) was an interval in Earth’s history when reconstructed temperatures and atmospheric CO ₂ concentrations were among the highest of the Cenozoic Era (that is, the last 66 million years). The causes of the high CO ₂ levels remain unclear. CO ₂ changes over recent ice age cycles appear to be largely controlled by the biologically mediated build-up and release of carbon from the Southern Ocean, a process that might have also played a role earlier in the Cenozoic. Here we test this possibility with foraminifera-bound nitrogen isotope measurements. Our data show higher surface nitrate concentrations in the Southern Ocean, and thus a more inefficient biological sequestration of carbon during peak Early Eocene Climatic Optimum. The associated transfer of CO ₂ from the deep ocean to the atmosphere would have been amplified by the low seawater buffering capacity of the Eocene, due to the lower pH and high Ca ion concentration. Previously proposed low silicate rock weatherability during the Eocene may have weakened the weathering thermostat, allowing the reconstructed change in biological pump efficiency in the Southern Ocean to explain at least half of the atmospheric CO ₂ increase observed during the Early Eocene Climatic Optimum.

Coral resilience is shaped by trophodynamic flexibility – the balance between photosymbiont-derived energy and feeding-based heterotrophy – yet quantifying this balance across taxa and through time remains difficult. Nitrogen (N) isotopes are a powerful tool to investigate trophic strategies, but their use requires information on local isotopic baselines. Here, we introduce the biomineral-bound N isotopes of crustose coralline algae (CCA) as an archive that closely tracks the N isotopic composition of the reef nitrate supply, making it a proxy for a reef’s isotopic baseline. Coupled N isotope measurements of co-occurring CCA, symbiont-bearing, and symbiont-barren corals further enable us to quantify a coral’s “trophic enrichment factor,” reflecting the efficiency of the internal N recycling between the coral and its photosymbionts. From this framework, we derive a Reliance on Symbionts Index (RSI) that captures taxonomic and regional variation in mixotrophy, enabling reconstruction of coral trophodynamics in modern and fossil reef systems.

The Early Eocene Climatic Optimum (EECO) was an interval in Earth’s history when reconstructed temperatures and atmospheric CO2 concentrations were among the highest of the Cenozoic Era (i.e., the last 66 million years). The causes of the high CO2 levels remain unclear. The Southern Ocean, a major “leak” in the global ocean’s biological carbon pump that has been implicated in the CO2 changes over recent ice age cycles, may also have played a role earlier in the Cenozoic. Here, we report foraminifera-bound δ15N measurements that show higher surface nitrate concentrations in the Southern Ocean during peak EECO. The associated venting of CO2 from the deep ocean to the atmosphere would have been amplified by the low seawater pH buffering capacity of the Eocene. Previously proposed low silicate rock weatherability during the Eocene may have weakened the “weathering thermostat”, allowing the reconstructed change in biological pump efficiency in the Southern Ocean to explain at least half of the atmospheric CO2 increase observed during the EECO.

The marine nitrogen (N) cycle plays a central role in regulating ocean productivity, ecosystem structure, and biogeochemical feedbacks on climate. However, our understanding of historical changes in the marine N cycle is limited by the short time span and spotty spatial coverage of direct observations. This study explores the use of crustose coralline algae (CCA) as a novel proxy for reconstructing past changes in the marine N cycle. We use the biomineral-bound nitrogen isotopic composition of tropical CCA (δ ¹⁵ N _CCA ) and compare it to the δ¹⁵N of subsurface nitrate (δ ¹⁵ N _Nitrate ) and corals (δ ¹⁵ N _coral ). Our results show that CCA reliably track subsurface nitrate δ¹⁵N and, therefore, can be used to reconstruct past changes in the N cycle. Additionally, anchoring δ ¹⁵ N _coral values to co-occurring CCA allows us to isolate the reliance on photosymbionts in coral species, revealing taxonomic and regional variation in trophic strategies. Therefore, δ ¹⁵ N _CCA also provides a tool to investigate coral trophodynamics in the modern ocean as well as in the historical record.

The Antarctic Zone has long been suspected to play a crucial role in the glacial-interglacial changes in atmospheric concentration of CO2. However, global climate has many possible influences on Antarctic Zone conditions, with the potential for interactions between ice, winds and circulation that, in turn, influence the biogeochemistry and carbon budget of the Antarctic Zone surface. In a sediment core from the polar Antarctic Zone, we analyzed diatom-bound nitrogen isotopes to reconstruct surface nitrate concentration, which reflects the balance between biological productivity and the flux of subsurface nitrate into the Antarctic surface. The record covers the last 150 kyr, which includes two peak glacial periods and the subsequent deglaciations and interglacials. During each glacial period, the data support prior interpretations of lower surface nitrate concentrations and reduced circulation-driven nitrate supply to the Antarctic surface, although the change appears to be weaker at this polar Antarctic Zone site than in records from further north in the open Antarctic Zone. Early in each deglaciation, there is a further decline in surface nitrate concentration, reflecting a rise in density stratification. This is followed by an increase in nutrient supply in each of the two interglacials, signaling more vigorous surface-subsurface exchange than during the glacials or the early deglaciations. Combining the data with other Antarctic records further from the continent, the deglacial changes echo model simulations of ongoing global warming, in which upwelling increases near the Polar Front, while subsurface influx to the surface closer to the Antarctic continent decreases in response to ice sheet melting. The findings have implications for the cause of the observed rise in atmospheric CO2 concentrations during deglaciations and also warrant consideration with regard to the future of the ocean’s uptake of global warming heat and fossil fuel-derived CO2.

Primary productivity is a critical parameter of marine ecosystems, yet in many coastal areas, it has been significantly altered by human activities. The Northern Adriatic Sea (NAS), a shallow epicontinental sea bordered by the Italian and Balkan peninsulas, exemplifies this phenomenon. In the 20th century, eutrophication caused by substantial fertilizer use, industrial discharge, and high riverine input led to frequent algal blooms, bottom hypoxia, and mucilage events. Over the past three decades, however, environmental regulations and declining river discharge have reduced nutrient input, leading to decreased eutrophication.These shifts in primary productivity have profoundly impacted marine communities. Understanding how communities respond to such changes is essential as climate change and anthropogenic pressures continue to shape the NAS. Fortunately, the NAS provides historical analogs due to marked fluctuations in freshwater, sediment, and nutrient input during the Holocene.This study employs nitrogen stable isotope values (δ15N) in shell-bound organic matter of bivalves as a proxy for past primary productivity. δ15N is fractionated by primary producers and reflects nutrient dynamics within an ecosystem. As low-level consumers, bivalves offer δ15N values indicative of the food web base, providing a more stable proxy than primary producers, which are highly sensitive to short-term environmental fluctuations. The robust (sub)fossil record of bivalves allows correlations between changes in primary productivity and community turnovers over time.Our research focuses on Varicorbula gibba, an infaunal filter feeder abundant in the NAS throughout the Holocene and increasingly dominant during 20th-century eutrophication due to its opportunistic nature. The first step of this study involves calibrating δ15N values from live bivalves against water samples collected across a productivity gradient in the NAS. This calibration will assess how well shell-bound δ15N reflects variations in primary productivity along an onshore-offshore gradient.δ15N analysis is conducted using the denitrifier method, wherein nitrogen species from bivalve and water samples are oxidized, bacterially transformed into N₂O, and analyzed via mass spectrometry. Understanding how δ15N of V. gibba relates to its environment enables us to extend this analysis to (sub)fossil specimens, reconstructing Holocene primary productivity changes and their ecological impacts.By providing a historical baseline, this study offers valuable insights into the NAS's past ecosystem dynamics and serves as an analog for predicting future changes under ongoing environmental pressures.

A growing body of observations has revealed rapid changes in both the total inventory and the distribution of marine oxygen over the latter half of the 20th century, leading to increased interest in extending oxygenation records into the past. The use of paleo-oxygen proxies has the potential to extend the spatial and temporal range of current records, constrain pre-anthropogenic baselines, provide datasets necessary to test climate models under different boundary conditions, and ultimately understand how ocean oxygenation responds beyond decadal-scale changes. This review seeks to summarize the current state of knowledge about proxies for reconstructing Cenozoic marine oxygen: sedimentary features, sedimentary redox-sensitive trace elements and isotopes, biomarkers, nitrogen isotopes, foraminiferal trace elements, foraminiferal assemblages, foraminiferal morphometrics, and benthic foraminiferal carbon isotope gradients. Taking stock of each proxy reveals some common limitations as the majority of proxies functions best at low-oxygen concentrations, and many reflect multiple environmental drivers. We also highlight recent breakthroughs in geochemistry and proxy approaches to constraining pelagic (in addition to benthic) oxygenation that are rapidly advancing the field. In light of both the emergence of new proxies and the persistent multiple driver problem, the need for multi-proxy approaches and data storage and sharing that adhere to the principles of findability, accessibility, interoperability, and reusability (FAIR) is emphasized. Continued refinements of proxy approaches and both proxy–proxy and proxy–model comparisons are likely to support the growing needs of both oceanographers and paleoceanographers interested in paleo-oxygenation records.

The continuous expansion of oxygen-deficient zones (ODZs) poses risks to marine ecosystems and societies dependent on fisheries for income and sustenance. However, the trajectory of this deoxygenation in response to 21st-century climate change remains uncertain. To gain a clearer understanding of future oxygen dynamics and processes leading to deoxygenation, we investigate the response of ODZs during Cenozoic global warming periods in the Miocene and Early Eocene, using a combination of oxygen-sensitive proxies including foraminifer-bound nitrogen isotopes (FB-δ15N) and iodine-to-calcium ratios in planktic foraminifer shells (I/Ca). Our findings reveal contracted, rather than expanded, tropical Pacific ODZs during all studied warm intervals. The increased oxygenation closely aligns with high-latitude warming and reduced meridional sea surface temperature gradients, indicating a climatic driver behind these observed changes. We discuss potential causes for the contraction of ODZs, including (i) diminished wind-driven equatorial upwelling and primary productivity, and/or (ii) increased deep-ocean ventilation. Finally, we compare the behaviour of Pacific vs. Indian Ocean ODZs during the Miocene and investigate potential teleconnections between these two wind-driven ODZs.

Ocean’s oxygen (O2) is essential to most marine life forms and represent a fundamental component of the biogeochemical cycling of nitrogen and carbon. Its inventory is declining in response to global warming. Contrasting predictions about the future of the tropical oxygen deficient zones (ODZs) in numerical simulations and palaeoceanographic evidence for contracted ODZs during Cenozoic’s warmest periods, make long-term predictions about the future of ocean O2 challenging. We present new evidence for tropical ocean oxygenation during the Paleocene-Eocene Thermal Maximum (PETM), a rapid warming event that serves as a geologic analogue to ongoing warming. Foraminifera-bound nitrogen isotopes indicate that the tropical North Pacific ODZ contracted during the PETM, implying higher O2. Metabolic modelling of aquatic ectotherms shows that a concomitant increase in planktic foraminifera size implies that seawater oxygen partial pressure (pO2) rose in the shallow subsurface throughout the tropical North Pacific, beyond the spatial extent of the ODZs. These findings call for an oceanographic mechanism capable of both enhancing subsurface oxygenation and operating beyond the regional scale of the North Pacific ODZs, on millennial timescales. These divergent changes are consistent with Ocean General Circulation Models under SSP5-8.5 scenario for 2300, in which a decline in biological productivity allows tropical subsurface oxygen to rise even as global ocean oxygen declines. The tropical upper ocean oxygen increase may have relieved physiological stress, helping to avoid a mass extinction in planktic organisms during the PETM, in spite of the largest benthic extinction of the Cenozoic.

The ability of stony corals to thrive in the oligotrophic (low-nutrient, low-productivity) surface waters of the tropical ocean is commonly attributed to their symbiotic relationship with photosynthetic dinoflagellates^1,2. The evolutionary history of this symbiosis might clarify its organismal and environmental roles³, but its prevalence through time, and across taxa, morphologies and oceanic settings, is currently unclear^4–6. Here we report measurements of the nitrogen isotope (¹⁵N/¹⁴N) ratio of coral-bound organic matter (CB-δ¹⁵N) in samples from Mid-Devonian reefs (Givetian, around 385 million years ago), which represent a constraint on the evolution of coral photosymbiosis. Colonial tabulate and fasciculate (dendroid) rugose corals have low CB-δ¹⁵N values (2.51 ± 0.97‰) in comparison with co-occurring solitary and (pseudo)colonial (cerioid or phaceloid) rugose corals (5.52 ± 1.63‰). The average of the isotopic difference per deposit (3.01 ± 0.58‰) is statistically indistinguishable from that observed between modern symbiont-barren and symbiont-bearing corals (3.38 ± 1.05‰). On the basis of this evidence, we infer that Mid-Devonian tabulate and some fasciculate (dendroid) rugose corals hosted active photosymbionts, while solitary and some (pseudo)colonial (cerioid or phaceloid) rugose corals did not. The low CB-δ¹⁵N values of the Devonian tabulate and fasciculate rugose corals relative to the modern range suggest that Mid-Devonian reefs formed in biogeochemical regimes analogous to the modern oligotrophic subtropical gyres. Widespread oligotrophy during the Devonian may have promoted coral photosymbiosis, the occurrence of which may explain why Devonian reefs were the most productive reef ecosystems of the Phanerozoic.