Lower Jurassic sedimentary successions from the Neuquén Basin, Argentina are unique in the abundance of radiometrically datable material (ash-beds) present, which can be tied to bio- and chemostratigraphic (carbon-isotope) zonations. Here, we present new U-Pb radio isotopic dates, integrated with carbon-isotope and Hg/TOC data, from three localities in Argentina (Arroyo Lapa, Arroyo Serrucho/Las Overas and Chacay Melehue) to generate a biostratigraphically calibrated composite carbon-isotope curve and geochronological framework for the Pliensbachian–Toarcian transition in South America. Using a Bayesian framework we present an age-depth model for this composite record and estimate the age and duration of key intervals extending from the Latest Pliensbachian carbon isotope excursion (CIE) through the Early Toarcian negative CIE. Using a statistical analysis of all available Karoo and Ferrar Large Igneous Province (LIP ) U-Pb and Ar-Ar radioisotopic ages we create a timeline of the key events and examine the timing of the carbon cycle perturbations specifically looking at potential links to peaks of extrusive emplacement of the Karoo and Ferrar LIP. The geochronological framework is further compared with other available radioisotopic dates from correlative sections, allowing for a more precise constraint and validation of the timing and duration of these Early Jurassic events.

The Toarcian Oceanic Anoxic Event (T-OAE, similar to 183 Ma) was marked by globally recognized environmental perturbations, most notably disturbances to the global carbon cycle and climate. To date, geochemical records providing information about the T-OAE have been largely generated from the warm temperate climate zone of the NW European realm. Coeval geochemical records from the Southern Hemisphere, providing a more global perspective on palaeoenvironmental changes associated with the T-OAE, are comparatively scarce. In this study, we present a biostratigraphically calibrated litho- and chemostratigraphic record of Lower Jurassic strata from the northern Neuquen Basin, Argentina, covering the Upper Pliensbachian and Toarcian upper tenuicostatum to lower Dumortieria Andean ammonite zones, equivalent to the uppermost tenuicostatum to pseudoradiosa European standard zones. The integrated stratigraphic data re-define the stratigraphic position of the Andean tenuicostatum-D. hoelderi ammonite Zone boundary and support near-synchroneity of this horizon with the tenuicostatum - serpentinum zonal boundary in NW Europe. The stratigraphic interval recording the negative carbon-isotope excursion associated with the T-OAE appears massively expanded and organic lean in contrast to the coeval organic-rich deposits in other parts of the Neuquen Basin and in European sections. At Las Overas, persistent sedimentary organic-matter enrichment was limited to brief intervals of black-shale deposition, possibly coinciding with reduced sedimentary organic matter dilution. Depositional rates and inorganic redox proxies suggest that the development of oxygen-depleted conditions may have been disrupted by the interplay between basin subsidence, sedimentation rate, relative sea-level change, depositional setting and deep-water currents.

Astronomical insolation forcing is well established as the underlying metronome of Quaternary ice ages and Cenozoic climate changes. Yet its effects on earlier eras (Mesozoic, Palaeozoic and pre-Cambrian) are less understood. In this Review, we explore how cyclostratigraphy can help to distinguish climate modes over the pre-Cenozoic era and aid our understanding of climate responses to astronomical forcing over geological time. The growing uncertainties with geologic age mean that pre-Cenozoic astronomical solutions cannot be used as tuning targets. However, they can be used as metronomes to identify the pacing of distinct climate states. Throughout the pre-Cenozoic, global average temperature differences between climate states were even more extreme (5–32 °C) than in the Cenozoic (14–27 °C), and these, combined with an evolving biosphere and changing plate tectonics, led to distinct Earth-system responses to astronomical forcing. The late Palaeozoic icehouse, for example, is characterized by a pronounced response to eccentricity, caused by nonlinear cryosphere and carbon-cycle behaviour. By contrast, the Devonian warmhouse and the Late Cretaceous hothouse featured recurrent episodes of marine anoxia that may have been paced by astronomical forcing. Formally defining 405,000-year eccentricity cycles as chronostratigraphic units (astrochronozones) throughout the Phanerozoic eon will enable a more comprehensive understanding of how astronomical forcing has shaped Earth’s climate over geologic time.

Abstract The Cretaceous Period was marked by the formation of numerous large igneous provinces (LIPs), several of which were associated with geologically rapid climate, environmental and biosphere perturbations, including the early Aptian and latest Cenomanian oceanic anoxic events (OAEs 1a and 2, respectively). In most cases, magmatic CO 2 emissions are thought to have been the major driver of climate and biosphere degradation. This work summarizes the relationships between Cretaceous LIPs and environmental perturbations, focusing on how volcanism caused climate warming during OAE 1a using osmium-isotope and mercury concentration data. The new results support magmatic CO 2 output from submarine LIP activity as the primary trigger of climate warming and biosphere stress before/during OAE 1a. This submarine volcanic trigger of OAE 1a (and OAE 2), two of the most climatically/biotically severe Cretaceous events, highlights the capacity of oceanic LIPs to impact Earth's environment as profoundly as many continental provinces. Cretaceous magmatism (and likely output of CO 2 and trace-metal micronutrients) was apparently most intense during those OAEs; further studies are needed to better constrain the eruption histories of those oceanic plateaus. Another open question is why the Cretaceous Period overall featured a higher rate of magmatic activity and LIP formation compared with before and afterwards.

Abstract The Cretaceous–Paleogene boundary is marked by a large impact and coeval mass extinction event that occurred 66 m.y. ago. Contemporaneous emplacement of the volcanic Deccan Traps also affected global climate before, during, and after the mass extinction. Many questions remain about the timing and eruption rates of Deccan volcanism, its precise forcing of climatic changes, and its signature in the marine geochemical sedimentary proxy record. Here, we compile new and existing mercury (Hg) concentration and osmium isotope (187Os/188Os) records for various stratigraphic sections worldwide. Both geochemical proxies have been suggested to reflect past variations in Deccan volcanic activity. New data from deep marine pelagic carbonate records are compared to contemporaneous records from shallower marine sites correlated through high-resolution cyclostratigraphic age models. The robustness of the proxy records is evaluated on a common timeline and compared to two different Deccan eruption history scenarios. Results show that the global 187Os/188Os signal is clearly reproducible, while the global Hg record does not form a consistent pattern. Moreover, the deep marine sections investigated do not record clear variations in the Hg cycle, particularly in the latest Cretaceous, prior to the extinction event. A detailed reevaluation of the precise depth of the redistribution of impactor-sourced platinum group elements does not exclude the possibility of a minor drop in 187Os/188Os corresponding with a pulse of Deccan volcanism ~50,000 years before the Cretaceous–Paleogene boundary. Simple Os isotope mass balance modeling indicates that the latest Cretaceous was marked by significant levels of basalt weathering. CO2 sequestration during this weathering likely overwhelmed the emission of Deccan volatiles, thereby contributing to the end of the late Maastrichtian warming.

Repeated carbon isotope excursions and widespread organic-rich shale deposition mark the Middle and Upper Devonian series. Various explanations such as extensive volcanism and land plant evolution have been given for these perturbations and the general sensitivity of the Devonian oceans to the development of anoxia, but their repeated nature suggests that astronomical forcing may have controlled their timing. Here, a cyclostratigraphic study of the Kellwasser Crisis at the Frasnian–Famennian stage boundary (ca. 372 Ma) is carried out. The Kellwasser Crisis was one of the most ecologically impactful of the Devonian perturbations and is ranked among the “Big Five” Phanerozoic mass extinctions. The studied site is the Winsenberg roadcut section in the Rhenish Massif, Germany, which represents a quiet tropical shelf basin setting. Centimetre-scale elemental records, generated by portable X-ray scanning, allow for testing of the hypothesis that a 2.4 Myr eccentricity node preceded the Upper Kellwasser event. The study's results are supportive of this hypothesis. We find enhanced chemical weathering (K2O / Al2O3) during the period leading up to the Upper Kellwasser and a peak in distal detrital input (SiO2 / CaO) and riverine runoff (TiO2 / Al2O3) just prior to the start of the Upper Kellwasser event. We interpret this pattern as the long-term eccentricity minimum facilitating excessive regolith build-up in the absence of strong seasonal contrasts. The Earth's system coming out of this node would have rapidly intensified the hydrological cycle, causing these nutrient-rich regoliths to be eroded and washed away to the oceans, where they resulted in eutrophication and anoxia. An astronomical control on regional climate is observed beyond this single crisis. Wet–dry cycles were paced by 405 kyr eccentricity, with both the Lower and Upper Kellwasser events taking place during comparatively drier times. A precession-sensitive monsoonal climate system prevailed on shorter timescales. Intensification of this monsoonal system following the node may have caused the widespread regolith erosion. We estimate the total duration of the Kellwasser Crisis at ca. 900 kyr, with the individual events lasting for ca. 250 and 100 kyr, respectively. If astronomical control indeed operated via regolith development in monsoonal climates, then land plants may have played an important role. This would not have been through evolutionary steps directly triggering Kellwasser perturbations but by gradually strengthening the climatic response to orbital forcing via soil development – creating soils thick enough to meaningfully respond to orbital forcing – and intensifying the hydrological cycle.

The Phanerozoic Aeon was marked by several variations in global oxygenation levels, occuring both gradually over multi-million year timescales and also more abruptly as transient perturbations lasting typically a million years or less. Following an overall trend of rising atmospheric oxygen levels in the mid–late Palaeozoic, marine redox conditions are thought to have been close to modern by the time of the Cretaceous Period (145–66 Ma). However, the Cretaceous Period also featured multiple episodes of geologically abrupt depletions in seawater oxygen levels, known as Oceanic Anoxic Events (OAEs), one of the most severe of which occurred during the Early Aptian (OAE 1a, ~120 Ma). This environmental crisis is thought to have been triggered by major carbon emissions related to the volcanic formation of the Greater Ontong-Java Plateau. Several OAE 1a sites are marked by the preservation of organic-rich laminated shales, indicative of oxygen-depleted conditions in the water column and at the sediment-water interface. However, the relative paucity of Early Aptian open-ocean sedimentary records means that the degree to which anoxic conditions spread throughout the global marine realm during OAE 1a remains poorly constrained.Here, we aim to verify the nature of seawater oxygen levels prior to, during, and after OAE 1a, using uranium-isotope (δ238U) records of that event. Under iron-reducing conditions, soluble U6+ transforms to insoluble U4+, which is associated with a pronounced isotopic fractionation in favour of 238U in U4+ ions. Thus, the subsequent sequestration of U4+ in organic-rich sediments causes depletion of 238U in the water column and a shift to an isotopically lighter δ238U composition of seawater. This change in marine δ238U is recorded by carbonates precipitated in seawater. Thus, δ238U trends across various records of OAE 1a enable the hypothesis that background Cretaceous ocean redox conditions were comparable to today to be tested, and the change in geographic extent of anoxic water masses during the environmental change quantified. By further comparing the δ238U data with other geochemical proxies (e.g., carbon-isotope evidence of organic-matter burial; osmium-isotope evidence of volcanism), we further explore the causes and environmental consequences of transient ocean redox fluctuations in the Early Cretaceous oceans.

Elevated mercury (Hg) in sedimentary strata are a widely used tracer for assessing the relationship between large igneous province (LIP) activity and global environmental change. A key unknown in applying this proxy is the extent to which Hg was sourced from contact metamorphism of sedimentary rocks during sill intrusions versus gaseous emissions of the magmas themselves. Here, we investigate Hg behaviour during contact metamorphism of shales. We show loss of 80–99% of the sedimentary Hg in contact aureoles in four case studies covering the interactions around dykes, sills and plutons associated the High Arctic LIP (Sverdrup Basin, Canada), the Karoo LIP (South Africa) and the Skagerrak-centred LIP (Oslo Rift, Norway). A combination of geochemical data and thermal modelling around a dyke from the High Arctic LIP shows 33% Hg volatilization in the aureole at 265–300 °C. The other cases show similar behaviours with significant lowering of organic-bound Hg, more significantly in the innermost 60% of the contact aureoles. We hypothesize that gaseous Hg is transported out of aureoles during metamorphism, together with CH4 and CO2. Furthermore, we estimate the thermogenic Hg mobilization from Karoo LIP aureoles as 72–192 t per km3 of aureole, which is between 1–3 times the estimated volumetric Hg release from Karoo magmas. When scaling our results to the size of the shale portions of the Karoo Basin affected by the LIP and a timescale of 100 kyr of sill emplacement, the average Hg flux is calculated to have been 78–207 t/y with maximum values up to ∼300 t/y. The pulsed nature of intrusive volcanism suggests that this thermogenic Hg flux could have dominated LIP Hg emissions during periods of their life span. Our results demonstrate that the global Hg cycle can be significantly perturbed following LIP-scale sill emplacement into organic-rich sedimentary rocks and our quantification of the emissions based on source-rock analysis provides important information for independent interpretation of the sedimentary Hg record.

Large perturbations in the global carbon cycle recorded as carbon-isotope (δ13C) excursions (CIEs) in both organic carbon and carbonate records have been linked to volcanism during the emplacement of Large Igneous Provinces (LIPs). This link is based primarily on the purported temporal coincidence between CIEs and LIP emplacement. Mercury (Hg) concentration in sedimentary rocks has been used as a regional to global tracer of large-scale volcanic activity, yet few studies have been undertaken on Upper Jurassic – Lower Cretaceous sediments from Boreal localities compared to those for Tethyan (northern mid-latitude) successions. This has limited our understanding of the regional-to-global spatial impact of volcanic activity during this period. This study examines the Hg record as a proxy for volcanism, and the δ13C records from organic matter (δ13Corg) of CIEs from the uppermost Jurassic to Lower Cretaceous (Callovian – Aptian) successions from Axel Heiberg and Spitsbergen in the Canadian Arctic and Svalbard archipelagos, respectively. This interval includes three regional- to global CIEs. These sections show no significant variation in the ratio of Hg to total organic carbon (TOC) across the Boreal-wide Volgian negative CIE (Volgian Isotopic Carbon Excursion, “VOICE”), which has not been associated with LIP volcanism. The examined successions spanning this interval all show some influence from changing environmental or post-burial parameters, however, which could have (partially) overprinted a volcanic signal. Despite some problems in stratigraphically constraining the Weissert Event, increased Hg/TOC ratios are observed across this interval, which may be partially driven by volcanism associated with the emplacement of the Paraná-Etendeka Traps. A spike in Hg/TOC is observed immediately prior to the negative peak of the Aptian Oceanic Anoxic Event (OAE1a) CIE, supporting recent evidence of a pulse of High Arctic Large Igneous Province (HALIP) volcanic activity preceding this oceanic anoxic event.

The Cenomanian–Turonian Oceanic Anoxic Event (OAE 2: 94 Ma) marked one of the most severe episodes of climate and environmental change during the Cretaceous Period. The primary feature of this interval was widespread marine anoxia/euxinia, leading to deposition of organic-rich, thinly-bedded, mudstones across the world’s oceans, which in turn caused a pronounced positive shift in δ13C of seawater that is recorded (and characterizes) OAE 2 strata worldwide. The event was also marked by climate warming (with superimposed cooling pulses), biotic stress, and terrestrial perturbations such as increased continental weathering. However, the majority of studied records of OAE 2 were deposited in deep-marine Atlantic and Tethyan settings or European epicontinental basins. Thus, the record of environmental change in other locations or environments is less clear. The Western Interior Seaway (WIS) represented a marine corridor across North America that connected the Arctic and Atlantic marine realms. Thus, understanding the environmental and oceanographic changes in the WIS during OAE 2 is crucial to resolving the wider impact of the event across the global marine realm. Several previous works have focused on sites towards the southern end of the WIS (e.g., Pueblo, Iona Core). In this study, we present a new multi-proxy geochemical dataset from a site in the central–northern part of the Western Interior Seaway: Pratts Landing (western Alberta, Canada). Previous palynological studies have highlighted a southward migration of boreal dinoflagellates during the Plenus Cold Event midway through OAE 2, as well as increased input of terrestrial organic matter. Here, we correlate these data with information from redox, nutrient, and volcanic proxies, and compare the Pratts Landing record with other deep- and shallow-marine records of OAE 2 to gain a wider perspective over the environmental changes that operated in different settings during that time interval. This viewpoint is key for understanding the differences and complexities in how surface phenomena were disturbed during OAE 2, and interpreting geochemical records of different settings during that time interval.

<p>The Late Devonian oceans were susceptible to the development of anoxic conditions, as evidenced by repeated widespread organic-rich shale deposition. Understanding how these anoxic facies were deposited will provide insight into Devonian climatic modes. To this end, we constructed a high-resolution cyclostratigraphic model based on portable XRF-generated elemental ratio records from a Frasnian-Famennian (~372 Ma) black shale section. These black shales are associated with the Kellwasser Crisis, one of the largest mass extinctions of the Phanerozoic, which is not fully understood to this day. The studied section at Winsenberg is located in the Rhenish Massif in Germany and represents a basinal setting at southern low paleolatitudes. Spectral analysis was carried out on the Si/Ca ratios generated by XRF, which is interpreted as the detrital (distal) vs carbonaceous (local) input. The resulting astrochronology suggests a duration of ca. 1 Myr from the base of the Lower Kellwasser to the F-F boundary at the top of the Upper Kellwasser level. This corresponds to an average sedimentation rate of 0.9 cm/kyr. Both the Lower and Upper Kellwasser shales occur at the onset of a 405 kyr eccentricity cycle. We further interpret the Ti/Al record as a riverine runoff signal, as Ti is associated with the coarse-grained fraction, and K/Al as a chemical weathering signal, as K is leached easier than Al. Both tuned records exhibit eccentricity-modulated precession cycles. On precession and short eccentricity timescales, Ti/Al and K/Al are positively correlated, suggesting an orbitally forced wet/dry monsoonal climate in the region where the section was deposited. On longer timescales, the weathering signal becomes decoupled from the riverine runoff signal, highlighting that K/Al (chemical weathering) decreased even during wetter periods. This decoupling is linked to soil maturation in the hinterland, as potassium leaching from mature soils became increasingly limited. Soil build-up and maturation forms a potential mechanism for nutrient storage and subsequent release into the ocean, potentially triggering eutrophication and anoxia.</p>

The Valanginian Weissert Event (ca. 134 Ma) has long been linked to emplacement of the Parana-Etendeka large igneous province (LIP). Although several Mesozoic crises were triggered by volcanic CO2 emissions and global warming, causing oceanic oxygen depletion, the Weissert Event featured climate cooling and limited marine anoxia. Here, the impact of silicate weathering on environmental change during the Weissert Event is investigated by presenting the first osmium isotope (Os-187/Os-188) data sets for the late Berriasian-Valanginian interval. These records document a possible rise in weathering and nutrient runoff during the latest Berriasian, followed by a switch to Parana-Etendeka basalt erosion during the Valanginian. The high weatherability of LIP basalts enhanced global silicate weathering despite limited coeval climate warming. This weathering triggered the documented cooling, with consequent micronutrient runoff potentially aiding ocean fertilization, highlighting a mechanism linking LIP formation with environmental change that was uniquely different than that of other Mesozoic crises.

Marine tetrapods occupy important roles in modern marine ecosystems and often gather in large aggrega-tions driven by patchy prey distribution,1,2 social or reproductive behaviors,3,4 or oceanographic factors.5 Here, we show that similar grouping behaviors evolved in an early marine tetrapod lineage, documented by dozens of specimens of the giant ichthyosaur Shonisaurus in the Luning Formation in West Union Canyon, Nevada, USA.6,7 A concentration of at least seven skeletons closely preserved on a single bedding plane received the bulk of previous attention. However, many more specimens are preserved across-106 square meters and-200 stratigraphic meters of outcrop representing an estimated >105-6 years. Unlike other marine-tetrapod-rich deposits, this assemblage is essentially monotaxic; other vertebrate fossils are excep-tionally scarce. Large individuals are disproportionately abundant, with the exception of multiple neonatal or embryonic specimens, indicating an unusual demographic composition apparently lacking intermediate -sized juveniles or subadults. Combined with geological evidence, our data suggest that dense aggregations of Shonisaurus inhabited this moderately deep, low-diversity, tropical marine environment for millennia dur-ing the latest Carnian Stage of the Late Triassic Period (237-227 Ma). Thus, philopatric grouping behavior in marine tetrapods, potentially linked to reproductive activity, has an antiquity of at least 230 million years.

The biogeochemical cycle of trace metals and oxyanions in marine sediments is mainly controlled by early diagenesis of organic material, but anthropogenic activities such as dumping of dredged sludge or shipyard activities can strongly perturb these natural processes. Therefore, the impact of dumping of dredged sludge, shipyard activities such as blasting of boat hulls and the mixing of polluted Scheldt estuary water with less polluted coastal water on the mobilization of trace metals and oxyanions in the sediments was studied in the pre- and post-bloom period (March and October) in Belgian Coastal Zone (BCZ). Vertical profiles of total and labile dissolved trace element concentrations, dissolved sulfide, pH and redox potential were assessed in sediment porewaters. Generally, total and labile dissolved element profiles in the porewaters are well correlated, showing higher concentrations in March when the redox potential is lower and sulfide concentrations higher. Low pH levels were documented together with high amounts of Fe and associated elements in March at the dumping station. At the coastal station in the vicinity of the shipyards, sediment porewaters are enriched in dissolved Zn while at the mouth of the Scheldt estuary the highest dissolved Cu levels were found. Theoretical calculations with Visual MINTEQ confirm the increased Fe and associated element concentrations in March (lower pH) and the lower metal concentrations in October (lower Eh). R values, ratio of labile dissolved to total dissolved concentrations, reflect the potential of the solid sediment phase to supply labile elements to the porewater. They were lowest for Cu and highest for V. This study showed that anthropogenic activities in the BCZ such as dumping of dredged sludge and blasting of boat hulls have a serious impact on the levels, distribution and bioavailability of contaminants in the sediments, which may form a real threat to the benthic ecosystem.

The Valanginian Weissert Event (~134 Ma) represented the first major carbon-cycle disturbance of the Cretaceous Period, characterized in the sedimentary record by a prolonged positive excursion in carbon-isotope ratios. The event has been widely linked with climate cooling, documented in several geographic regions; however, some areas show minimal evidence of temperature change at that time, or even warming and enhanced humidity around the onset of the event. Moreover, although the carbon-isotope excursion has generally been attributed to enhanced burial of organic matter, there is no evidence of widespread marine anoxia that would have promoted such deposition. Consequently, key questions remain regarding the causes of climate and environmental degradation during the early Valanginian. Understanding changes in volcanic activity and silicate weathering rates through late Berriasian to early Valanginian times is crucial for resolving this debate, as both processes influence atmospheric pCO2 levels and global temperatures. In particular, volcanism associated with formation of the Paraná-Etendeka large igneous province (LIP) during the Valanginian has long been proposed as the ultimate trigger of the Weissert Event via carbon emissions and greenhouse warming,but weathering of juvenile LIP basalts could equally have caused climate cooling. In this study, we investigated the osmium-isotope composition (187Os/188Os) of deep-marine organic-rich Berriasian–Valanginian sediments from two proto-Atlantic Ocean archives (DSDP sites 534 and 603). Given the palaeoenvironmental setting of the two sites, the recorded 187Os/188Os seawater compositions of the proto-Atlantic should be representative of the global ocean. We find that seawater 187Os/188Os shifted from ~0.6 to ~0.75 during the latest Berriasian, suggestive of an increased flux of radiogenic osmium to the ocean during that time, likely resulting from enhanced weathering of the continental crust. Interestingly, however, there is no evidence of global climate warming during the late Berriasian that would have caused this weathering. Following the late Berriasian radiogenic shift, seawater osmium gradually changed to a more unradiogenic isotopic composition (~0.45) during the early Valanginian; the lowest 187Os/188Os values correlating with both the peak in the Weissert Event carbon-isotope excursion and evidence for climate cooling. This unradiogenic shift could reflect a decline in weathering of radiogenic crustal material; however, it also stratigraphically correlates with geochronological and geochemical evidence for the time of maximum igneous activity on the western (Paraná) part of the Paraná-Etendeka LIP. Therefore, we conclude that the early Valanginian shift to unradiogenic 187Os/188Os seawater compositions resulted from erosion of juvenile primitive basalts, suggesting that combined weathering of the continental crust and the Paraná-Etendeka LIP played a key role in causing the global cooling associated with the Weissert Event.

Widespread marine anoxia triggered by the runoff and recycling of nutrients was a key phenomenon associated with the Frasnian–Famennian (FF) mass extinction. However, the relative importance of global‐scale processes versus local influences on site‐specific environmental change remains poorly understood. Here, nitrogen‐isotope (δ 15 N) trends are combined with organic‐biomarker, phosphorus, and Rock‐Eval data in FF sites from the USA (H‐32 core, Iowa), Poland (Kowala Quarry), and Belgium (Sinsin). Up‐to‐date cyclostratigraphic age models for all three sites allow the nature and timing of changes to be precisely compared across the globe. Negative δ 15 N excursions across the FF interval from the H‐32 core and Kowala correlate with geochemical evidence for euxinic, phosphorus‐rich, water columns, and possible cyanobacterial activity, suggestive of increased diazotrophic N fixation, potentially coupled with ammonium assimilation at the latter site. By contrast, previously studied sites from Western Canada and South China document enhanced water‐column denitrification around the onset of the Upper Kellwasser (UKW) Event, re‐emphasizing the geographical heterogeneity in environmental perturbations at that time. Moreover, environmental degradation began >100 kyr earlier in Poland, coeval with a major increase in bioavailable phosphorus supply, than in Iowa, where no such influx is recorded. These regional differences in both the timing and nature of marine perturbations during the FF interval likely resulted from the variable influx of terrigenous nutrients to different marine basins at that time, highlighting the importance of local processes such as terrestrial runoff in driving environmental degradation during times of climate cooling such as the UKW Event.