Denis Scholz

Primary production in the northeastern Arabian Sea is a key component of regional biogeochemical cycling and oxygen minimum zone development, linking monsoon circulation to nutrient cycling and carbon export. Today, primary production is influenced by both components of the South Asian monsoon system, with the South Asian summer monsoon (SASM) affecting coastal nutrient supply, whereas winter monsoon (SAWM) winds promote surface cooling and convective mixing. However, how the relative influence of these monsoonal systems changes under different climatic and oceanographic boundary conditions remains poorly understood. By combining plant-wax stable carbon and hydrogen isotopes, alkenone-derived sea surface temperature (SST), hyperspectral imaging of chloropigments- a , and alkenone concentration from the sediment core SO130-289KL, we reconstruct hydroclimate, SST, and marine primary production between 23,000 and 2,000 years before 2000 (b2k). Our reconstruction indicates progressive strengthening of SASM precipitation from the Last Glacial Maximum into the Holocene, interrupted by pronounced aridity during Heinrich Stadial 1 and Greenland Stadial 1. SST broadly followed deglacial warming in the Northern Hemisphere but showed substantial regional variability associated with SAWM strength. We further identify two intervals of increased winter precipitation between 14,000–10,500 and 4,000–2,000 years b2k. Primary production shows no clear correspondence with either monsoon system alone. During glacial conditions, lower sea level placed the core site closer to the productive coastal zone, increasing its sensitivity to changes in SASM strength. During the Early Holocene increased oxygenation of intermediate waters suppressed low-oxygen nutrient recycling, where after ~6,000 years b2k weaker ventilation promoted higher production despite weaker monsoons.

The ongoing global decline of coral reefs underscores the need to identify key mechanisms driving reef deterioration. The Flower Garden Banks (FGB) reefs in the northern Gulf are widely regarded as some of the healthiest in the continental United States, but recent disease outbreaks in the FGB have raised concerns about their long-term resilience. As human and agricultural expansion intensified across the Mississippi River basin, the river has become the dominant source of nutrients to the northern Gulf; however, the evolution of this influence is poorly understood. Here, we reconstruct nitrogen inputs to the FGB using annually resolved coral-bound nitrogen isotopes (CB-δ ¹⁵ N) from two coral cores that collectively span the period from 1753 to 2023. Our results show that anthropogenic nitrogen began to rise around 1850, coinciding with the early use of fertilizers, and increased sharply after the 1960s with the onset of the Green Revolution and the use of synthetic fertilizers, suggesting the Mississippi discharge as the source. Fractional endmember modeling further reveals that the accumulation and mobilization of legacy nitrogen stored in Midwestern soils and groundwater have amplified modern riverine fluxes, driving Mississippi-derived nitrate to more than 60 to 80% of the nitrogen reaching the FGB in recent decades. The highest riverine contributions of the past 270 years occurred in 2016 and 2022–2023, coinciding with disease outbreaks and thermal stress events. These findings suggest that the combined effects of unprecedented nutrient loading and extreme heat are key synergetic drivers of the recent deterioration in coral health observed for this historically resilient reef system.

Reconstructing past South Asian monsoon dynamics, which governs wind and rainfall patterns across the Indian subcontinent, is crucial for understanding low-latitude climate. Constraining monsoon drivers requires separating summer and winter components in palaeoclimate archives, but this remains challenging because proxy signals usually integrate seasonal signals. The northeastern Arabian Sea provides a unique setting in which sedimentary and geochemical proxies are driven by the summer monsoon, while sea surface temperature is primarily controlled by winter monsoon winds. Here we employed mass spectrometry and hyperspectral imaging on a sediment core off Pakistan to reconstruct subdecadal changes in sea surface temperature and marine primary production during the last deglaciation (~16,000–12,000 years ago). Atmospheric humidity and vegetation changes were additionally assessed by plant-wax isotope analyses on a subset of samples. We show that summer monsoon winds were driven by the Northern Hemisphere high-latitude climate on centennial-to-millennial timescales. Winter monsoon was primarily characterized by a millennial-scale decline in wind strength driven by increasing global temperatures, with superimposed centennial-scale variability. We identified an inverse relationship between winter monsoon wind strength and the amount of winter non-monsoonal precipitation. Mechanistic insights of seasonal monsoon dynamics improved interpretation of regional palaeoprecipitation records and may enhance climate model performances in low latitudes.

Extreme rainfall events (ERE) are projected to increase in intensity and frequency worldwide. Southern Brazil, a densely populated region, is strongly affected by these events with severe socioeconomic impacts, such as the catastrophic flooding in Rio Grande do Sul State in April/May of 2024. Understanding the intensity and frequency of past ERE is fundamental for predicting future impacts in a global warming scenario. The lack of records of past ERE variability beyond the instrumental period hampers our ability to assess the relationship of ERE variability and large-scale climate drivers over longer timescales. Here we present the first ERE record from southern Brazil, based on the frequency of detrital layers within stalagmites from Malfazido cave, spanning the past 7500 years. Our results suggest a strong influence of Antarctic-tropical teleconnections on the long-term modulation of ERE and important contribution of strong/moderate El Niño events to the frequency of ERE in the last millennium. In the 20th century, ERE frequency is in the upper range of the variability of our record, suggesting an influence of anthropogenic forcing on ERE modulation.

The modern Western Mediterranean climate is characterised by strong seasonality and, especially in south-eastern Spain, by limited rainfall. In contrast, terrestrial and marine archives indicate that the peak of the last interglacial was marked by warmer conditions and potentially more variable climate, partly driven by meltwater outbursts in the North Atlantic. High insolation during interglacials forced a northward displacement of the Intertropical Convergence Zone, increasing rainfall over Northern Africa. Enhanced fluvial input leads to stratification of the Eastern Mediterranean Sea and ultimately to the deposition of sapropels. However, the climatic consequences of these large-scale processes for south-eastern Iberia have not yet been systematically investigated.Several speleothems from Cueva Victoria, south-eastern Spain, cover the mid-Pleistocene to the Holocene. During the last glacial period, Dansgaard/Oeschger (D/O)-type variability is expressed in the speleothem stable isotope records. Variations in speleothem stable isotope values are interpreted in terms of changes in temperature (δ18O) and vegetation cover (δ13C). In general, warmer temperatures during D/O events are associated with lower δ18O values due to temperature and moisture source effects. Increased effective precipitation (precipitation-evaporation) is reflected by more negative δ13C values, resulting from higher soil microbial activity and denser vegetation cover.Here, we present a compilation of speleothem records from Cueva Victoria spanning several interglacials and encompassing the timing of multiple sapropel layers in the Eastern Mediterranean. A comparison of speleothem stable isotope signatures during the formation of different sapropels reveals contrasting climatic responses in south-eastern Spain. During glacial phases, speleothem growth coincided with the timing of the sapropel deposition, indicating more humid conditions in south-eastern Spain. In contrast, during the Holocene Climate Optimum and the formation of sapropel 1, elevated δ13C values point to a decline in vegetation cover, interpreted as a result of enhanced seasonality. Speleothem formation is almost completely absent during sapropel 5 (≈ 122-128 ka), which may reflect enhanced seasonality with warmer temperatures associated with a reduction in precipitation-evaporation compared to the Holocene.

Millennial-scale climate variability is a prominent feature of the last glacial cycle, intensively studied in Greenland ice cores as well as other marine and terrestrial climate archives. The most widely recognised expressions of abrupt millennial- to centennial- scale climate oscillations during this period are Dansgaard – Oeschger (D/O) events. For the past 125 ka, Greenland ice cores provide a benchmark for studying D/O events, but their restriction to the last glacial cycle limits investigations of the timing, duration and amplitude of D/O-type events during previous glacial cycles. Synthetic Greenland ice core data suggest that D/O-type millennial-scale climate variability occurred across all glacial phases of the past 800 ka. However, a major limitation for understanding the timing and dynamics of millennial-scale climate variability in preceding glacial cycles is the progressively more challenging dating of older material and the general lack of absolutely and precisely dated high resolution climate records beyond the last interglacial.Here we present a new speleothem record from Cueva Victoria in SE Spain covering Marine Isotope Stages (MIS) 11a and 10, showing millennial-scale climate variability in both temperature and precipitation. Previous studies confirm that speleothems from Cueva Victoria are sensitive archives of past atmospheric and hydrological changes on both millennial and orbital timescales. For the last glacial cycle, numerous D/O events have been identified in Cueva Victoria speleothem stable isotope records, demonstrating their strong connection to North Atlantic climate patterns.During MIS 11a and 10, millennial-scale variability is evident in multiple high-resolution proxies in the Cueva Victoria speleothems, such as stable carbon and oxygen isotopes, Mg concentrations, as well as TEX86-derived cave temperatures. The structure and timing of those millennial-scale events align closely with millennial-scale variability in marine sediment records, especially from the Iberian Margin, enabling direct comparison of temperature and precipitation dynamics in the marine and terrestrial realm. All events are characterised by a rapid increase in temperature and moisture availability, followed by a more gradual cooling and drying trend. This results in distinct stadial-interstadial D/O-type oscillations, particularly pronounced during MIS 10. The timing of these oscillations matches with the predicted occurrence of D/O events based on the synthetic Greenland ice core record, highlighting the potential of Cueva Victoria speleothems to reconstruct millennial scale climate variability beyond the last glacial cycle.

New high-resolution trace element records combining stalagmites from Southeastern Brazil (SEBRA) evidence persistent multi-centennial shifts in hydroclimate conditions over the past 7,500 years, with wet anomalies associated with North Atlantic cooling events, including Bond events and the Little Ice Age (LIA). Our analysis reveals a coupling between the Bond events and increased South Atlantic Convergence Zone (SACZ) rainfall over SEBRA, with a persistent pattern over the Middle and Late Holocene. The most pronounced wet anomalies in SEBRA are synchronous with these events, and present a coherent structure with other records from the South American Summer Monsoon (SASM) region and the SACZ, and are in antiphase with Southern Brazil (SB) resembling the multi-centennial dipole between SEBRA and SB. This pattern indicates that large-scale reorganizations of the Intertropical Convergence Zone (ITCZ) are induced by North Atlantic cooling and a strengthened SASM/SACZ convection through changes in cross-equatorial heat transport related to a weakening of the AMOC. Furthermore, the interhemispheric antiphase relationship between SEBRA wet anomalies and drying across the Asian monsoon region evidences the global expression of AMOC–ITCZ modulation under North Atlantic cooling events. These findings demonstrate the pronounced response of SEBRA hydroclimate to even modest perturbations in the interhemispheric energy balance, evidencing the sensitivity of the region towards potential impacts under AMOC weakening scenarios.

Cueva Victoria is located in the semi-arid region of south-eastern Spain, one of the driest regions in Europe with mean annual precipitation of 200–300 mm and pronounced seasonality. The cave is hosted in Triassic dolomites and limestones of the Alpujarride Complex, part of the Inner Betic Cordillera, where karstification has enabled the development of extensive cave systems and flowstone formation. These flowstone deposits provide a sensitive archive of past climate variability.Previous studies established robust ²³⁰Th/U chronologies spanning the last ~450 ka, demonstrating that speleothem growth occurred during both interglacial and warmer glacial periods, such as Marine Isotope Stage (MIS) 3, reflecting phases of enhanced regional moisture availability.Here, we investigate flowstone samples VIC-III-4 and VIC-III-5, covering MIS 11c to MIS 7a (~430–190 ka), to evaluate their potential as quantitative paleotemperature archives. Preliminary nucleation-assisted (NA) fluid inclusion microthermometry measurements of a few flowstone samples yielded cave temperature estimates in agreement with the range of independently derived TEX₈₆-based temperature reconstructions from the same samples.Detailed petrographic thin section analysis of the two flowstones indicates the presence of several fluid-inclusion-bearing growth layers that appear promising for NA fluid inclusion microthermometry. This provides the basis for targeted selection of additional microthermometry measurements and a more detailed analysis of the two flowstones. Although assisted (NA) fluid inclusion microthermometry has successfully been applied to speleothems from other regions, this study represents the first application to the Cueva Victoria flowstones and one of the first applications to a semi-arid cave system.The combination of precisely dated high-resolution speleothem proxy records (stable isotopes, trace elements) with direct temperature reconstructions significantly enhances the potential of the Cueva Victoria flowstones for palaeoclimate reconstruction and will contribute to improving terrestrial paleoclimate reconstructions for the western Mediterranean region, an area highly sensitive to future hydroclimate change.

The Arabian Desert experienced multiple periods of wetter and greener conditions that sustained human populations and allowed the dispersal of mammal fauna across the Arabian Peninsula. A recently published speleothem-based paleoclimate reconstruction of central Arabia extends the record of such recurrent short-lasting humid periods over at least the past 8 million years. Here, we applied multiple recently developed paleothermometers to this late Miocene to late Pleistocene speleothem record: Fluid inclusion stable isotopes, microthermometry and dual-clumped isotopes. The data indicate that in the late Miocene and Pliocene, wetter episodes in central Arabia were up to ~4 °C warmer than current Mean Annual Air Temperature (MAAT). These temperature estimates imply that potential evapotranspiration was significantly higher during the late Miocene and Pliocene than during the late Pleistocene. From these temperature estimates, we calculated Pliocene potential evapotranspiration and estimated precipitation amounts for the humid periods in central Arabia. All the evidence from the speleothems combined (temperature, precipitation, δ¹³C values) suggests that over the past 8 million years, the wetter phases in central Arabia typically led to savanna-like environments.Modern climate data show that our study area has already reached Pliocene MAATs in recent years due to anthropogenic warming. The concomitant drying trend in modern settings indicates that higher temperatures are not the key factor in creating wetter conditions on the Arabian Peninsula. Previously proposed orbital control on the incursion of monsoonal moisture from the south into the Arabian Peninsula remains the most important driver of humidity during these past humid periods. In the modern orbital configuration, monsoonal moisture advection is displaced to the south, and increasing temperatures will likely lead to increased potential evaporation and aridity in central Arabia.

Reconstructing past temperature variations is essential for understanding climate systems and improve projections for future climate changes. In central-east South America, modern warming has been shown to progress faster than global average. Nonetheless, paleotemperature records remain sparse in central South America, which limits our ability to evaluate the response of this region to rapid shifts in global forcings, such as during the deglacial period. Studies show that temperature evolution during the deglaciation was characterized by high-latitude rapid warming episodes associated with major reorganizations of the Atlantic Meridional Overturning Circulation (AMOC), which led to perturbations in inter-hemispheric heat distribution. Yet, how these perturbations affect temperatures in tropical South America and the thermal evolution of this region is still largely unknown. Here we present a new 15k-year paleotemperature reconstruction from a precisely dated speleothem collected in central-eastern Brazil. The temperature record is based on the glycerol dialkyl glycerol tetraether (GDGT) paleothermometer, revealing a total of 6.1°C±0.81 (2std = 0.81°C) of temperature shifts over the last 15k years. Our findings provide evidence of a non-linear temperature increase since the last deglaciation with abrupt warming and cooling events in response to high-latitude forcings, shifts in South Atlantic sea-surface temperatures (SSTs), and increases in atmospheric CO2. Finally, we present a temperature gradient within central-east Brazil and show how paleoclimate models might underestimate rapid temperature changes.