Abstract African rainforests store large quantities of carbon within below‐ground reservoirs, yet these stocks are threatened by land use and climate disturbances. Previous studies revealed that the peatlands of the Cuvette Centrale in the Congo Basin accumulated less organic carbon (OC) between 7,500 and 2,000 yrs BP (so‐called “Ghost Interval”), likely caused by drier climate at this time and potentially linked to the “Late Holocene Rainforest Crisis” (LHRC)—a period of forest fragmentation around 3,000 yrs BP. This crisis event provides an ideal opportunity to reconstruct the effects of climatological and vegetational changes on OC burial rates in the Congo Basin using lake sediment records. To further constrain the spatial extent of the LHRC, we collected sediment cores from 10 lakes across the rainforest‐savanna boundary in the Kasaï Basin. We derived apparent sediment accumulation rates (SARs), which we with total organic carbon (TOC) and bulk density measurements to calculate apparent OC mass accumulation rates (OC MARs). Most lakes show a decrease in SAR during the LHRC, mirroring observations in existing cores from the peatlands of the Cuvette Centrale. The LHRC is thus likely a wider‐spread phenomenon than previously postulated, that is, not limited to central‐western Africa. Furthermore, OC mineral stabilization and thermal lability measurements on two representative lake records suggest that OC deposited over the last 5,000 cal yrs BP is highly susceptible to remineralization upon oxygen exposure. Our findings demonstrate the broad sensitivity of Congo Basin carbon stocks to precipitation patterns and land‐use change.
Abstract The Late Miocene was a critical interval in the evolution of the South Asian monsoon system and the Himalayan foreland. This study presents a quantitative paleoclimatic reconstruction based on fossil leaf assemblages from the Lower (10.5–9.2 Ma) and Middle (9.2–7.0 Ma) Siwalik sediments of the Daldale section, western Nepal, using the Climate Leaf Analysis Multivariate Program (CLAMP). The reconstructed mean annual temperatures are 22.2 ± 2.3°C for the Lower Siwalik and 21.5 ± 2.3°C for the Middle Siwalik, with corresponding cold month mean temperatures of 15.4 ± 3.3°C and 15.0 ± 3.3°C. Mean annual precipitation ranges from 228 ± 48 to 238 ± 48 cm, and the annual vapor pressure deficit remains close to 6 ± 2 hPa, indicating persistently warm and humid monsoon‐influenced conditions throughout the study interval. Although several climatic variables exhibit coherent directional trends, including slightly lower temperatures, reduced rainfall concentration during the wettest months, and subtle changes in seasonal moisture availability in the Middle Siwalik, these differences largely overlap within the uncertainties of the CLAMP reconstructions. Consequently, the Daldale record indicates broad climatic stability rather than a major climatic transition between the Lower and Middle Siwalik intervals within the studied section. The study provides the first quantitative paleoclimatic benchmark for the Daldale Siwalik succession and demonstrates that climatic responses within the Himalayan foreland were spatially heterogeneous, with western Nepal maintaining persistently humid conditions despite broader regional environmental changes.
Abstract Although foraminiferal oxygen isotope records (δ 18 O F ) have long been used to develop time‐continuous estimates of past global ice volume, growing evidence indicates that the ice volume and temperature contributions to δ 18 O F diverge considerably over the last glacial cycle. Across Marine Isotope Stage 3 (MIS 3; 57 to 29 ka), estimates of peak global sea level based on δ 18 O F are ∼30 m lower than reconstructions based on recent glacial geology, geologic sea‐level records, and glacial isostatic adjustment (GIA) modeling approaches. Here we address this mismatch by quantifying uncertainties in converting δ 18 O F to global sea level during MIS 3. We review the geochemical tools to assess the contributions of ocean temperature, diagenesis, and the isotopic composition of ice sheets to δ 18 O F records across MIS 3. We show that recent advances in our understanding of these and other factors that contribute uncertainty (e.g., global sea level during the Last Glacial Maximum) help to explain the MIS 3 gap between global sea level estimates based on δ 18 O F and GIA analyses of geologic sea‐level records. We find that the accuracy and precision of MIS 3 ocean temperature reconstructions is likely a key contributor, and various MIS 3 temperature reconstructions can reduce the “gap” by 30%–70% (∼10–20 m). Future improvements in the precision of temperature reconstructions will better constrain the ice volume component of δ 18 O F records. Ultimately, assessing the fidelity of δ 18 O F as a proxy for global ice volume is key to disentangling the feedbacks between climate and ice sheets over the Pleistocene.
Abstract Reconstructing past interactions between the West Antarctic Ice Sheet (WAIS) and changing environmental conditions in the Ross Sea region can help determine long‐term WAIS stability. Existing sedimentary records reveal changes to oceanography, sea ice and ice sheet extent over the glacial‐interglacial cycles of the last ∼800 kyr. There are, however, few high‐resolution, ice‐proximal records extending to before the Last Glacial Maximum. Here, we examine a broad suite of lithological, biogenic, physical property and geochemical data from International Ocean Discovery Program (IODP) Site U1524 from the continental rise in the central Ross Sea. An age model spanning the last ∼800 kyr is established within constraints provided by biostratigraphy, palaeomagnetism and foraminifera amino acid racemization. Lithofacies analysis reveals the pattern of glacial advance and interglacial retreat. Enhanced delivery of ice rafted debris and preservation of radiolaria occurs during deglacial and interglacial periods, with fine‐grained diatom‐bearing to diatom‐rich sediments deposited during glacial periods. Foraminifera are preserved during the latest parts of the Marine Isotope Stage (MIS) 11 and 9 interglacials. Proxy data apparently suggest higher productivity during glacial periods, which is explained by reworking and redeposition of older detritus from the continental shelf. A change in the character of glacial period sediments after MIS 7 (∼200 kyr ago) suggests there was a less extensive grounded ice sheet during the last two glacial periods. Together, these results provide a detailed reconstruction of mid‐ to late‐Pleistocene ocean–ice sheet interactions, improving understanding of how the Ross Sea region may respond to ongoing climate change.
Abstract Orbital changes have shaped Earth's climate system and influenced the evolution and spatial distribution of marine plankton, including coccolithophores. Some studies have assessed eccentricity's impact on the diversity and morphological evolution of Noelaerhabdaceae taxa during the Quaternary. However, the response of pre‐Pleistocene Noelaerhabdaceae species remains largely unexplored. Here, we examine how the extinct group Reticulofenestra , a dominant pre‐Quaternary Noelaerhabdaceae taxon, responded to eccentricity during the Early Pliocene. This interval, characterized by relatively stable climate, offers a unique opportunity to investigate how orbital variations may have influenced coccolithophore morphological evolution. We analyzed sediments retrieved from the SW Iberian Margin (International Ocean Discovery Program 397 Expedition, Site U1587), a region with high sediment accumulation rates where astronomically forced climate mechanisms strongly influenced sedimentation. Calcareous nannofossil data show co‐occurring high abundances of Reticulofenestra <3 μm and Reticulofenestra >7 μm, with open and closed central areas, during 400‐kyr eccentricity minima. However, medium size Reticulofenestra morphotypes exhibit a gradual decline throughout the interval, punctuated by several abrupt decreases coinciding with 400‐kyr eccentricity minima. We suggest that during these minima, reduced seasonality in daylight hours favored more stable conditions for K‐strategist coccolithophores ( Reticulofenestra <3 μm), allowing them to dominate the ecological niche and displace less resilient groups (medium size Reticulofenestra ). However, the gradual decrease in Reticulofenestra size observed throughout the Early Pliocene could also reflect an adaptive response to improve nutrient assimilation under decreasing CO 2 concentrations. Synchronous Reticulofenestra size reduction at 4.45 Ma in Agulhas Plateau (Southwest Indian Ocean) records suggests a large‐scale teleconnection in nannofossil community dynamics.
