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1. Carbon isotope values for grasses in Madagascar's Central Highlands establish baselines for historical and paleoecological research

   https://doi.org/10.1002/ppp3.10358  

   Crowley, Brooke E. ; Schmidt, Heidi H. ; Vorontsova, Maria S. ( January 2023 , PLANTS, PEOPLE, PLANET)

   Grasses are significant drivers of fires and are the primary food source for cattle in Madagascar's Central Highlands. However, their extent and importance to animals and people in the past remain poorly understood. Clarifying the history of Malagasy grasslands is necessary for building climate resilient food systems and supporting carbon stores that also conserve biodiversity. We generated chemical data for grasses that grow in open habitats in central Madagascar, which will help improve our understanding of the ecological and economic importance of modern grassy ecosystems, reconstruct the regional history of grasses, and anticipate how vegetation may respond to changing climate and rising atmospheric carbon dioxide levels.

   Stable carbon isotope (δ¹³C) data for Malagasy grasses are needed to establish expected values for C₃and C₄grasses from particular regions in Madagascar, and possible differences among different grass lineages, or species with different distributions or adaptations. These data, in turn, may help inform how widespread grasses were in the past, and the importance of grasses to endemic and domesticated animals as well as people over time.

   We analysed both δ¹³C and weight %C:N from 63 Poaceae species that grow in open grassy biomes in Madagascar's Central Highlands and explored how these values relate to multiple variables, including encounter frequency, distribution, lineage, adaptations to grazing and fire and the typical floral assemblage in which each species occurs.

   Of the species sampled, 56 are C₄and seven are C₃. There are no differences in δ¹³C or weight %C:N among either C₃or C₄species with different distributions or adaptations, from different assemblages, or that are frequently or infrequently encountered. However, there are differences in both δ¹³C and weight %C:N among C₄lineages, and the single C₃arundinoid (Styppeiochloa hitchcockii) has larger weight %C:N than C₃Paniceae.

   Our results provide a foundation for evaluating reliance on C₄resources by people, as well as domesticated and endemic animals both today and in the past. We encourage gathering additional comparative data for co‐occurring individual plants from the same open grassy biome localities, as well as other species, habitats and regions in Madagascar.

    

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2. Late Miocene C 4 Grassland Fire Feedbacks on the Indian Subcontinent

   https://doi.org/10.1029/2020PA004106  

   Karp, Allison T. ; Uno, Kevin T. ; Polissar, Pratigya J. ; Freeman, Katherine H. ( April 2021 , Paleoceanography and Paleoclimatology)

   Fire dynamics potentially account for the asynchronous timing of the expansion of C₄grasslands throughout the Mio‐Pliocene world. Yet how fire, climate, and ecosystems interacted in different settings remain poorly constrained because it is difficult to quantify fires and fuel source over these timescales. Here, we apply molecular proxies for fire occurrence alongside records of vegetation change and paleohydrology in Bengal Fan sediments (ODP Leg 116) to examine fire feedbacks on the south Asian continent. We employ abundances of polycyclic aromatic hydrocarbons (PAHs) to reconstruct fire occurrence and δ¹³C measurements of pyrogenic PAHs to constrain fuel source and grassland burning. This combination allowed us to test whether: (1) a fire‐seasonality forcing facilitated the expansion of grassland ecosystems and (2) a fire‐C₄grass burning feedback maintained these systems. PAHs can be sourced from weathered fossil carbon (i.e., a petrogenic source) and from burned terrestrial biomass (i.e., a pyrogenic source). Alkylated and non‐alkylated structure abundance data distinguished pyrogenic from petrogenic sourced samples. A sharp increase in pyrogenic PAHs along with increases in δ²H and δ¹³C values of plant waxes at 7.4 Ma indicates increased fire coincided with the onset of C₄expansion and hydrologic change in South Asia. The correlated¹³C enrichment in PAHs,¹³C enrichment in plant waxes, and increased abundances of PAHs suggest burning of C₄grasslands likely maintained open ecosystems. Our results link fire to the initial opening of grassland ecosystems on a subcontinental‐scale and support disturbance as a critical mechanism of terrestrial biome transition.

    

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3. What can hippopotamus isotopes tell us about past distributions of C 4 grassy biomes on Madagascar?

   https://doi.org/10.1002/ppp3.10402  

   Crowley, Brooke Erin ; Godfrey, Laurie Rohde ; Samonds, Karen Elizabeth ( July 2023 , PLANTS, PEOPLE, PLANET)

   Today, expansive C₄grassy biomes exist across central, western, and northern Madagascar. Some researchers have argued that the island's now‐extinct pygmy hippopotamuses belonged to a megaherbivore grazing guild that maintained these grasslands prior to human arrival. However, the chemistry of hippo bones indicates that C₄grasses were only a minor part of hippo diet. This, in turn, suggests that C₄grasses were present but not widespread when hippos were alive and that grasses expanded only after Malagasy people shifted from hunting and foraging to agropastoralism approximately 1000 years ago. These results have important implications for environmental reconstructions and biodiversity management.

   Extinct hippopotamuses (Hippopotamusspp.) were part of Madagascar's megaherbivore guild. Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in radiometrically dated bone collagen track spatial and temporal variation in diet and habitat. If hippos helped maintain C₄grassy biomes, then they should have regularly consumed C₄grasses, which have high δ¹³C values. However, if expansive C₄grassy biomes are anthropogenic, then forests would have been more extensive in the past, and hippos would have predominantly consumed C₃plants with low δ¹³C values. Nitrogen isotopes can clarify foraging habitat (moist or dry).

   We assessed δ¹³C and δ¹⁵N values for hippos from different ecoregions of Madagascar and compared these with data for extinct herbivorous lemurs from the same ecoregions. We further explored the effects of wet/dry transitions on isotopic trends for hippos from the central highlands and spiny thicket ecoregions.

   Carbon isotopes suggest (1) limited C₄consumption by hippos in the central highlands, dry deciduous forest, and succulent woodland ecoregions; and (2) moderate consumption of C₄resources in the spiny thicket. Nitrogen data indicate that hippos foraged in wetter habitats than sympatric lemurs in all regions.

   Malagasy hippos did not regularly graze C₄grasses in dry, open habitats, even in regions blanketed by C₄grassy biomes today. Malagasy grasses are adapted to grazing and fire, but these are likely ancient adaptations that accompanied grasses when they initially spread to Madagascar. C₄grassy biomes were spatially limited in extent in the past and only expanded after the Late Holocene introduction of domesticated ungulates.

    

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4. Synchronous Marine and Terrestrial Carbon Cycle Perturbation in the High Arctic During the PETM

   https://doi.org/10.1029/2020PA003942  

   Cui, Ying ; Diefendorf, Aaron F. ; Kump, Lee R. ; Jiang, Shijun ; Freeman, Katherine H. ( April 2021 , Paleoceanography and Paleoclimatology)

   The Paleocene‐Eocene Thermal Maximum (PETM; 56 Ma) is considered to be one of the best analogs for future climate change. The carbon isotope composition (δ¹³C) ofn‐alkanes derived from leaf waxes of terrestrial plants and marine algae can provide important insights into the carbon cycle perturbation during the PETM. Here, we present new organic geochemical data and compound‐specific δ¹³C data from sediments recovered from an early Cenozoic basin‐margin succession from Spitsbergen. These samples represent one of the most expanded PETM sites and provide new insights into the high Arctic response to the PETM. Our results reveal a synchronous ∼−6.5‰ carbon isotope excursion (CIE) in short‐chainn‐alkanes (nC₁₉; marine algae/bacteria) with a ∼−5‰ CIE in long‐chainn‐alkanes (nC₂₉andnC₃₁; plant waxes) during the peak of the PETM. Although δ¹³C_n‐alkanesvalues were potentially affected via a modest thermal effect (1‰–2‰), the relative changes in the δ¹³C_n‐alkanesremain robust. A simple carbon cycle modeling suggests peak carbon emission rate could be ∼3 times faster than previously suggested using δ¹³C_TOCrecords. The CIE magnitude of both δ¹³C_n‐C19and δ¹³C_n‐C29can be explained by the elevated influence of¹³C‐depleted respired CO₂in the water column and increased water availability on land, elevatedpCO₂in the atmosphere, and changes in vegetation type during the PETM. The synchronous decline in δ¹³C of both leaf waxes and marine algae/bacteria argues against a significant contribution to the sedimentary organic carbon pool from the weathering delivery of fossiln‐alkanes in the Arctic region.

    

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5. Miocene C 4 Grassland Expansion as Recorded by the Indus Fan

   https://doi.org/10.1029/2020PA003856  

   Feakins, Sarah J. ; Liddy, Hannah M. ; Tauxe, Lisa ; Galy, Valier ; Feng, Xiaojuan ; Tierney, Jessica E. ; Miao, Yunfa ; Warny, Sophie ( June 2020 , Paleoceanography and Paleoclimatology)

   In the late Miocene, grasslands spread across the forested floodplains of the Himalayan foreland, but the causes of the ecological transition are still debated. Recent seafloor drilling by the International Ocean Discovery Program (IODP) provides an opportunity to study the transition across a larger region as archived in the Indus submarine fan. We present a multiproxy study of past vegetation change based on analyses of the carbon isotopic composition (δ¹³C) of bulk organic carbon, plant waxn‐alkanes andn‐alkanoic acids, and quantification of lignin phenols, charcoal, and pollen. We analyze the hydrogen isotopic composition (δD) of plant wax to reconstruct precipitation δD. We use the Branched and Isoprenoid Tetraether (BIT) index to diagnose shifts between terrestrial versus marine lipid inputs between turbidite and hemipelagic sediments. We reconstruct ocean temperatures using the TEX₈₆index only where marine lipids dominate. We find evidence for the late Miocene grassland expansion in both facies, confirming this was a regional ecosystem transformation. Turbidites contain dominantly terrestrial matter from the Indus catchment (D‐depleted plant wax), delivered via fluvial transport as shown by the presence of lignin. In contrast, hemipelagic sediments lack lignin and bear D‐enriched plant wax consistent with wind‐blown inputs from the Indian peninsula; these show a 7.4–7.2 Ma expansion of C₄grasslands on the Indian subcontinent. Within each facies, we find no clear change in δD values across the late Miocene C₄expansion, implying consistent distillation of rainfall by monsoon dynamics. Yet, a cooling in the Arabian Sea is coincident with the C₄expansion.

    

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