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 wax
Combining magnetostratigraphic and carbon isotopic data for the late Miocene can provide a temporal framework for an isotopic shift first documented in soil carbonate nodules of northern Pakistan. The shift has been interpreted as a change in vegetation from trees and shrubs (using the C3photosynthetic pathway) to grassland (using the C4pathway). The cause of the event has been hotly debated and its timing is close to a shift in carbon isotopes in the marine realm. Further understanding depends critically on chronology. Unfortunately, temporal calibration of the various records published over decades relied on different time scales. To address the lack of a consistent chronology, we have reevaluated the constraints for the carbon isotopic shifts recorded from the Indian subcontinent. These show a diachronous transition ranging in age from about 7.8 Ma in Pakistan to as late at 6 Ma in Nepal. The record from IODP Expedition 355 Site U1457, drilled on the Indus fan shows that the transition in peninsular India began at about 7.2 Ma. Similar records from the African margin saw an earlier shift to C4dominance starting around 10 Ma and those from Australia and South America transitioned later, during the Pliocene. The diachroneity around the globe does not invalidate pCO2as a driver, but is consistent with it being one of several drivers of the global C4expansion.
more » « less- NSF-PAR ID:
- 10456607
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Paleoceanography and Paleoclimatology
- Volume:
- 35
- Issue:
- 7
- ISSN:
- 2572-4517
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract n ‐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. -
more » « less
Abstract Modern tropical and subtropical C4grasslands and savannas were established during the late‐Miocene and Pliocene, over 20 Myr after evolutionary originations of the C4photosynthetic pathway. This lag suggests environmental factors first limited and then favored C4plants. Here, we examine the timing and drivers for the establishment of C4grasslands on the Indian Subcontinent using carbon and hydrogen isotope signatures of plant‐wax
n ‐alkanes recovered from turbidites in the Bengal Fan. Like prior studies, we find C4ecosystems in the Ganges‐Brahmaputra catchment first emerged at 7.4 Ma and subsequently expanded between 6.9 to ∼6.0 Ma. Hydrogen isotope values varied from 10.2 to 7.4 Ma and then increased after 7.4, which suggests intermittent drying began before the establishment of C4grasslands with further drying at the onset of C4expansion. Synthesis of published plant fossil data from the Siwalik Group of the Himalayan foreland basin documents an ecosystem trajectory from evergreen tropical forests to seasonally deciduous forests, and then expansive C4grasslands. This trajectory coincided with a seasonally uneven drying trend due to both increased evaporation of plant leaf and soil waters and reduced rainfall, as identified in soil carbonate and tooth enamel data sets. Collectively the fossil, biomarker, and isotopic evidence reveal the development of modern C4ecosystems on the Indian Subcontinent followed a series of ecosystem transformations driven by drying and fire feedbacks, and possibly declining atmospheric pCO2, beginning at 10.2 Ma and strengthening through the late Miocene. -
more » « less
New approaches to the study of early hominin diets have refreshed interest in how and when our diets diverged from those of other African apes. A trend toward significant consumption of C4foods in hominins after this divergence has emerged as a landmark event in human evolution, with direct evidence provided by stable carbon isotope studies. In this study, we report on detailed carbon isotopic evidence from the hominin fossil record of the Shungura and Usno Formations, Lower Omo Valley, Ethiopia, which elucidates the patterns of C4dietary utilization in the robust hominin
Paranthropus . The results show that the most important shift toward C4foods occurred at ∼2.37 Ma, within the temporal range of the earliest known member of the genus,Paranthropus aethiopicus , and that this shift was not unique toParanthropus but occurred in all hominins from this fossil sequence. This uptake of C4foods by hominins occurred during a period marked by an overall trend toward increased C4grazing by cooccurring mammalian taxa from the same sequence. However, the timing and geographic patterns of hominin diets in this region differ from those observed elsewhere in the same basin, where environmental controls on the underlying availability of various food sources were likely quite different. These results highlight the complexities of dietary responses by hominins to changes in the availability of food resources. -
more » « less
Abstract Paleosols preserved in the Red Clay depositional sequence of the Chinese Loess Plateau record information about vegetation and regional hydrology responses to global temperature variation throughout the late Miocene and Pliocene. Reconstructing spatial and temporal patterns of environmental change across the Loess Plateau from carbon isotopes of pedogenic carbonate (δ13Ccarb) is complicated because multiple factors affect δ13Ccarbvalues and higher resolution records do not exist along the northern margin of the Loess Plateau. To address these needs, we present paired carbon isotope records of pedogenic carbonate and occluded organic matter (δ13Corg) from 697 discrete nodules sampled from 119 different depths at the Jiaxian section, North Central China. Between 7.6 and 2.4 Ma, δ13Ccarbvalues increase by nearly 5‰, while δ13Corgvalues increase by 2.5‰. These increases are explained by a progressive decline in moisture availability through time, and there is no definitive evidence from these δ13C data for C4vegetation at the Jiaxian site until after 3.6 Ma. Comparison of the Jiaxian record to other Loess Plateau sections reveals a consistent spatial gradient with δ13Ccarbvalues becoming higher and more variable to the N‐NW. Additionally, an independent index of monsoonal precipitation from a southern site corresponds to fluctuations in δ13Ccarbvalues at Jiaxian, while southern δ13Ccarbrecords remain more stable. These spatial patterns are explained by a progressive decline in moisture availability across the Loess Plateau through the Late Miocene and Pliocene, with δ13Ccarbvalues being more sensitive to moisture availability under consistently more arid conditions to the NW.
-
more » « less
Societal Impact Statement 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.
Summary Extinct hippopotamuses (
Hippopotamus spp.) 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.
