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Northwest Africa transitioned from a wet/vegetated landscape toward drier/sparser conditions sometime between the late‐Pliocene and the late‐Pleistocene. However, our understanding of the precise timing and nature of this transition is hampered by a paucity of paleo‐records which bridge these two intervals. Here we report new plant‐wax isotope as well as dust and opal flux records from the relatively brief interval ∼1.1–1.0 million years ago (Ma) to evaluate the astronomical timescale controls of Northwest African hydroclimate and vegetation during the Mid‐Pleistocene Transition (MPT) and, in context with published records, the drivers of long‐term climate and ecological trends over the Plio‐Pleistocene. The tempo and amplitude of the Northwest African monsoon rainfall swings closely track low latitude insolation forcings over the last 5 Ma. However, we demonstrate that a pronounced mean state decline in monsoon strength likely occurred following the MPT most likely instigated by increasing Atlantic meridional sea surface temperature gradients or declines in the strength of the meridional overturning circulation. The northward extent of vegetation does not track changes in monsoon strength over the Plio‐Pleistocene and thus may be more strongly influenced by changes in monsoon rainfall extent or ecosystem disturbances. Progressively diminished dust fluxes following a decline in monsoon strength after 1.0 Ma is consistent with reduced production and subsequent depletion of fine‐grained sediments in the Sahara. Synchroneity between dust and opal fluxes across timescales suggests nutrient delivery to the surface ocean via dust plays a key role in marine primary productivity off the coast of Northwest Africa. Northwest African monsoon intensity declined substantially sometime between 1.0 and 0.5 MaRainfall penetration or disturbances may be more consequential than monsoon strength for ecosystem biogeography over 105–106yearsSedimentary dust and opal fluxes drop following monsoon intensity decline, linking dust production, dust flux, and marine productivity Northwest African monsoon intensity declined substantially sometime between 1.0 and 0.5 Ma Rainfall penetration or disturbances may be more consequential than monsoon strength for ecosystem biogeography over 105–106years Sedimentary dust and opal fluxes drop following monsoon intensity decline, linking dust production, dust flux, and marine productivity

The hydrogen (δDwax) and carbon (δ13Cwax) isotope compositions of long‐chain alkanes derived from plant waxes record hydrological and environmental conditions. However, the integration of plant n‐alkanes into the sedimentary cycle, the variability of δDwaxand δ13Cwaxin soils, and the paleoclimate applicability in paleosols and archaeological sediments are poorly constrained. We sampled plants and soils across a steep climate transect in Israel to understand how plant type and environmental parameters shape δ13Cwaxand δDwax. This transect has three advantages: existence of long‐term precipitation isotopic composition (δDr) records, a single wet season potentially reduces variability due to seasonality, and abandoned Byzantine period (~300–600 AD) agricultural terraces that reduce modern and ancient soil mixing and provide age constraints. We find that soil δ13Cwaxis constant (0.4‰, 1σ) across a 500‐ to 1,300‐mm/year rainfall gradient and appears insensitive to rainfall amount, unlike bulk plant δ13C. The absence of a rainfall effect suggests that δ13Cwaxmay be better suited to reconstructing C3/C4plant ratios than bulk δ13C. Homologue average soil δDwaxsignificantly correlate with δDr, and the offset between δDrand soil δDwax(εapp) correlates with growing season relative humidity. The seasonality of leaf production accounted for at most ~10% of total plant δDwaxvariability. Lastly, soil δDwaxand δ13Cwaxvariability is reduced by ~80% relative to plant δDwaxand δ13Cwaxvariability. Our results show that soil δDwaxand δ13Cwaxfaithfully record δDrand landscape C3‐C4plant contributions and thus support the utility of these proxy data in paleosols and archaeological sites. The influence of climate on biological and social human evolution has been an important research topic in recent decades. Isotopic analysis of leaf wax molecules from soil horizons and sediments from paleontological and archaeological sites can potentially offer climatic and environmental context directly associated with the sites. We conducted a calibration test across a climate transect in Israel to understand how plant type and environmental parameters impact the isotopic composition of leaf wax molecules in soils. Our results show that leaf wax carbon isotopes in soils are constant in the range of 500 to 1,300 mm/year and thus are suitable for reconstructing the contribution of C3and C4plants to the soils. The leaf wax hydrogen isotopes in soils are correlated with the isotopic composition of rainfall and the relative humidity of the growing season. Our results support the utility of these proxy data in paleosols and archaeological sites. We measured leaf wax hydrogen (δDwax) and carbon (δ13Cwax) isotopes in plants and soils across a climate transect in IsraelIn areas containing only C3plants (rainfall 500 to 1,300 mm/year), soil δ13Cwaxis constantSoil δDwaxcorrelates with rainfall isotopic composition, and the rainfall to leaf wax isotopic offset correlates with relative humidity

Antarctic ice sheet margin extent and the sensitivity of benthic δ18O to orbital forcing have varied on million‐year timescales during the Oligocene to Early Miocene. However, few sea surface temperature (SST) records for this time interval exist to evaluate links between polar processes and mean temperature outside polar regions. Here, we present a new record of SST for the time interval 30 to 17 Ma derived from the long‐chain alkenone unsaturation ratio ( U37k′) at Integrated Ocean Drilling Program Site 1406A in the midlatitude North Atlantic. Results confirm that warm temperatures from 24°C to over 30°C prevailed in midlatitudes in this time and suggest a transition from colder early‐middle Oligocene to warmer average conditions after 24.5 Ma. The global significance of this transition is highlighted by the coincidence with changes in the dominance from marine‐ to terrestrial‐terminating ice sheets in the Ross Sea around Antarctica. The longest continuous section of the record (20.6 to 26.6 Ma) contains multiple 2 million‐year cycles in SST, potentially paced by long obliquity modulation. Complex and temporally varying relationships are observed between North Atlantic SST and benthic δ18O in paired samples; significant covariation is only observed around the Oligocene‐Miocene transition, coincident with a lower average marine ice extent. These North Atlantic U37k′temperature records provide a new context in which to examine the stability of climate and the Antarctic ice sheet during the Oligocene and early Miocene. Transition from colder early‐middle Oligocene to warmer average conditions after 24.5 MaComplex and temporally varying relationships of global significance of paired temperature and benthic δ18O reconstructions

Grasslands expanded globally during the late Cenozoic and the development of these ecosystems shaped the evolution of many faunal groups, including our hominin ancestors. The emergence of these ecosystems has been dated in many regions, but the origins of the iconic African C4savannah grasslands remain poorly known, as do the causal factors that led to their establishment. Here we document their origins with the distinct carbon isotope signature from the hot-, arid- and low-CO2-adapted C4grasses that dominate modern savannahs and grasslands. We use the carbon isotope values of leaf-wax molecules in deep-sea drill cores to measure the rise of African C4ecosystems. We also reconstruct African palaeohydroclimate change from leaf-wax hydrogen isotope values and dust deposition rates in these cores. We find that C4-dominated ecosystems expanded synchronously across Northwestern and East Africa after 10 million years ago. This was not accompanied by substantial changes in palaeohydrology or dust deposition, precluding aridification as a causal factor. The expansion of C4grasses was coincident, however, with dramatic high-latitude cooling and increased pole–Equator temperature gradients. We suggest that declining atmospheric CO2levels were a direct cause of the C4grassland expansion.

Since the late Miocene, plants using the C4photosynthetic pathway have increased to become major components of many tropical and subtropical ecosystems. However, the drivers for this expansion remain under debate, in part because of the varied histories of C4vegetation on different continents. Australia hosts the highest dominance of C4vegetation of all continents, but little is known about the history of C4vegetation there. Carbon isotope ratios of plant waxes from scientific ocean drilling sediments off north‐western Australia reveal the onset of Australian C4expansion at ~3.5 Ma, later than in many other regions. Pollen analysis from the same sediments reveals increasingly open C3‐dominated biomes preceding the shift to open C4‐dominated biomes by several million years. We hypothesize that the development of a summer monsoon climate beginning in the late Pliocene promoted a highly seasonal precipitation regime favorable to the expansion of C4vegetation. This study documents for the first time that C4vegetation initially expanded on the Australian continent in the late Pliocene, several million years later than in Asia, Africa, North America, and South America. The expansion of C4plants displaced C3open habitat vegetation. Understanding the timing and sequence of expansion of C4‐dominated biomes enables us to better constrain the key environmental and evolutionary factors in their development and provides a basis for future conservation of these widespread and important biomes. Carbon isotope ratios of plant waxes reveal the onset of C4expansion during the late Pliocene in Australia, later than other geographic regionsPalynological analysis reveals increasingly open landscapes in the lead‐up to C4expansionNorthern Australian monsoon initiation linked to East Asian winter monsoon intensification is hypothesized as a driver

El Niño–Southern Oscillation (ENSO) dominates interannual climate variability; thus, understanding its response to climate forcing is critical. ENSO's sensitivity to changing insolation is poorly understood, due to contrasting interpretations of Holocene proxy records. Some records show dampened ENSO during the early to mid‐Holocene, consistent with insolation forcing of ENSO amplitude, but other records emphasize decadal‐centennial fluctuations in ENSO strength, with no clear trend. To clarify Holocene ENSO behavior, we collected proxy data spanning the last ~12 kyr and find relatively low El Niño amplitude during the early to mid‐Holocene. Our data, together with published work, indicate both a long‐term trend in ENSO strength due to June insolation forcing and high‐amplitude decadal‐centennial fluctuations; both behaviors are shown in models. The best supported mechanism for insolation‐driven dampening of ENSO is weakening of the upwelling feedback by insolation‐forced warming/deepening of thermocline source waters. Elucidating the thermocline's role will help predict future ENSO change. Our new data show a reduced amplitude of El Nino events during the early and mid‐HoloceneOverall, proxy data supports both a June insolation‐forced trend in ENSO strength and decadal‐centennial variability in ENSO strengthThe best supported mechanism for insolation forcing of Holocene ENSO is a weaker upwelling feedback, due to a warmer/deeper thermocline

Methods are described for studying the metabolic cost of increased and diminished gravitational work done by the human subject during normal locomotion at various speeds and slopes on the treadmill. It is shown that the energy expenditure is a linear function of the gravitational work and, as long as the gait is of a smooth and natural character, appears to be dependent upon the true vertical lift per step multiplied by the number of steps per minute. The true vertical lift is defined as the lift resulting from muscle action, as contrasted with components due to treadmill motion. Methods are also described for recording the vertical and translational motions of the torso during a single step, and for analyzing the flow of mechanical energy into and out of the torso during each phase of the walking cycle. Implications for calculation of efficiency are briefly discussed.

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 correlated 13C 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. Alkylated polycyclic aromatic hydrocarbon (PAH) distributions can constrain relative pyrogenic and petrogenic sources in settings with mixed recalcitrant inputsCarbon isotopes of PAHs indicate that C4grasses preferentially burned relative to C3vegetation on Indian SubcontinentFire both facilitated and maintained open C4grasslands at local and subcontinental scales Alkylated polycyclic aromatic hydrocarbon (PAH) distributions can constrain relative pyrogenic and petrogenic sources in settings with mixed recalcitrant inputs Carbon isotopes of PAHs indicate that C4grasses preferentially burned relative to C3vegetation on Indian Subcontinent Fire both facilitated and maintained open C4grasslands at local and subcontinental scales

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. Late Miocene expansion of C4grasslands in the Bengal Fan catchment followed prior ecological change from evergreen to dry deciduous forestsSynthesis of isotopic and ecological records from Himalayan foreland basin reveals ecological change and multifaceted hydrologic change preceded C4grassland expansionSeasonal reduction in rainfall and increased evaporation promoted ecosystem change, accelerated by ecological feedbacks Late Miocene expansion of C4grasslands in the Bengal Fan catchment followed prior ecological change from evergreen to dry deciduous forests Synthesis of isotopic and ecological records from Himalayan foreland basin reveals ecological change and multifaceted hydrologic change preceded C4grassland expansion Seasonal reduction in rainfall and increased evaporation promoted ecosystem change, accelerated by ecological feedbacks

C4grasslands proliferated later in Australia than they did on other continents (∼3.5 Ma vs. 10–5 Ma). It remains unclear whether this delay reflects differences in climate conditions or ecological feedbacks, such as fire, that promote C4ecosystems. Here, we evaluated these factors using terrestrial biomarkers from marine sediments off western Australia. Fire‐derived polycyclic aromatic hydrocarbons (PAH) indicate fire ecology did not substantially change during or following C4expansion. The presence of fire‐adapted C3woody vegetation likely diminished the role of fire and delayed C4expansion until it was prompted by climate drying between 3.5 and 3.0 Ma. At the same time, mass accumulation rates of weathered PAHs increased 100‐fold, which indicates a significant loss of soil carbon accompanied this ecosystem shift. The tight couplings between hydroclimate and carbon storage altered boundary conditions for Australian ecosystems, and similar abrupt behavior may shape environmental responses to climate change. Today, grassland ecosystems are characterized by frequent fires. This study tested the role of fire in the establishment of these ecosystems 3 Myr ago in Australia using molecular fossils. Results show that fire occurrence did not drastically change as C4ecosystems were established. Instead, changes in rainfall caused massive amounts of soil carbon to be transported off of the continent, which may have favored C4plants. These dynamics are different than what is seen on other continents, but have important implications for how Australian ecosystems may respond to future climate changes. Molecular markers indicate that fire preexisted in the Australia system and did not further increase with C4grassland expansionA 100‐fold increase in weathered Polycyclic aromatic hydrocarbons at ∼3.5 Ma indicates increased soil carbon remobilization off AustraliaIncreasingly variable hydrology drove both the expansion of C4biomes and diminished carbon storage in the Australian terrestrial system Molecular markers indicate that fire preexisted in the Australia system and did not further increase with C4grassland expansion A 100‐fold increase in weathered Polycyclic aromatic hydrocarbons at ∼3.5 Ma indicates increased soil carbon remobilization off Australia Increasingly variable hydrology drove both the expansion of C4biomes and diminished carbon storage in the Australian terrestrial system