Jason F. Hicks

The structure and rate of late Miocene expansion of C4 plants: Evidence from lateral variation in stable isotopes in paleosols of the Siwalik Group, northern Pakistan

This study uses stable isotope variation within individual Mio-Pliocene paleosols to investigate subkilometer-scale phytogeography of late Miocene vegetation change in southeast Asia between ca. 8.1 and 5 Ma, a time interval that coincides with dramatic global vegetation change. We examine trends through time in the distribution of low-latitude grasses (C4 plants) and forest (C3 plants) on Indo-Gangetic floodplains using carbon (δ13C) and oxygen isotopic (δ18O) values in buried soil carbonates in Siwalik Series sediments exposed in the Rohtas Anticline, north-central Pakistan. Revised, high-resolution magnetostratigraphy and a new 40Ar/39Ar date provide improved age control for the 2020 m Rohtas section. Carbon isotope results capture lateral variability of C3 versus C4 plants at five stratigraphic levels, R11 (8.0 Ma), R15 (6.74–6.78 Ma), R23 (5.78 Ma), R29 (4.8–4.9 Ma), and upper boundary tuff (UBT; 2.4 Ma), using detailed sampling of paleosols traceable laterally over hundreds of meters. Paleosols and the contained isotopic results can be assigned to three different depositional contexts within the fluvial sediments: channel fill, crevassesplay, and floodplain environments. δ13C results show that near the beginning (8.0 Ma) and after (4.0 Ma) the period of major ecological change, vegetation was homogeneously C3 or C4, respectively, regardless of paleo-landscape position. In the intervening period, there is a wide range of values overall, with C4 grasses first invading the drier portions of the system (floodplain surfaces) and C3 plants persisting in moister settings, such as topographically lower channel swales. Although abrupt on a geologic timescale, changes in abundance of C4 plants are modest (∼2% per 100,000 yr) compared to rates of vegetation turnover in response to glacial and interglacial climate changes in the Quaternary. Earlier research documented a sharply defined C3 to C4 transition in Pakistan between 8.1 and 5.0 Ma, based on vertical sampling, but this higher-resolution study reveals a more gradual transition between 8.0 and 4.5 Ma in which C3 and C4 plants occupied different subenvironments of the Siwalik alluvial plain. δ18O values as well as δ13C values of soil carbonate increase up section at Rohtas, similar to isotope trends in other paleosol records from the region. Spatially, however, there is no correlation between δ13C and δ18O values at most stratigraphic levels. This implies that the changes in soil hydrology brought about by the shift from forest to grassland (i.e., an increase in average soil evaporation) did not produce the shift through time in δ18O values. We interpret the trend toward heavier soil carbonate δ18O values as a response to changes in external climatic factors such as a net decrease in rainfall over the past 9 Ma.

Magnetostratigraphy and geochronology of the Hell Creek and basal Fort Union Formations of southwestern North Dakota and a recalibration of the age of the Cretaceous-Tertiary boundary

To estimate the age and duration of the Maastrichtian Hell Creek Formation, an extensive program of paleomagnetic sampling was carried out on six surface sections that span the Maastrichtian and Paleocene interval on the southern portion of the Cedar Creek anticline in Slope and Bowman Counties in North Dakota. The magnetic polarity sequence measured can be correlated with confidence to that part of the geomagnetic polarity time scale that ranges from polarity subchron C30n through C29n. A revised age estimate for the Cretaceous-Tertiary boundary of 65.51 ± 0.10 Ma has been obtained by normalizing the most recently published isotopic ages for the boundary to a standard monitor age of 28.02 Ma for the Fish Canyon Tuff and 28.32 Ma for the Taylor Creek Rhyolite. Orbital chronology gives very precise estimates for the duration of C29r that range from 570 to 673 k.y. The most recently published estimate gives a value of 603 k.y. for C29r, with a level of precision that is much higher than ages obtained by conventional dating methods. By extrapolating the measured sediment accumulation rate of the Cretaceous portion of C29r (333 k.y.) through to the base of the Hell Creek, we estimate the Hell Creek Formation to be 1.36 m.y. in duration. If the range of precessional age estimates for C29r and the differences in thickness of the reversal between the sections are taken into account, then the duration of the Hell Creek could range from 1.05 to 1.90 m.y.

A New Calibration Point for the Late Cretaceous Time Scale: The

We present a new calibration point for the latest Cretaceous time scale, an interval that at present contains no well-dated polarity reversals. The magnetostratigraphy of Campanian-aged sediments in the northern Bighorn basin of Wyoming documents a geomagnetic reversal in the upper part of the Judith River Formation that occurs in direct association with ash fall layers dated by the

Isotopic Age of the C33r/C33n Geomagnetic Reversal from the Judith River Formation (Upper Creataceous), Elk Basin, Wyoming, USA

^{40}Ar/^{39}Ar

Research on late Pliocene Oldowan sites at Kanjera South, Kenya

method. Extrapolation based on three new

Magnetostratigraphy, isotopic age calibration and intercontinental correlation of the Red Bird section of the Pierre Shale, Niobrara County, Wyoming, USA

^{40}Ar/^{39}Ar