Bulk stable carbon and oxygen isotopes from ODP Site 1258, and Fe intensities from ODP Site 1262, and minimal tuning age model options for ODP Site 1258 and 1262

Timing is crucial to understanding the causes and consequences of events in Earth history. The calibration of geological time relies heavily on the accuracy of radioisotopic and astronomical dating. Uncertainties in the computations of Earth's orbital parameters and in radioisotopic dating have hampered the construction of a reliable astronomically calibrated time scale beyond 40 Ma. Attempts to construct a robust astronomically tuned time scale for the early Paleogene by integrating radioisotopic and astronomical dating are only partially consistent. Here, using the new La2010 and La2011 orbital solutions, we present the first accurate astronomically calibrated time scale for the early Paleogene (47-65 Ma) uniquely based on astronomical tuning and thus independent of the radioisotopic determination of the Fish Canyon standard. Comparison with geological data confirms the stability of the new La2011 solution back to ~54 Ma. Subsequent anchoring of floating chronologies to the La2011 solution using the very long eccentricity nodes provides an absolute age of 55.530 {plus minus} 0.05 Ma for the onset of the Paleocene/Eocene Thermal Maximum (PETM), 54.850 {plus minus} 0.05 Ma for the early Eocene ash -17, and 65.250 {plus minus} 0.06 Ma for the K/Pg boundary. The new astrochronology presented here indicates that the intercalibration and synchronization of U/Pb and 40Ar/39Ar radiometric geochronology is much more challenging than previously thought.

Revised calibration of the MBT-CBT paleotemperature proxy

The MBT-CBT proxy for the reconstruction of paleotemperatures and past soil pH is based on the distribution of branched glycerol dialkyl glycerol tetraether (brGDGT) membrane lipids. The Methylation of Branched Tetraether (MBT) and the Cyclisation of Branched Tetraether (CBT) indices were developed to quantify these distributions, and significant empirical relations between these indices and annual mean air temperature (MAT) and/or soil pH were found in a large data set of soils. In this study, we extended this soil dataset to 278 globally distributed surface soils. Of these soils, 26% contains all nine brGDGTs, while in 63% of the soils the seven most common brGDGTs were detected, and the latter were selected for calibration purposes. This resulted in new transfer functions for the reconstruction of pH based on the CBT index: pH = 7.90-1.97 × CBT (r**2 = 0.70; RMSE = 0.8; n = 176), as well as for MAT based on the CBT index and methylation index based on the seven most abundant GDGTs (defined as MBT'): MAT = 0.81-5.67 × CBT + 31.0 × MBT' (r**2 = 0.59; RMSE = 5.0 °C; n = 176). The new transfer function for MAT has a substantially lower correlation coefficient than the original equation (r**2 = 0.77). To investigate possible improvement of the correlation, we used our extended global surface soil dataset to statistically derive the indices that best describe the relations of brGDGT composition with MAT and soil pH. These new indices, however, resulted in only a relatively minor increase in correlation coefficients, while they cannot be explained straightforwardly by physiological mechanisms. The large scatter in the calibration cannot be fully explained by local factors or by seasonality, but MAT for soils from arid regions are generally substantially (up to 20 °C) underestimated, suggesting that absolute brGDGT-based temperature records for these areas should be interpreted with caution. The applicability of the new MBT'-CBT calibration function was tested using previously published MBT-CBT-derived paleotemperature records covering the last deglaciation in Central Africa and East Asia, the Eocene-Oligocene boundary and the Paleocene-Eocene thermal maximum. The results show that trends remain similar in all records, but that absolute temperature estimates and the amplitude of temperature changes are lower for most records, and generally in better agreement with independent proxy data.

Radiometric age determinations for basements of ODP Sites 123-765 and 123-766

The results of experiments in 40Ar/39Ar age dating using fresh basement material from Sites 765 and 766 of Leg 123 of the Ocean Drilling Program are inconsistent and cannot be used to constrain the basement age of the Argo Abyssal Plain in the Indian Ocean. However, a celadonite sample, which was precipitated during a low-temperature alteration event that affected the basement at Site 765, yielded a K-Ar age of 155.3 ±3.4 Ma. Celadonites, which have been dated using Rb-Sr methods for basement in the Atlantic Ocean (Staudigel et al., 1981, doi:10.1016/0012-821X(81)90186-2) and by K-Ar methods for the Troodos Ophiolite (Staudigel et al., 1986, doi:10.1130/0091-7613(1986)14<72:AASAOC>2.0.CO;2), and for sediments from the Pacific Ocean (Peterson et al., 1986, doi:10.2973/dsdp.proc.92.132.1986) yield ages that are up to 15 Ma younger than the age for the formation of basement. Thus, the celadonite age is retained as a reliable minimum age for basement at Site 765. This radiometric age is inconsistent with biostratigraphic ages, which indicate a maximum of late Berriasian (approximately 140 Ma) for Site 765, but is consistent with geophysical interpretations of marine magnetic anomalies and with the early north-south seafloor spreading history of the Argo Abyssal Plain region of the Indian Ocean.

Geochemistry, mineralogy, and 40K-40Ar radiometric dating of DSDP Leg 84 basalts - Guatemala Trench

Geochemical (atomic absorption, neutron activation analyses), mineralogical (microprobe), and radiometric (40K - 40Ar) data are presented for five basalts from the Guatemala Trench area (Deep Sea Drilling Project, Leg 84). Strong geochemical and mineralogical differences distinguish two types among these basalts: (1) One basalt (Sample 567A-19,CC), recovered below Upper Cretaceous limestone has the following characteristics: it is quartz normative and has low TiO2, content, as well as low amounts of Cr, Ni and other transition metals, an LREE depleted pattern, and affinities of clinopyroxene phenocryst plotted into the field of tholeiitic and calc-alkalic pyroxenes. (2) Four alkaline basalts, recovered from the mafic and ultramafic acoustic basement, are nepheline normative and show high TiO2 content, high amounts of Cr, Ni and so on, an LREE enriched pattern and compositions of clinopyroxene phenocryst plotted close to or within the field of alkali basalt pyroxenes. These basalts are comparable to those recognized in the lower part of the Santa Elena complex and are clearly different from the oceanic basalts of the Cocos Plate. The radiometric age of the orogenic basalt seems to be close to 80 Ma. The alkaline basalts are clearly older, even if a discrepancy appears between the results of different analyses because of the secondary effects of alteration.

Orbital control on carbonate-lignite cycles on sediment core KAP-107 in the Ptolemais Basin, northern Greece

Time control is essential for the reconstruction of geological processes. We use a combination of relative and absolute methods to establish the chronology and related paleoclimatic processes for Late Neogene lacustrine sediment from the Ptolemais Basin, northern Greece. We determined changes in magnetic polarity and correlated them to the global magnetic polarity time scale, which again is calibrated by radiometric methods, to provide a low-resolution age model for the Upper Miocene to Lower Pliocene (7 - 3 Ma). Sedimentary successions show rhythmic alterations of lignites, clays, and marls. Using photospetrometry we measured this variability at 1-cm resolution, and correlated the pattern to known changes in earth's orbital parameters, namely to eccentricity and precession. For 230-m long borehole KAP-107 from the Amynteon Sub-Basin we obtained a high-resolution age model that spans 2 myr from 5.1 to 3.1 Ma, with age control points at insolation maxima (20-kyr resolution). We recommend using photospectrometry as reliable tool to establish orbital-based chronologies and to reconstruct paleoclimate variability at high resolution.