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The stratotype section for the base of the Miocene is at a reversed (below) to normal (above) magnetic transition that is claimed to represent magnetic chron C6Cn.2n (o). Deep Sea Drilling Project (DSDP) Site 522 is the only location we are aware of that unambiguously records the three normal events of C6Cn. We have quantitatively determined the range of the short-lived nannofossil Sphenolithus delphix and the lower limit of S. disbelemnos in DSDP Holes 522 and 522A in order to calibrate their precise relationship to the magnetostratigraphy and to confirm the completeness of the record at this site. Astronomical tuning of Ocean Drilling Program (ODP) Sites 926, 928, and 929 shows that S. disbelemnos appears at 22.67 Ma and that the entire range of S. delphix is from about 22.98 Ma to 23.24 Ma. Using these ages, linear interpolation in DSDP Site 522 suggests that the age of C6Cn.2n (o) and of the Oligocene-Miocene boundary is 22.92 ± 0.04 Ma. Our value, conservatively expressed as 22.9 ± 0.1 Ma, is 0.9 m.y. younger than the currently accepted age of the Oligocene-Miocene boundary and of C6Cn.2n (o), which was assigned an age of 23.8 Ma, based on an estimate of 23.8 ± 1 Ma for the Oligocene-Miocene boundary. The bulk-sediment carbon isotope data from DSDP Site 522 is correlated to the record from benthic foraminifera at ODP Site 929 to refine the calibration of magnetic reversals from C6Cn.1n (o) to C7n.2n (o) at DSDP Site 522 on the astronomical time scale.

High-resolution (-3-4 kyr) stable isotope stratigraphies from sites drilled along a depth transect on Ceara Rise (Ocean Drilling Program Leg 154, Sites 925 and 929) are used to reconstruct the deep water circulation response to the long- and short-term climate changes of the early Pliocene (3.2-4.7 Ma). Over the long term, benthic foraminiferal carbon isotope records show that the vertical fil3c gradient in this region was similar to that of the late Holocene, implying a steady flux of Northern Component Deep Water (NCDW) into the deep Atlantic during most of the early Pliocene. The vertical benthic foraminiferal oxygen isotope gradient is reversed with respect to that of the late Holocene. On the basis of density constraints imposed by seawater stability along this depth transect we attribute the reversed gradient to warmer and more saline NCDW (5oC and 35.1). On orbital timescales we find that the phase relationship between fi180 and fil3c values at the deeper Site 929 differs from the late Pliocene/Pleistocene, while that at the shallower Site 925 was essentially the same.

Characterizing polar climates during past warm intervals is important for understanding ‘greenhouse’ climate dynamics because high-latitude surface temperatures and precipitation patterns are extremely sensitive to global climatic conditions. Model-data comparisons of high-latitude climates during past warm intervals (Cretaceous–Eocene, Pliocene) are currently at odds. Specifically, simulations of past warm climates produce polar regions characterized by sub-freezing temperatures and significant seasonality, whereas limited fossil proxy data indicate higher mean annual temperatures and low seasonality (i.e. an equable climate). We have constructed a data set to infer northern hemisphere polar marine temperatures during the late Paleocene. Seasonal and mean annual temperature and the oxygen isotopic composition of precipitation are reconstructed for an Arctic coastal setting during the Thanetian (57–58 Ma) using fossil shell stable isotope profiles. We estimate that coastal water temperatures varied between 10 and 15°C during the seasons of growth, presumably spring, summer and fall. These findings support paleontological evidence, implying Northern Hemisphere polar climates were seasonally warm during the late Paleocene. In addition, estuarine fossil oxygen isotope profiles show periodic excursions to low values (as low as −19‰ VPDB), which indicate seasonal pulses of isotopically-depleted freshwater.

Exposed near Braggs, Alabama, is one of the few well-studied, nearly continuous shallow-marine Cretaceous/Tertiary boundary sections; it allows a glimpse of the biotic and environmental changes that occurred in the latest Cretaceous to earliest Paleocene. Paleomagnetic, strontium isotopic, and biostratigraphic data closely constrain the age of a series of lithologic, geochemical, and biotic variations and suggest that no more than 100–200 ka could be missing at the boundary. A major reduction in macrofaunal diversity associated with lithofacies changes occurs prior to but within 300 ka of the nannofossil-defined boundary. Approximately 40% of the apparent faunal reduction is attributed to the “Lazarus effect.” Faunal and floral assemblages, trends in carbon isotopic composition of benthic invertebrates, and lithologic characteristics indicate that a latest Maestrichtian regression culminated near the boundary (Chron C29R; Micula murus zone), significantly later than recent estimates. Water depths at this site remained shallow during the subsequent early Paleocene (zone NP1) transgression and did not reach depths equivalent to those of the late Maestrichtian until zone NP2. Relatively minor climatic changes across the boundary are suggested by a ≤4 °C cooling trend seen in the oxygen-isotope paleotemperatures. A high-resolution 87 Sr/ 86 Sr record from well-preserved macrofossil calcite shows a pattern of smooth variation and elevated values near the boundary; however, the early Paleocene “spike” of other workers was not found.