Erosion and reworking of Pacific sediments near the Eocene‐Oligocene boundary

Abstract

[1] The Eocene-Oligocene (E/O) boundary interval marks one of the largest and most rapid changes in climate during the last 50 Myr. Because of a very shallow calcium carbonate compensation depth in the Eocene, as well as the reworking of sediments and hiatuses in the boundary zone, it has also been one of the most difficult stratigraphic boundaries to study in deep water marine sections, especially in the Pacific Ocean. Recently, three drill sites have recovered complete sections of the E/O boundary interval in the tropical Pacific. A detailed study of these sections shows a series of pulses of reworked older radiolarians in the upper Eocene and lowermost Oligocene. The two largest pulses are coincident with the two sharp steps in carbon and oxygen isotope values that bracket the E/O boundary. Several smaller peaks in reworked material precede these two maxima. It is proposed that immixing of the older radiolarian species results from erosion and redeposition, possibly linked to pulses of new bottom water formation and the formation of a deep pycnocline.

Radiolarian stratigraphy near the Eocene–Oligocene boundary

“…Most of the average LAD ages from Nigrini et al (2006) lie slightly to the left (younger) side of the plot of the revised datum ages. We attribute these small offsets to differences in the Eocene and Oligocene time scale used in this study, to the reworking of older Eocene radiolarians into younger sediments (Moore, 2013), or to the more closely spaced sampling in the Moore and Kamikuri (2012) study. A few of the datums defined by Nigrini et al (2006) are substantially offset from the plot of revised datum ages.…”

“…Of the sites studied here, Site U1334 appears, on average, to have the fewest immixed older microfossils in the samples studied (Moore, 2013). Here we take the last continuous occurrence of a species to be its LAD, relying primarily on the last continuous occurrence of the species in Site U1334.…”

“…Coxall et al (2005) determined that Site 1218 shows a characteristic "two-step" shift in all major lithostratigraphic and isotopic variables across the E/O boundary, which they linked to a two-phase development of the Antarctic ice sheets (DeConto and Pollard, 2003); but, of the three sites studied by Nigrini et al (2006), only Site 1218 is regarded as having a complete section. The other two sites studied have short hiatuses across the E/O boundary, and all three sites show a substantial amount of reworked radiolarians in samples near this boundary, especially in the latest Eocene Moore and Kamikuri, 2012;Moore, 2013). This made the definition of the LADs particularly difficult for many Eocene species that became extinct near the E/O boundary.…”

“…Although the stratigraphic sections of all three sites studied are judged to be complete, their sediments appear to contain varying amounts of reworked older microfossils Moore, 2013). Many radiolarian species become extinct near the close of the Eocene, but radiolarian stratigraphy of the uppermost Eocene has always been plagued by the fact that the reworking of older microfossils makes the exact level of extinction of these species suspect.…”

“…Acknowledging the difficulties raised by the presence of reworked older radiolarians near the E/O boundary interval (Moore, 2013), they also redefined the base of RP19 (Cryptocarpium ornatum interval zone) as determined by the first occurrence of Lophochytris (C.) hadra (rather than the last occurrence of Thyrsocyrtis (P.) tetracantha). In the revised zonation, all of the boundaries are now based on FADs or the transition from the ancestor to the descendent species, which avoids much of the difficulty in using LADs for such definitions in this interval of common hiatuses and reworking of older microfossils (Moore, 2013). These boundaries along with the paleomagnetic chrons and the more reliable radiolarian first occurrences are shown in Fig.…”