S. Cooke

Reconstructing past planktic foraminiferal habitats using stable isotope data: a case history for Mediterranean sapropel S5

Abstract A high-resolution stable O and C isotope study is undertaken on all planktic foraminiferal species that are reasonably continuous through an Eemian sapropel S5 from the western side of the eastern Mediterranean. The data are considered within a context of high-resolution isotope records for two further S5 sapropels from the central and easternmost sectors of the basin, alkenone-based sea surface temperature records for all three sapropels, and planktic foraminiferal abundance records for the same sample sets through all three sapropels. Results are compared with similar data for Holocene sapropel S1. The adopted approach allows distinction between species that are most suitable to assess overall changes in the climatic/hydrographic state of the basin, including depth-related differentiations and the main seasonal developments, and species that are most affected by variable biological controls or local/regional and transient physico–chemical forcings. It is found that a-priori assumptions about certain species’ palaeohabitats, based on modern habitat observations, may become biased when non-analogue conditions develop. In the case of Mediterranean sapropel S5, these consisted of enhanced freshwater dilution, elevated productivity, shoaling of the pycnocline between intermediate and surface waters, and stagnation of the subsurface circulation. Under these conditions, some species are found to ‘shift’ into habitat settings that differ considerably from those occupied today. The present multiple-species approach can identify such ‘anomalous responses’, and thus offers a sound background for further shell-chemistry investigations and quantitative interpretation of the isotopic profiles. We capitalise on the latter potential, and offer the first quantitative estimates of monsoon flooding into the Mediterranean during the deposition of Eemian sapropel S5.

High-resolution stratigraphic framework for Mediterranean sapropel S5: defining temporal relationships between records of Eemian climate variability

A high-resolution stratigraphic framework is presented for sapropel S5, which represents the low-mid latitude climate optimum of the previous interglacial period (Eemian). The framework is based on three sites along a transect from west to east through the eastern Mediterranean, and is further validated using a fourth site. This method allows expression of S5-based proxy records of Eemian climate variability along a standardised depth scale that offers unprecedented possibilities for assessment of spatial gradients and signal leads and lags in an interval where high-resolution (radiocarbon-style) dating cannot be performed. Our lateral comparison of S5 sapropels suggests that the onset of S5 in ODP site 967C (Eratosthenes seamount) was 1–6 centuries delayed relative to the onsets in more westerly sites.

Strontium isotope dating of the New Zealand Oligocene

Abstract One of the least well resolved portions of the New Zealand Cenozoic time‐scale is that centred on and about the Oligocene Epoch, internationally regarded as spanning c. 10 m.y. from 33.7 to 23.8 Ma. We have determined the 87Sr/86Sr ratios and derived absolute ages for 77 macrofossil samples collected from several biostratigraphically dated mid‐Tertiary sections in the South Auckland (North Island) and North Otago/South Canterbury (South Island) regions. While the site‐specific stratigraphic significance of our ages remains to be assessed, we present them here to foster wider consideration and discussion in relation to evolving absolute age schemes for the New Zealand Oligocene biostratigraphic stages. Initial results suggest: 1. The approximate boundary ages for the mid‐Tertiary Stages are: Runangan/Whaingaroan, 34.8 Ma; early Whaingaroan/late Whaingaroan, 31.0 Ma; Whaingaroan/ Duntroonian, 28.5 Ma; Duntroonian/Waitakian, 25.5 Ma; Waitakian/Otaian, 22.2 Ma. These values are mainly older than ages assigned over the past decade. 2. The early Whaingaroan Stage, traditionally held to be entirely within the Oligocene, and to define its base, extends back across the Eocene/Oligocene boundary at 33.7 Ma into the late Eocene by up to 1.1 m.y., as previously suspected by Morgans et al. (1996) on biostratigraphic grounds. 3. There has been considerable uncertainty about placement of the Waitakian Stage over the past two decades, whether entirely in the Miocene, entirely in the Oligocene, or straddling both epochs. Our Sr dating shows that the Oligocene/Miocene boundary (23.8 Ma) lies about midway through the Waitakian Stage, in agreement with Graham et al. (2000). 4. The Whaingaroan/Duntroonian boundary approximates the international early‐late Oligocene one (28.5 Ma). Comparisons with recently published Oligocene stable oxygen isotope records suggest that δ18O maxima and attendant sea‐level lowering, with possibly significant unconformity development, may be anticipated on three occasions in the early Whaingaroan, two or three in the late Whaingaroan, two in the Duntroonian, and at least two in the Waitakian. The unconformities bounding formations and members in the Oligocene successions may relate to these sea‐level changes, and so be regionally correlatable, including the well publicised Marshall Paraconformity of latest Whaingaroan (c. 29 Ma) age.