Late Pliocene and early Pleistocene sea-level timing and amplitudes derived from fossil ostracod assemblages: Canterbury Basin, New Zealand

IODP Expedition 317 cruise drilled cores at three shelf sites (U1353, U1354 and U1351) and one slope site (U1352), in water depths between 85 and 344 m, to understand relationships between sea-level change and sequence stratigraphy. The shelf sites are well suited to reconstruction of high-resolution sea-level fluctuations because of high sedimentation rates from the uplifting Southern Alps. We examined fossil ostracod assemblages from the shelf sites to reconstruct paleo-water depth fluctuations and their amplitudes. We identified 178 ostracod species and 70 genera from more than 160 samples. Q-mode factor analysis was performed on ostracod taxa with abundances of >3.5 % in each sample containing >50 specimens. Six varimax factors were explained 70.8% of the total variance. Paleo-water depths in each factor were calibrated with reference to recent ostracodes occurring around the sites as follows: first factor, middle shelf (50-80 m); second factor, middle to outer shelf (60-130 m); third factor, middle to outer shelf (55-115 m); fourth factor, lagoon, estuary and inner shelf (0-50 m); fifth factor, middle to outer shelf (80-200 m); sixth factor, outer shelf (130-200 m). Factor analysis of ostracod assemblages reveal at least, eight transgressive- and regressive-cycles at Site U1353, seventeen at Site U1354 and two at Site U1351. These cycles probably correspond to a subset of MIS stages between MIS M2 and MIS 40. Furthermore, amplitudes of these paleo-water-depth cycles are expected to equate to eustatic amplitudes because shelf sedimentation has been continuous and minimal subsidence can have occurred during the short time period involved. We therefore estimate that eustatic amplitudes were: 10 - 30 m from 3.1 to 2.8 Ma, ca. 100 m from 2.8 to 2.6 Ma, and 30 - 115 m from 1.8 to1.2 Ma. These amplitudes, together with the timing of the increase in amplitudes (~2.7 Ma), agree with estimates derived from oxygen isotopic records (Raymo et al., 2005), suggesting that the Canterbury Basin sequences responded to global climate change. However, comparison with sea-level amplitude estimates from the North Island of New Zealand (Naish, 1997) suggests that sea-level amplitudes increased in the Canterbury Basin 200 thousand years earlier than in the North Island.