The stratigraphic record of the extinct Antarctic diatom Thalassiosira torokina Brady demonstrates significant morphological change over its stratigraphic range, from 9.36 Ma to 1.06 Ma. This study documents these changes across a range of ages and latitudes from [82° 55' 48'' S. lat - CIR to 50° 13.670' S - Leg 119 site 737A].
T. torokina samples used in this study are from the ANDRILL McMurdo Ice Shelf Project core (AND-1B) from beneath the northwestern margin of the Ross Ice Shelf in West Antarctica, ODP (Ocean Drilling Project) Legs 113 (Weddell Sea), 119 (Kerguelen Plateau - Prydz Bay) and 120 (Central Kerguelen Plateau) as well as material recovered from the Crary Ice Rise from beneath the central Ross Ice Shelf.
Morphometric measurements were taken of each T. torokina specimen's diameter (µm), central area (µm²), robustness (total number of areolae per 10 µm nearest the central area), degree of "domeness" or how arched the diatom valve is, and marginal width (µm). The number of strutted processes found within the central area was counted.
A size range from a diameter as small as 26 µm, near the end of its stratigraphic range in the Pleistocene, to as much as 152 µm in the Late Miocene was found, illustrating a pattern of size reduction over time. This study provides quantitative morphologic data on T. torokina that demonstrates a wider morphologic range than previously reported.
The central area size, number of strutted processes and robustness of T. torokina all decrease through its stratigraphic range. Average margin width and degree of domeness both increase over this time interval. These changes broadly correlate with climatic change, coincident with decreasing ocean surface temperatures and increasing sea-ice. At this point it is unclear if climate change or evolutionary processes unrelated to environmental change were the cause for their morphological variations, and if so, to what extent.
Thalassiosira torokina appeared as Antarctica entered a markedly cooler phase, in the early Late Miocene, approximately coincident with the development of extensive grounded ice in West Antarctica. It thrived through the Pliocene and went extinct at the onset of the Mid-Pleistocene transition, where lower frequency, higher amplitude climatic cycles took hold. T. torokina abundance declined in the early Pleistocene, with the last confirmed in situ occurrence being during Marine Isotope Stage 31, 1.06 Ma ago, which is younger than that of most published last occurrences. Occurrences in younger strata are inferred to be reworked.