[1] The valve area of Fragilariopsis kerguelensis, the most abundant diatom species in the Southern Ocean, strongly changes in size in response to varying conditions in the surface ocean. We examined the link, both in two iron fertilization experiments and in sediment samples covering several glacial Terminations, between size variability in this species and environmental conditions across the Antarctic Polar Front, including sea ice extent, sea surface temperature, and the input of eolian dust. The iron fertilization experiments show valve area to be positively correlated with iron concentrations in ambient waters, which suggests the possibility of a causal relation between valve size of Fragilariopsis kerguelensis and ambient surface water iron concentration. Larger valves are usually found during glacial times and thus seem to be related to lower sea surface temperature and wider sea ice coverage. Moreover, our results indicate that there usually is a strong correlation between larger valve size and increased input of eolian dust to the Southern Ocean. However, this correlation, obvious for the fertilization experiments and for glacial Terminations I, II, III, and V, does not seem to be valid for Termination VI, where size appears to be inversely correlated to dust input.

The southwestern region of the Dominican Republic (Enriquillo Valley) contains exceptionally well-preserved, relict marine and saline lake deposits of mid-Holocne age. Abundant euryhaline ostracodes found in this deposit include Cyprideis salebrosa, C. mexicana, C. similis, and C. edentata. Morphometric and geochemical analyses performed on Cyprideis spp. provide high d 18 O and d 13 C values that are coincident with relative abundances of irregularly shaped pores that permeate the ostracode carapace. We recognize three stratigraphic intervals with distinct ostracode pore shape and stable isotope trends: (I) a 4.5-5.0m interval that contains ostracodes with highly irregular shaped pores (multiradiate) and high amplitude variability in d 18 O and d 13 C values; (II) a 5.0-5.6m interval comprised of ostracodes with circular pores and an overall trend towards low d 18 O and d 13 C values; and (III) a 5.6-6.5m interval containing ostracodes with an upward increasing abundance of circular pore shapes coincident with decreasing d 18 O and d 13 C values. When the Enriquillo lagoon was first separated from the Caribbean Sea approximately 4.3 ka, an arid and evaporative climate led to hypersaline water in a restricted lagoon environment. By the middle to late-Holocene, increased precipitation in the valley resulted in a coastal lake system that became progressively oligohaline. Moderate to small amplitude variability in the salinity proxy data (d 18 O) suggest short-term oscillations in the precipitation-evaporation budgets at that time. At least two marine incursions likely contributed to the observed variability in ostracode d 18 O and d 13 C values. Evidence for abrupt changes in base level indicate that climatic factors or also tectonic activity may have contributed to the observed paleoenvironmental trends recorded in these deposits.

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