Four large, open-ocean diatoms from the Southern Ocean (Actinocyclus sp., Thalassiosira sp., Fragilariopsis kerguelensis, and Corethron pennatum) were grown in natural (low iron) Southern ocean seawater with increasing Fe concentrations. With increasing dissolved iron (Fe diss ) concentrations, the growth rates increased three-to sixfold. The species with the smallest cells had the highest growth rates. The half-saturation constants (K m ) for growth were low (0.19-1.14 nmol L Ϫ1 Fe diss ), and close to the ambient Fe diss concentrations of 0.2 nmol L Ϫ1 . The range in K m with respect to Fe diss also varied with the size of the diatoms: the smallest species had the lowest K m and the largest species had the highest K m . As Fe diss concentrations decreased, silicate consumption per cell increased, but nitrate consumption per cell decreased. Phosphate consumption per cell varied without clear relation to the dissolved iron concentrations. The differences in nutrient consumption per cell resulted in marked differences in elemental depletion ratios in relation to Fe diss concentrations, with the depletion ratios being most affected by iron limitation in the largest cells. These experimental findings are in agreement with previous laboratory and field studies, showing the relatively high requirements of large diatoms for Fe. The size-dependent response of the diatoms with respect to nutrient depletion is a good illustration of the effects of Fe on silicate, nitrate, and phosphate metabolism.

Blooms of large diatoms dominate the CO 2 drawdown and silicon cycle of the Southern Ocean in both the past and present. The growth of these Antarctic diatoms is limited by availability of iron (and light). Here we report the first assessment of growth rates in relation to iron availability of two truly oceanic Antarctic diatom species, the large, chain-forming diatom Chaetoceros dichaeta and the small, unicellular diatom C. brevis. In filtered natural, untreated Southern Ocean water, a maximum specific growth rate of 0.62 Ϯ 0.09 d Ϫ1 and a K m for growth of 1.12 ϫ 10 Ϫ9 M dissolved iron was calculated for C. dichaeta. This response could only be seen during a long-day light period. C. brevis maintained growth rates of 0.39 Ϯ 0.09 d Ϫ1 with and without iron addition, even under short-day light conditions, and could only be forced into iron limitation by adding the siderophore desferri-ferrioxamine B (DFB), an iron immobilizing agent. Using this approach, the low K m value for growth of 0.59 ϫ 10 Ϫ12 M dissolved Fe was calculated for this species. The size-class dependent growth response to iron (and light) confirms the key role of these parameters in structuring Southern Ocean ecosystems and thus the CO 2 dynamics and the silicon cycle.

Three cleaning techniques that remove external contamination of human hair are assessed to investigate the potential use of Sr and Pb isotope composition of hair for human provenancing. These techniques are; (i) a centrifugation technique using diiodomethane where hair and soil particles are separated by density difference; (ii) a leaching technique of the hair surface using 2 M HNO 3 acid and (iii) cleaning the hair with chloroform, methanol and ultra pure water. These techniques are validated and are successfully applied to demonstrate the capability of modern and archaeological human scalp hair and modern human facial hair to record Sr and Pb isotope variations related to geographic location. In this study, Sr isotope ratios analysed in modern human scalp hair from a female vegetarian and non-smoker register marked isotopic change on a monthly timescale when an individual moves to locations with contrasting Sr isotope compositions. Pb isotope ratios do not show significant changes after moving locations, possibly due to comparable Pb isotope ratios in the two environments. In contrast, Pb isotopes ratios analysed in facial hair from a male omnivore and smoker record isotopic changes within a two week period when moving between locations with significant differences in environmental Pb isotope compositions. Further research is needed to determine the exact rate of change in isotopic composition in scalp and facial hair in humans with different diets and life styles that move from geographical locations with isotopic contrasting composition.

The interaction between iron and light limitation was investigated in 3 marine diatom species: 2 Antarctic diatom isolates, Chaetoceros brevis and C. dichaeta, and an isolate from temperate waters, C. calcitrans. In C. calcitrans and C. brevis, grown in the laboratory using trace metal buffered medium, both iron and light limitation affected growth rates, cellular chlorophyll a fluorescence, spinal index (i.e., the number and size of spines) and cell size. Growth rates and cell size declined at lower iron concentrations and lower light intensities. Cellular chlorophyll a fluorescence increased with lower light, but decreased due to iron limitation. The spinal index, based on the ratio of side scatter to forward scatter, increased in iron-and light-limited cells. The large diatom C. dichaeta, grown in natural Southern Ocean water (without ethelenediaminetetraacetic acid [EDTA]) showed qualitatively similar responses to co-limitation by iron and light. C. dichaeta only grew under long-day light conditions. This response was further modified by the availability of iron. Addition of iron resulted in higher growth rates. In contrast, ambient iron concentrations did not limit the growth rate of the small Antarctic diatom C. brevis. However, iron limitation could be induced by addition of the natural iron binding ligand desferrioxamine B (DFOB). Addition of iron to Fe-depleted cultures of C. dichaeta and C. brevis reversed the effects of DFOB, as evidenced by rapid increases (within 24 h) in photochemical quantum efficiency (F v /F m ) and decreases in effective absorption of the crosssection of photosystem II (σ PSII ), the turnover time of the photosynthetic unit (τ) and the electron transfer rate (1/ τ), followed by an increase in growth rates after 48 h. The interactions between iron and light in Antarctic diatoms are sufficient to explain the observation that in the Southern Ocean some species (i.e., the small C. brevis) thrive under low iron and low light conditions, whereas other species (i.e., the large C. dichaeta) can bloom only under conditions of relatively high iron concentrations and favorable high light intensities or a long-day light period. These differences in physiological responses will have consequences for primary production, the carbon cycle and biogeochemical cycles.

Silicon isotope ratios ( 28 Si, 29 Si and 30 Si) can be measured with high precision by multi-collector inductively coupled plasma mass spectrometers (MC-ICP-MS). However, the problematic extraction of silicon from geological materials has been a major disadvantage in previous silicon isotope studies with conventional gas source mass spectrometry, whereas available silicon isotope results obtained by MC-ICP-MS techniques have been mainly restricted to waters and high purity silica. We show here that high yields of silicon (>97%) can be achieved from samples ranging from pure silica to basaltic compositions (45-52 wt.% SiO 2 ) via a three-step digestion and purification procedure. Silicon isotope measurements, performed with a Finnigan Neptune MC-ICP-MS used in medium-resolution mode (resolving power: 2500), indicate that polyatomic interferences can be resolved and that both d 29 Si and d 30 Si can be determined with high accuracy and precision on interference-free peak plateaux in the mass spectrum. Instrumental blanks (20-65 mV) were reduced to acceptable values with a Cetac Aridus desolvating device fitted with a sapphire injector in the torch. Sensitivity in medium-resolution mode is in the range of B6 V per mg g À1 for 28 Si. d 29 Si and d 30 Si have been determined for silicon isotope standards IRMM-018 (d 30 Si = À1.75%), IRMM-018-76 (d 30 Si = À1.42%), Diatomite (d 30 Si = 1.34%) and Big Batch (d 30 Si = À10.52%), for USGS standards BHVO-2 (d 30 Si = À0.09%) and AGV-2 (d 30 Si = À0.01%), and for Aldrich pure silica powder (d 30 Si = À0.32%). Precision on d 30 Si is 0.18-0.41% (2 s.d.). Our combined procedure for sample preparation followed by high-resolution MC-ICP-MS analysis facilitates straightforward and safe measurement of silicon isotope ratios in silicate materials.