Alkenone fluxes in the water column of the Cariaco Basin ranged from 12 to 20 μg m −2 d −1 and were inversely related to upwelling strength. The U 37 K′ ratios of sinking particles varied from 0.78 to 0.96 and exhibited seasonal changes that were coherent with a 7°C variation in sea surface temperature (SST). The correlation between SST and U 37 K′ ratios closely overlapped with the calibration of Prahl et al. [1988] . Alkenone burial fluxes in Cariaco Basin sediments varied markedly over the past ∼6000 years, ranging from 0.2 to 5 μg m −2 d −1 . The U 37 K′ ratios of surface sediments indicate SST was higher (26.3°C) during the last 50 years of deposition than in the previous 300 years (∼25°C), signaling an upwelling decrease in the latter part of the twentieth century. The lowest U 37 K′ ‐derived temperatures (∼25°C) were measured in sediments deposited during the little ice age (LIA). These compositions, coupled with relatively low alkenone fluxes (≤2 μg m −2 d −1 ), are consistent with conditions of enhanced upwelling, decreased SST and reduced haptophyte production. The highest U 37 K′ ‐derived SST estimates (over 26.5°C) were measured during the Medieval Warm Period (MWP) and suggest reduced upwelling at this time. Prior to the MWP, the alkenone record indicates temperatures of ∼26°C and burial fluxes of ∼2 μg m −2 d −1 . These compositions indicate stronger upwelling conditions during the Holocene relative to the last 50 years and the MWP but annual SSTs above those estimated for the LIA.
An interlaboratory study of Mg/Ca and Sr/Ca ratios in three commercially available carbonate reference materials (BAM RS3, CMSI 1767, and ECRM 752‐1) was performed with the participation of 25 laboratories that determine foraminiferal Mg/Ca ratios worldwide. These reference materials containing Mg/Ca in the range of foraminiferal calcite (0.8 mmol/mol to 6 mmol/mol) were circulated with a dissolution protocol for analysis. Participants were asked to make replicate dissolutions of the powdered samples and to analyze them using the instruments and calibration standards routinely used in their laboratories. Statistical analysis was performed in accordance with the International Standardization Organization standard 5725, which is based on the analysis of variance (ANOVA) technique. Repeatability (RSD r %), an indicator of intralaboratory precision, for Mg/Ca determinations in solutions after centrifuging increased with decreasing Mg/Ca, ranging from 0.78% at Mg/Ca = 5.56 mmol/mol to 1.15% at Mg/Ca = 0.79 mmol/mol. Reproducibility (RSD R %), an indicator of the interlaboratory method precision, for Mg/Ca determinations in centrifuged solutions was noticeably worse than repeatability, ranging from 4.5% at Mg/Ca = 5.56 mmol/mol to 8.7% at Mg/Ca = 0.79 mmol/mol. Results of this study show that interlaboratory variability is dominated by inconsistencies among instrument calibrations and highlight the need to improve interlaboratory compatibility. Additionally, the study confirmed the suitability of these solid standards as reference materials for foraminiferal Mg/Ca (and Sr/Ca) determinations, provided that appropriate procedures are adopted to minimize and to monitor possible contamination from silicate mineral phases.
Detailed analyses of well-preserved carbonate samples from across the Cretaceous/Tertiary boundary in Hole 577 have revealed a significant decline in the δ 13 C values of calcareous nannoplankton from the Maestrichtian to the Danian Age accompanied by a substantial reduction in carbonate accumulation rates.Benthic foraminifers, however, do not exhibit a shift in carbon composition similar to that recorded by the calcareous nannoplankton, but actually increase slightly over the same time interval.These results are similar to the earlier findings at two North Pacific Deep Sea Drilling Project locations, Sites 47.2 and 465, and are considered to represent a dramatic decrease in oceanic phytoplankton production associated with the catastrophic Cretaceous/Tertiary boundary extinctions.In addition, the change in carbon composition of calcareous nannoplankton across the Cretaceous/Tertiary boundary at Hole 577 is accompanied by only minor changes in the oxygen isotope trends of both calcareous nannoplankton and benthic foraminifers, suggesting that temperature variations in the North Pacific from the late Maestrichtian to the early Danian Age were insignificant.
