986 resultados para Proclus, ca. 410-485.
Resumo:
We present and examine a multi-sensor global compilation of mid-Holocene (MH) sea surface temperatures (SST), based on Mg/Ca and alkenone palaeothermometry and reconstructions obtained using planktonic foraminifera and organic-walled dinoflagellate cyst census counts. We assess the uncertainties originating from using different methodologies and evaluate the potential of MH SST reconstructions as a benchmark for climate-model simulations. The comparison between different analytical approaches (time frame, baseline climate) shows the choice of time window for the MH has a negligible effect on the reconstructed SST pattern, but the choice of baseline climate affects both the magnitude and spatial pattern of the reconstructed SSTs. Comparison of the SST reconstructions made using different sensors shows significant discrepancies at a regional scale, with uncertainties often exceeding the reconstructed SST anomaly. Apparent patterns in SST may largely be a reflection of the use of different sensors in different regions. Overall, the uncertainties associated with the SST reconstructions are generally larger than the MH anomalies. Thus, the SST data currently available cannot serve as a target for benchmarking model simulations.
Resumo:
The euphotic depth (Zeu) is a key parameter in modelling primary production (PP) using satellite ocean colour. However, evaluations of satellite Zeu products are scarce. The objective of this paper is to investigate existing approaches and sensors to estimate Zeu from satellite and to evaluate how different Zeu products might affect the estimation of PP in the Southern Ocean (SO). Euphotic depth was derived from MODIS and SeaWiFS products of (i) surface chlorophyll-a (Zeu-Chla) and (ii) inherent optical properties (Zeu-IOP). They were compared with in situ measurements of Zeu from different regions of the SO. Both approaches and sensors are robust to retrieve Zeu, although the best results were obtained using the IOP approach and SeaWiFS data, with an average percentage of error (E) of 25.43% and mean absolute error (MAE) of 0.10 m (log scale). Nevertheless, differences in the spatial distribution of Zeu-Chla and Zeu-IOP for both sensors were found as large as 30% over specific regions. These differences were also observed in PP. On average, PP based on Zeu-Chla was 8% higher than PP based on Zeu-IOP, but it was up to 30% higher south of 60°S. Satellite phytoplankton absorption coefficients (aph) derived by the Quasi-Analytical Algorithm at different wavelengths were also validated and the results showed that MODIS aph are generally more robust than SeaWiFS. Thus, MODIS aph should be preferred in PP models based on aph in the SO. Further, we reinforce the importance of investigating the spatial differences between satellite products, which might not be detected by the validation with in situ measurements due to the insufficient amount and uneven distribution of the data.
Resumo:
Records of Cd/Ca in planktonic foraminiferal calcite of Globigerinoides bulloides in cores from the Subantarctic region of the Southern Ocean show large glacial-interglacial variations with lower Cd/Ca (by 0.06-0.10 µmol/mol) at glacial times. Interpretation of these records in terms of lower dissolved phosphate and inferred higher glacial nutrient utilization has significant implications for glacial atmospheric carbon dioxide (pCO2) draw-down. However, box core-top data for G. bulloides in the North Atlantic suggest that the incorporation of Cd into planktonic foraminifera relative to seawater (DCd) is temperature sensitive (DCd=0.637 exp 0.15T). When the Subantarctic planktonic Cd/Ca records are corrected for this temperature dependence, they show little or no glacial-interglacial diferences. If, as seems likely, this observation can be interpreted to indicate a minimal change (< 0.5 µmol/kg) in surface water phosphate concentrations, then the explanation for lowered glacial pCO2 must be looked for elsewhere.
