Sonja Schulte

Global temperature calibration of the alkenone unsaturation index (UK′37) in surface waters and comparison with surface sediments

In this paper, we compile the current surface seawater C37 alkenone unsaturation (UK′37) measurements (n = 629, −1 to 30°C temperature range) to derive a global, field-based calibration of UK′37 with alkenone production temperature. A single nonlinear “global” surface water calibration of UK′37 accurately predicts alkenone production temperatures over the diversity of modern-day oceanic environments and alkenone-synthesizing populations (T = −0.957 + 54.293(UK′37) − 52.894(UK′37)2 + 28.321(UK′37)3, r2 = 0.97, n = 567). The mean standard error of estimation is 1.2°C with insignificant bias in estimated production temperature among the different ocean regions sampled. An exception to these trends is regions characterized by strong lateral advection and extreme productivity and temperature gradients (e.g., the Brazil-Malvinas Confluence). In contrast to the surface water data, the calibration of UK′37 in surface sediments with overlying annual mean sea surface temperature (AnnO) is best fit by a linear model (AnnO = 29.876(UK′37) − 1.334, r2 = 0.97, n = 592). The standard error of estimation (1.1°C) is similar to that of the surface water production calibration, but a higher degree of bias is observed among the regional data sets. The sediment calibration differs significantly from the surface water calibration. UK′37 in surface sediments is consistently higher than that predicted from AnnO and the surface water production temperature calibration, and the magnitude of the offset increases as the surface water AnnO decreases. We apply the global production temperature calibration to the coretop UK′37 data to estimate the coretop alkenone integrated production temperature (coretop IPT) and compare this with the overlying annual mean sea surface temperature (AnnO). We use simple models to explore the possible causes of the deviation observed between the coretop temperature signal, as estimated by UK′37, and AnnO. Our results indicate that the deviation can best be explained if seasonality in production and/or thermocline production as well as differential degradation of 37:3 and 37:2 alkenones both affect the sedimentary alkenone signal.

Stable Carbon Isotopic Composition of the C37:2 Alkenone: A Proxy for CO2(aq) Concentration in Oceanic Surface Waters?

We tested the applicability of the carbon isotopic composition of C37:2 alkenones (δ13C37:2) as a proxy for dissolved carbon dioxide CO2(aq) in oceanic surface waters. For this purpose we determined (δ13C37:2 in suspended particulate organic matter (POM) and surface sediments from the South Atlantic. In opposite of what would be expected from a diffusive CO2 uptake model for marine algae we observed a positive correlation between 1/[CO2(aq)] and the isotopic fractionation (ep) calculated from (δ13C37:2. This clearly demonstrates that CO2(aq) is not the primary factor controlling ep at the sites studied. On the other hand we found a negative correlation between ep and the phosphate concentration in the surface waters (0–10 m) supporting the assumption of (1997) that ep is primarily related to nutrient-limited algal growth rather than to [CO2(aq)]. Reconstructing past CO2(aq) levels from (δ13C37:2 thus requires additional proxy information in order to correct for the influence of haptophyte growth on the isotopic fractionation. In the eastern Angola Basin, we previously used δ15N of bulk organic matter as proxy for nutrient-limited growth rates. As an alternative the Sr/Ca ratio of coccoliths has been recently suggested as growth-rate proxy which should be tested in future studies.

Delta 13C measured on alkenones of surface sediment samples GeoB1008-6 to GeoB3603-1 from the South Atlantic

We have analyzed the stable carbon isotopic composition of the diunsaturated C37 alkenone in 29 surface sediments from the equatorial and South Atlantic Ocean. Our study area covers different oceanographic settings, including sediments from the major upwelling regions off South Africa, the equatorial upwelling, and the oligotrophic western South Atlantic. In order to examine the environmental influences on the sedimentary record the alkenone-based carbon isotopic fractionation (Ep) values were correlated with the overlying surface water concentrations of aqueous CO2 ([CO2(aq)]), phosphate, and nitrate. We found Ep positively correlated with 1/[CO2(aq)] and negatively correlated with [PO43-] and [NO3-]. However, the relationship between Ep and 1/[CO2(aq)] is opposite of what is expected from a [CO2(aq)] controlled, diffusive uptake model. Instead, our findings support the theory of Bidigare et al. (1997, doi:10.1029/96GB03939) that the isotopic fractionation in haptophytes is related to nutrient-limited growth rates. The relatively high variability of the Ep-[PO4] relationship in regions with low surface water nutrient concentrations indicates that here other environmental factors also affect the isotopic signal. These factors might be variations in other growth-limiting resources such as light intensity or micronutrient concentrations.