Peer Helmke

A study of the alkenone, TEX86, and planktonic foraminifera in the Benguela Upwelling System: Implications for past sea surface temperature estimates

Suspended particulate matter from seawater and core top sediments were collected during 2003 Meteor cruise M57/1 in January–February from the continental margin off western South Africa for analysis of alkenones, glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, and planktonic foraminifera. Alkenone analysis of suspended particulates in seawater and core top sediments indicates that U37K′ temperatures were representative of annual mean sea surface temperature. In contrast, GDGT analysis suggests that TEX86 temperatures are cold-biased due to upward transports of GDGTs produced below the mixed layer. The analysis of plankton tow samples revealed that the δ18O of Globorotalia inflata in core top sediments could be biased toward lower temperatures due to subsurface calcification. Accordingly, our study shows that each paleotemperature proxy may record different temperature signals in the Benguela upwelling system emphasizing the general need to constrain potential biases in each proxy for better interpreting paleoclimate records.

Assessment of geochemical and micropaleontological sedimentary parameters as proxies of surface water properties in the Canary Islands region

Abstract The Canary Islands region occupies a key position with respect to biogeochemical cycles, with the zonal transition from oligotrophic to nutrient-rich waters and the contribution of Saharan dust to the particle flux. We present the distribution of geochemical proxies (TOC, carbonate, δ15N, δ13Corg, C/N-ratio) and micropaleontological parameters (diatoms, dinoflagellates, foraminifera, pteropods), in 80 surface-sediment samples in order to characterise the influence of coastally upwelled water on the domain of the subtropical gyre. Results of the surface-sediment analyses confirmed the high biomass gradient from the coast to the open ocean inferred from satellite data of surface chlorophyll or SST. The distribution of total dinoflagellate cysts, the planktic foraminifera species Globigerina bulloides, the diatom resting spore Chaetoceros spp., and TOC concentration coincided well with the areas of strong filament production off Cape Ghir and Cape Yubi. The warm-water planktic foraminifera Globigerinoides ruber (white), the diatom Nitzschia spp., and the δ15N-values showed the opposite trend with high values in the open ocean. Factor analyses on the planktic foraminifera species distribution indicated three major assemblages in the Canary Islands region that represent the present surface-water conditions from the upwelling influenced region via a mixing area towards the subtropical gyre.

Decadal analysis of hydrography and in situ nutrient budgets in the western and eastern North Atlantic subtropical gyre

Received 29 June 2006; revised 30 January 2007; accepted 5 April 2007; published 24 July 2007. [1] The current debate about the mechanisms and magnitude of new nutrient input to the euphotic zone in subtropical gyres calls for studies which consider large and mesoscale perspectives by combining in situ time series and remote observations. We carried out a first of its kind comparative analysis of hydrography and sea level anomaly (SLA) at the oligotrophic time series stations BATS (Bermuda Atlantic Time Series Study) and ESTOC (European Station for Time Series, Canary Islands) using concomitant 10-yr in situ and satellite altimetry data. The stations are located at about the same latitude in the western and eastern boundaries of the subtropical North Atlantic gyre, respectively, and provide the opportunity to study differences that may exist between both regions. Observed SLA was 0.25 m at BATS, compared with 0.12 m at ESTOC, a consequence of the higher eddy kinetic energy in the western compared with the eastern subtropical gyre. We quantified a detailed in situ nutrient budget for both time series stations; ESTOC received about 75% of the nutrients available for new production at BATS (in average 0.28 mol N m � 2 yr � 1 compared with 0.38 mol N m � 2 yr � 1 , respectively), but the difference was not significant. However, significant differences in input mechanisms existed between both stations; eddy pumping constituted the main new nutrient source BATS, whereas wintertime convection was the main nutrient supply mechanism at ESTOC. In addition, the nutricline was significantly shallower at ESTOC compared with BATS, partly compensating for shallower mixed-layer depths and SLA variability at the western station. Wefoundconsiderableinterannualvariabilityinbotheddypumpingandwintertimeconvection which may be related to NAO-induced changes in the pattern of the subtropical gyre.

Correction to “Decadal analysis of hydrography and in situ nutrient budgets in the western and eastern North Atlantic subtropical gyre”

[1] In the paper ‘‘Decadal analysis of hydrography and in situ nutrient budgets in the western and eastern North Atlantic subtropical gyre’’ (Journal of Geophysical Research, 112, C07025, doi:10.1029/2006JC003788), a number of errors were introduced during processing. [2] Susanne Neuer’s sole affiliation is School of Life Sciences, Arizona State University, Tempe, Arizona, USA. The corresponding authors are Andres Cianca (Andres. ciancaaguilar@asu.edu) and Susanne Neuer (Susanne. neuer@asu.edu). [3] The word ‘‘at’’ was left out of the following sentence in the abstract, corrected here: ‘‘However, significant differences in input mechanisms existed between both stations; eddy pumping constituted the main new nutrient source at BATS, whereas wintertime convection was the main nutrient supply mechanism at ESTOC.’’ [4] In paragraph 5 in section 1, the final sentence should read as follows: ‘‘The authors hypothesized that this difference was due to a smaller new nutrient input into the mixed layer in the eastern NASTG, especially due to lower eddyinduced mixing inferred from eddy-resolving models [McGillicuddy et al., 2003; Oschlies, 2002b] and tracerbased observations [Jenkins, 1988; Lewis et al., 1986].’’ [5] In paragraph 29 in section 4.2, the final sentence should read as follows: ‘‘The direction of the isopycnal displacement in the upper ocean is the same as for a cyclonic eddy, causing upwelling of nutrients into the euphotic zone when they are forming or intensifying [Sweeney et al., 2003].’’ [6] In paragraph 53 in section 5, the first sentence should read as follows: ‘‘In addition, we would like to note the high nutrient concentrations during 1999 at ESTOC in the subsurface water at 300 m of up to 17 mmol kg .’’ [7] In paragraph 54 in section 6, the first sentence should read as follows: ‘‘Using a 10-yr time series of in situ data and satellite altimetry, we compared the hydrography and quantified new nutrient budgets of the two oligotrophic time series stations BATS and ESTOC located at about the same latitude in the western and eastern subtropical North Atlantic gyre, respectively.’’ JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, C03005, doi:10.1029/2008JC004762, 2008 Click Here for Full Article