Quaternary surface water temperature estimations: New diatom transfer functions for the Southern Ocean
Introduction
The estimation of paleoenvironmental variables from micropaleontological data sets has become an important tool for paleoceanographic reconstructions since the development of the Imbrie and Kipp transfer function method (IKM; Imbrie and Kipp, 1971) and the Modern Analog Technique (MAT; Hutson, 1980). After first comprehensive and successful use of the IKM on various microfossil groups in the Climate: Long-Range Investigation Mapping and Prediction (CLIMAP) project for the reconstruction of the last glacial maximum (LGM) and the last interglacial optimum (Climate: Long-Range Investigation, Mapping and Prediction (CLIMAP), 1976, Climate: Long-Range Investigation, Mapping and Prediction (CLIMAP), 1981, Climate: Long-Range Investigation, Mapping and Prediction (CLIMAP), 1984), and the successful application of the MAT proposed by Hutson (1980), Sea Surface Temperature (SST) estimations derived from transfer functions (TFs) are broadly accepted and applied in paleoceanography (e.g. Crosta et al., 1998, Crosta et al., 2004, Zielinski et al., 1998, Gersonde and Zielinski, 2000, Gersonde et al., 2005). Further improvement of TFs to deal with non-linear species responses to environmental gradients led to the development of methods based on Weighted Averaging (WA; Birks et al., 1990) and Weighted Averaging Partial Least Squares (WAPLS; ter Braak and Juggins, 1993), successfully applied in diatom-based SST reconstructions (e.g. Birks and Koc, 2002). Potentials and limitations concerning the application of TFs are summarized e.g. in Birks (2003), Telford and Birks (2005) and Juggins (2013). A method for assessing the statistical significance of quantitative reconstructions inferred from microfossil assemblages is proposed by Telford and Birks (2011).
In the Southern Ocean, diatom assemblages are useful tools for the reconstruction of past SSTs because diatoms are widely distributed, they are phototrophic algae living in the euphotic zone, thus representing environmental conditions of the upper water column, and their biogeographic distribution patterns are closely related to surface water summer temperature (e.g. Zielinski and Gersonde, 1997, Armand et al., 2005, Crosta et al., 2005, Romero et al., 2005, Esper et al., 2010). However, in order to reconstruct Pleistocene summer SSTs based on diatom assemblages, several limitations have to be taken into account. This includes selective dissolution of less robustly silicified diatom valves resulting in an overall dominance of a few robust species in the surface and the down-core sediment record (Zielinski et al., 1998, Esper and Gersonde, 2014). Another limitation are so-called non-analog conditions, where fossil assemblages are composed by taxa at numbers that exceed the abundance in the surface sediment reference data set or in case of the prominent occurrence of extinct species. A typical non-analog situation in Southern Ocean diatom assemblages is related to the abundant occurrence of Eucampia antarctica in glacial sediments, leading to the exclusion of such taxa from reference data sets (Zielinski et al., 1998, Esper and Gersonde, 2014). Nevertheless, the previous studies on quantitative SSST reconstructions based on diatoms show a down-core pattern congruent with radiolarian- and foraminifera-derived SSSTs in accordance with ice core temperature variation for the last 150,000 yr (e.g. Gersonde et al., 2004), proving a close diatom assemblage/SSST-relationship for at least this time span.
The previous diatom TFs for the Southern Ocean comprised Atlantic, Indian and a few Pacific sector surface samples (Crosta et al., 1998, Zielinski et al., 1998, Gersonde et al., 2005), calling for a more comprehensive Pacific sector extension of diatom sample collections to allow for reliable quantitative summer SST reconstructions in this largest Southern Ocean sector (Gersonde et al., 2005). The new reference data set presented herein comprises 336 samples with 29 diatom taxa from the Pacific, Atlantic, and western Indian sectors of the Southern Ocean (Fig. 1, Table 1) and expands the reference data set D93-29 of Zielinski et al. (1998). The combination of new diatom data from the Pacific and Atlantic sectors with published data allows for the separation of regional sub-sets for the Atlantic (151 samples) and the Pacific sector (107 samples). A further aim of this study is to compare corresponding sea surface temperatures derived from different hydrographic collections to identify reference SSTs with the best correlation with the sediment-derived diatom assemblages. Additionally, we elaborate, how statistical techniques are applied to optimize the estimation capacity and reliability of four different TF approaches for their application in circum-Antarctic environments. Interpolation techniques based on linear (IKM) or unimodal (WA, WAPLS) species-environment response models will be compared with MAT, a method directly relating modern reference data with down-core diatom assemblages. Finally, we apply the four transfer functions to two Pleistocene sediment cores from the Atlantic (PS1768-8) and eastern Pacific (PS58/271-1) sectors of the Southern Ocean (Fig. 1) to document the past 150 kyr (kyr = 1000 yr) summer SST variability in the Permanent Open Ocean Zone of the Southern Ocean with emphasis on a performance comparison between regional sub-sets and the full reference data set.
Section snippets
Samples, preparation and counting
To generate a new diatom calibration set for summer sea surface temperature (SSST) reconstruction the diatom assemblage data from 467 surface samples recovered in the Atlantic, the western Indian, and the Pacific sectors of the Southern Ocean were examined and tested for their applicability to TF-based SSST estimates. This comprises data sets obtained from 230 sites in the Atlantic sector (Zielinski and Gersonde, 1997), 29 sites in the Atlantic and western Indian sectors (Zielinski et al., 1998
Establishment of a diatom reference data set for TF-based temperature reconstruction
To establish the new diatom reference data set for the SO, the diatom assemblage composition preserved in a total of 467 surface sediment samples was reviewed. As a first step, the number of taxa/taxa groups, originally amounting to 85, was reduced to 35 (Table 1), following general rules proposed by Imbrie and Kipp (1971) and findings concerning the environmental significance of Antarctic diatom taxa established by Zielinski et al. (1998). Thus, taxa occurring at low abundance (generally below
Conclusions
The development of the new reference data set D336/28, including regional subsets D151/29 (Atlantic sector) and D107/27 (Pacific sector), and statistical tests for several pre-settings of transfer functions based on IKM, MAT, WA, and WAPLS led to reliable diatom transfer functions for paleo-SSST reconstruction in the Southern Ocean. Limitations for selecting a valuable reference data set are (1) parameters which alter the diatom assemblage in surface sediment samples (reworking, selective
Acknowledgments
We acknowledge the technical assistance of U. Bock and R. Cordelair. Finally we thank the reviewers for the constructive reviews and comments on the manuscript. The research has received funding from the European Union's Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 243908, ‘Past4Future’ Climate change — Learning from the past climate, the AWI program PACES (“Polar Regions and Coasts in the Changing Earth System”) and MARUM (“The Ocean in the Earth System”).
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