From species abundance to opal input: Simple geometrical models of radiolarian skeletons from the Atlantic sector of the Southern Ocean

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Abstract

Radiolarian-based paleoceanographic reconstructions generally use the abundance of selected radiolarian species. However, the recent focus on the opal flux and the development of isotope measurements in biogenic opal and the organic matter embedded in it demands a better knowledge of the origin of the opal. We present here an estimation of the opal content of the skeleton of 63 radiolarian species from two sites in the Southern Ocean. The skeletons are modelled as associations of simple geometrical shapes, and the volume thus obtained is combined with opal density to obtain the amount of opal. These data are, thus, used to determine the most important opal carriers in the radiolarian assemblage in both cores.

Introduction

The distribution and abundance of radiolarians species in the ocean are commonly used to reconstruct paleoceanographic conditions in different regions, such as sea surface temperature in the Southern Ocean (Abelmann et al., 1999; Brathauer and Abelmann, 1999; Cortese and Abelmann, 2002), upwelling intensity in the Indian and southern Atlantic oceans (Caulet et al., 1992; Vénec-Peyré and Caulet, 2000; Jacot Des Combes et al., 2005; Jacot Des Combes and Abelmann, 2007), pelagic export production in the Indian Ocean (Jacot Des Combes et al., 1999a, Jacot Des Combes et al., 1999b), and the inflow of water masses into a given region (Jacot Des Combes and Abelmann, 2007). However, the recent development of opal-based proxies, such as the measurement of stable isotopes (C, N, O, and Si) in opal (Shemesh et al., 1992, Shemesh et al., 1993, Shemesh et al., 1994, Shemesh et al., 1995, Shemesh et al., 2002; Crosta and Shemesh, 2002; Crosta et al., 2005; De La Rocha and Bickle, 2005; De La Rocha, 2006; Jacot Des Combes et al., 2008; Abelmann et al. subm.), and new questions on the main opal carriers in the opal cycle in the past (Cortese et al., 2004) bring new interest on the opal flux and its composition.

Although some studies of the radiolarian seasonal export flux have been published (Abelmann and Gersonde, 1991; Abelmann, 1992), all these data present species abundance and/or global silica flux, but no information on the amount of opal carried by the radiolarians. The composition of the radiolarian population varies geographically, seasonally, and with depth (Abelmann and Gersonde, 1991; Abelmann, 1992; Abelmann and Gowing, 1997; Abelmann et al., 1999). Recently established studies show that the decoupling of biogenic silica and organic carbon is steered by specific species and can also bias the isotopic signal (Smetacek et al., 2004; Abelmann et al. 2006; De La Rocha, 2006; Jacot Des Combes et al., 2008). This especially affects radiolarians, first because they are present in (and can thus give information on) different environmental conditions (temperature, depth, productivity regime), and second because their size, and consequently their opal contribution, greatly varies with species (50–500 μm). The isotopic composition of radiolarians will provide information about salinity changes and nutrient supply in surface but also in intermediate and deep waters. Further their C and N isotopic composition will give information on their feeding habits.

Combining the species distribution and opal flux and isotopic composition data would greatly help to improve paleoceanographic reconstruction, but that demands a link between the species composition of an assemblage and the opal it exports to the sediments.

One possibility to establish this link is to create models of the fossil skeletons. Geometrical modelling of siliceous fossils was already been used to estimate the biovolume of different diatoms species (Hillebrand et al., 1999). In this paper, we use a similar method to establish relatively simple models of radiolarian skeletons for the species present in the assemblage in two cores from the South Atlantic. These models are then used to transform the species abundance in the sediments into species-specific opal input, thus determining the main opal carrier species in the assemblage at these sites over the last 35 kyr.

Section snippets

Material and methods

The radiolarian species assemblage from the >63 μm fraction of the sediments is studied in two piston cores from the Southern Ocean covering the last glacial and the Holocene. Core PS 1768-8 (52.6°S, 4.5°E) was taken south of the present day Antarctic Polar Front and hosted 49 radiolarian taxa in its last glacial and Holocene sediments (Table 1). The sediments from this core are made of a diatomaceous ooze containing ice rafted debris, low amount of organic matter and of carbonates. Core PS

Results

The models are illustrated in Fig. 1, Fig. 2, Fig. 3, and the dimensions of each species and the corresponding opal weight are presented for the different models in Table 3, Table 4, Table 5, Table 6, Table 7, Table 8, Table 9, Table 10, Table 11, Table 12, Table 13, Table 14, Table 15.

The calculated opal weight of the radiolarian species present in the sediments of both cores ranges from 12 μg (Pseudodictyophymus gracilipes, Lipmanella sp.) to almost 15 mg (Spongotrochus glacialis). The

The inter-species difference in opal input

The wide range of opal input of the radiolarian species preserved in sediments from the Atlantic sector of the Southern Ocean reflects the large variety of shape and size of the species from the radiolarian assemblage.

Among the four major parameters of the radiolarian skeletons: their size, structure, thickness, and opal coverage, the size is of primary importance in the amount of opal it carries when taken on the whole studied assemblage. The strong size–opal content relation is logically

Conclusions

We present here the first quantitative estimate of the amount of opal carried by several radiolarian species present in the sediments from two cores in the Southern Ocean. These estimates are based on geometrical models reconstructing the radiolarian shape with simple forms. The results of this modelling show that, in the >63 μm fraction, the range of opal carried by radiolarian species is wide and controlled mainly by the size and the structure of the skeleton. Other parameters, such as

Aknowledgements

Thanks are owed to Giuseppe Cortese and Rainer Gersonde for helpful discussions. We also thank the two anonymous reviewers for helping us to improve the manuscript.

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