Research paperRepresentativity and applicability of small amounts of sediment samples in the determination of absolute diatom abundances (ADA): Evidence from an orbital shaking method
Introduction
Diatom remains in oceanic and freshwater sediments are valuable indicators of past environments (Round et al., 1990). Although diatom assemblages that are expressed as relative diatom abundances have long been used for paleoenvironmental and paleoclimatic reconstructions, various methods for determining the absolute diatom abundance (ADA) in marine or lacustrine sediments have been developed over the past decades (Battarbee, 1973; Schrader, 1973; Schrader and Gersonde, 1978; Koizumi, 1980; Kaland and Stabell, 1981; Battarbee and Kneen, 1982; Pokras and Molfino, 1986; Bodén, 1991; Scherer, 1994; Gersonde and Zielinski, 2000; Warnock and Scherer, 2015), and ADA estimation is gradually becoming a routine step in diatom paleoecological studies (e.g., Nees et al., 1999; Lopes et al., 2006; Konfirst et al., 2012; Nair et al., 2015; Barbara et al., 2016).
The technology of aliquot subsampling has long been applied for the determination of diatom abundances in sediments. However, during the process of aliquot subsampling, most ADA results obtained by previous methods are affected by necessary but arbitrary assumptions which introduce uncertainties into slide preparation or diatom counting, including pipetting a ‘representative’ aliquot of the diatom suspension from the middle or near the top of the water column onto the coverslip after 5–10 s of settling (e.g., Koizumi, 1980), or generating a presumed ‘well-mixed’ diatom suspension before aliquot subsampling (e.g., Battarbee, 1973), or adding a known concentration of foreign plant spores or palynomorphs that are assumed to evenly mix with diatoms (e.g., Kaland and Stabell, 1981).
In recent years, a new method that does not use aliquot subsampling for the determination of diatom abundance was proposed by Scherer (1994) and subsequently revised by Warnock and Scherer (2015). It was claimed that accurate and reproducible ADA estimates of the processed sediment samples were obtained using sediment samples in milligrams, and sample sizes did not significantly affect results of ADA estimates (Scherer, 1994). However, given that small amounts of sediments represent only a rather small proportion of the original sample, it is debatable whether or not they are truly representative of the original sample.
This paper proposes a new method that applies an orbital shaking technology to obtain ADA estimates in deep-sea fine-grained sediments. From the comparative study of ADA estimates of the new method and two other methods we try to determine whether or not a small sample in milligrams is representative of the original sediment in grams.
Section snippets
Sediment samples
Core LV63–23 (61.149°N, 176.761°E, 1891 m water depth, 20 cm in length) was collected from the Bering Sea with a multi-corer (98 mm diameter). Only the subsurface sediment to a depth of 11 cm to 12 cm (BS) was used in the present study. Results of 210Pb and 137Cs dating indicate that this layer of sediment spans a period from ca. 1942 to 1950 CE and the bioturbational mixing depth in this core is not deeper than 6 cm (Hu et al., unpublished data).
Core MD01–2416 (51.268°N, 167.725°E, 2317 m
Results of diatom counting along diameters
The number of diatom valves along each diameter varies from 2.5 to 777.5, with a mean of 5.5 for NP 2–3 and NP 2–4 and a mean of 376.2 for the rest (Table 2). The intra-assay coefficients of variation were generally below 12.2% except for NP 2–3 and NP 2–4. Coefficients of variation of NP 2–3 and NP 2–4 are relatively high (from 34.0% to 41.0%) since it is difficult to reach statistically significant counts even if all diatom valves on the coverslip have been counted (Table 1).
Accuracy of diatom counting along diameters
In order to test
Influences of sediment micro-heterogeneity on the results of ADA estimates
There have been few studies, so far, on whether the degree of homogenization of marine or lacustrine sediment samples has an impact on the results of absolute and/or relative diatom abundance. Once it was inferred by Scherer (1994) that sample size did not significantly affect results of ADA estimates since reproducible results were obtained even when the sample mass varied from 0.0007 g to 0.0640 g. In that vein, a question can be raised concerning homogeneity in the sediments as well as
Conclusions
- (1)
To asses the representativity of small amounts of sediment samples for the determination of diatom abundances in fine-grained marine sediment, a new method using an orbital shaking technology is proposed in this study. The most distinguishable difference between the orbital shaking method and previous methods is that during the preparation of diatom slides all particles in the settling cylinders are orbital shaken (not involving aliquot subsampling or random settling) before they settle onto
Acknowledgments
The authors are grateful to Michael Sarnthein of Kiel University and Rainer Gersonde of Alfred Wegener Institute for providing three layers of sediments in the core MD01-2416 and to the crew on board of RV Marion Dufresne and R/V Akademik M. A. Lavrentyev for obtaining the sediments. This work was jointly supported by the National Natural Science Foundation of China (41106166, 41606052), and NSFC-Shandong Joint Fund for Marine Science Research Centers (U1606401, U1606405). Comments from the
References (38)
- et al.
Living diatom assemblages from North Pacific and Bering Sea surface waters during summer 1999
Deep-Sea Res. II
(2005) - et al.
Diatom distribution in southeastern Pacific surface sediments and their relationship to modern environmental variables
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2010) - et al.
The reconstruction of late Quaternary Antarctic Sea-ice distribution -the use of diatoms as a proxy for sea-ice
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2000) - et al.
A marine diatom record from the Amundsen Sea - Insights into oceanographic and climatic response to the Mid-Pleistocene transition in the West Antarctic sector of the Southern Ocean
Mar. Micropaleontol.
(2012) - et al.
Diatoms in Northeast Pacific surface sediments as paleoceanographic proxies
Mar. Micropaleontol.
(2006) - et al.
A diatom and benthic foraminiferal record from the South Tasman rise (southeastern Indian Ocean): implications for palaeoceanographic changes for the last 200,000 years
Mar. Micropaleontol.
(1999) - et al.
Oceanographic control of diatom abundances and species distributions in surface sediments of the tropical and Southeast Atlantic
Mar. Micropaleontol.
(1986) - et al.
Diatom distributions in northern North Pacific surface sediments and their relationship to modern environmental variables
Palaeogeogr. Palaeoclimatol. Palaeoecol.
(2014) - et al.
Micro-scale biogeochemical heterogeneity in sediments: a review of available technology and observed evidence
Earth Sci. Rev.
(2009) - et al.
Environmental responses of the Northeast Antarctic Peninsula to the Holocene climate variability
Paleoceanography
(2016)