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Palaeogeography, Palaeoclimatology, Palaeoecology 149 (1999) 199–211
Taphonomy of tidal marsh foraminifera: implications of surface sample thickness for high-resolution sea-level studies R. Timothy Patterson a,Ł , Jean-Pierre Guilbault b , John J. Clague c,1 a
Ottawa–Carleton Geoscience Centre and Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada b BRAQ–Stratigraphie, 10545 Meilleur, Montreal, QC H3L 3K4, Canada c Geological Survey of Canada, Suite 101, 605 Robson Street, Vancouver, BC V6B 5J3, Canada Received 11 December 1996; revised version received 20 September 1997; accepted 8 June 1998
Abstract Previous research has shown that intertidal foraminiferal faunas can be used to document Holocene relative sea-level change and large prehistoric earthquakes. Applications like these, however, require an understanding of the impact of infaunal habitat and taphonomic processes on foraminiferal assemblages. To evaluate these effects, we analyzed surface sediment samples collected along a transect across a tidal marsh at Zeballos on Vancouver Island, British Columbia. Samples of the uppermost 10 cm of sediment in the marsh contain foraminiferal assemblages that permit recognition of a greater number of elevation-controlled marsh assemblages than samples of the top centimeter, which are generally used in sea-level studies. This is because the upper 10 cm contain most infaunal foraminifera species, whereas the top centimeter commonly lacks some of these species. A 10-cm thickness is somewhat arbitrary, but most foraminiferal taphonomic biasing occurs in the top 10 cm of the marsh. 1999 Elsevier Science B.V. All rights reserved. Keywords: taphonomy; foraminifera; sea level; tidal marsh; British Columbia
1. Introduction During the past ten years, paleontologists have worked with geologists in an attempt to reconstruct the history of sea-level change and large earthquakes by studying plant and animal microfossils recovered from tidal marsh sediments on the coasts of British Columbia, Washington, and Oregon (Williams, 1989; Patterson, 1990; Jennings and Nelson, 1992; Jonasson and Patterson, 1992; MathŁ Corresponding
author. Tel.: C1 613 520 2600, ext. 4425; Fax: C1 613 520 4490; E-mail:
[email protected] 1 Present address: Earth Sciences, Simon Fraser University, Burnaby, BC V5A 1S6, Canada.
ewes and Clague, 1994; Guilbault et al., 1995, 1996; Hemphill-Haley, 1995; Hutchinson et al., 1995; Reinhardt et al., 1996; Ozarko et al., 1997). The impetus for this research has been the recognition that the region is located at an active plate margin (Riddihough and Hyndman, 1976) and is subject to great earthquakes (magnitude 8 or more) which are larger than any occurring during the historical period (Atwater et al., 1995, and references therein). Were such an earthquake to occur today, it would cause extensive damage to cities and the economic infrastructure of the region (Clague, 1996). Episodic crustal subsidence during great earthquakes has played an important role in shaping Holocene sedimentary sequences at tidal wetlands
0031-0182/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 8 ) 0 0 2 0 1 - 6
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along the Pacific coasts of Vancouver Island, Washington, Oregon, and northern California (Atwater, 1987; Nelson, 1992). Abrupt submergence of wetlands, which is inferred to result from coseismic subsidence, is indicated by marsh or forest soils that are abruptly overlain by intertidal muds or lowmarsh peats. Mid- and high-marsh foraminiferal and diatom assemblages associated with the soils are sharply overlain by low-marsh and tidal flat assemblages. The amount of submergence inferred from the fossil evidence is a meter or more in some areas (Guilbault et al., 1995, 1996; Atwater and HemphillHaley, 1997). An inherent assumption in reconstructing past sea-level change using foraminiferal and diatom data is that modern assemblages collected at the surface of the marsh are accurate analogues of microfossil assemblages in subsurface sediments. However, this assumption may not be correct, at least in the case of foraminifera, because many species are primarily infaunal and also because of preservational biases. Ozarko et al. (1997) studied foraminifera faunas in cores from a marsh at Nanaimo on eastern Vancouver Island and determined that the fauna in the uppermost centimeter of the marsh sequence does not provide an accurate analogue of fossil assemblages. Goldstein and Watkins (1999) obtained similar results in studies carried out at marshes in the southeastern United States. These authors and Walker and Goldstein (1999) provide detailed discussions of infaunal habitat and taphonomic processes affecting marsh foraminifera. In particular, Walker and Goldstein (1999) identify the top 10 cm of the marsh as the taphonomic active zone where most preservational biasing occurs. All of the above-mentioned authors independently conclude that samples of the top 10 cm of the marsh sequence are more useful for paleoenvironmental studies than samples of the top 1 cm interval. Such samples enable researchers to recognize smaller sea-level changes than would otherwise be the case. The increased sensitivity is required if researchers are to document, in detail, late Holocene relative sea-level change and discriminate changes produced by earthquakes from those caused by nonseismic processes. Our study is an extension of the work of Ozarko et al. (1997). Whereas those researchers demonstrated that samples of the top 10 cm of the marsh rep-
resent modern foraminifera assemblages better than traditional 0–1 cm samples, we assess the practical value of this result for sea-level reconstruction. To be deemed a better approach, it must be shown that analysis using the proposed methodology provides vertical resolution of assemblage zones at least equivalent to that provided by surficial (0–1 cm) data. Practically speaking, the larger the number of modern assemblage zones one can recognize, regardless of the interval analyzed, the more precise will be the vertical resolution obtained from analogous fossil assemblage zones. To this end, we documented the foraminiferal fauna along a transect across a tidal marsh at Zeballos on northwestern Vancouver Island (Fig. 1). The site was chosen in part because the marsh, although relatively small, bears evidence of one or more earthquakes (our unpublished data).
2. Methods 2.1. Field and laboratory On June 13, 1995, 76 samples were collected for foraminiferal analysis at 38 stations along three legs of a single transect across the marsh, just west of the town of Zeballos (Figs. 1 and 2). The relative elevations of stations were measured with a surveying level and are accurate to within 1 cm. Absolute elevations were calculated by tying high-tide measurements in the marsh to high-tide level recorded at the nearby Zeballos tide gauge. Fortuitously, sampling was done at the time of the highest spring tide of the year, which allowed for a precise determination of the maximum elevation reached by the sea within the marsh. A 10-cm3 surface sample (0–1 cm) and a 10-cm3 sample of the top 10 cm were collected at each of the 38 stations. Most samples were stored in Ziploc plastic bags and treated in the field with isopropyl alcohol to prevent microbial decay of living protoplasm. In the laboratory, approximately 10 cm3 of each sample were washed on a 63-µm sieve. Samples were then fixed in a solution of Rose Bengal stain and buffered formalin and allowed to sit for several hours. This procedure allows for distinguishing foraminifera that were living at the time of sample collection from those that were dead (Scott
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Fig. 1. Map of the Zeballos area (left) showing the location of the marsh transect and floodplain, marsh, tidal flat, and upland environments. Details of the specific legs of the transect are depicted on the right.
and Medioli, 1980a). This procedure is also effective in documenting infaunal marsh foraminiferal microhabitats (see Goldstein et al., 1995, for a discussion). After staining, the samples were washed in tap water and preserved in a 5% isopropyl alcohol solution. They were then washed through a 500-µm screen to remove large plant debris that might inhibit counting. The residue was split using the wet splitter method of Scott and Hermelin (1993) until a fraction of countable size. This fraction represented approximately 500 specimens which is commonly about one-sixth of the original sample. Wet samples were examined under a binocular microscope, generally at around 40ð magnification. Water immersion helped in the identification of marsh species, because organic matter found in marsh samples tends to adhere to foraminiferal tests if samples are dried. Of the 76 samples, all but one, ZTT13, contained statistically significant numbers of foraminifera (Tables 1 and 2; see Patterson and Fishbein, 1989, for background on estimating statistical significance). Separate tallies were made of both total and live species (Tables 1 and 2).
2.2. Quantitative analytical procedures The foraminiferal data were converted into fractional abundances, and standard errors were then calculated according to the following formula proposed by Patterson and Fishbein (1989): S X i D [X i .1
X i /=N ]1=2
where S X i is the standard error, X i is the estimated fractional abundance for each i D 1; 2; 3; : : :; I species, where I D the total number of species in the sample, i is each species, and N is the total number of specimens counted in a sample. When making N counts, the actual fractional abundance f i lies between Xi
1:96S X i fi X i C 1:96S X i
95% of the time, regardless of the number of species (Patterson and Fishbein, 1989). Therefore, the 95% confidence interval on the estimated fractional abundances is X i C 1:96S X i . The standard error for samples having no specimens of a particular species was calculated using the standard error equation (S X i ; see
Miliammina fusca (>183 µm) live dead total Miliammina fusca (183 µm) live dead total Miliammina fusca (183 µm) live dead total Miliammina fusca (183 µm) live dead total Miliammina fusca (183 µm and juveniles
