Paleoclimatic Information From Biological Material In Ocean Cores

Paleoclimatic inferences from biogenic material in ocean sediments derive from assemblages of dead organisms (thanatocoenoses), which make up the bulk of all but the deepest of deep-sea sediments (biogenic ooze). However, thanatocoenoses are generally not directly representative of the biocoenoses (assemblages of living organisms) in the overlying water column. For example, selective dissolution of thin-walled specimens at depth (see Section 6.6), differential removal of easily transported species by scouring bottom currents, and occasional contamination by exotic species transported over long distances by large-scale ocean currents all contribute elements of uncertainty. Because of these problems, sediments over much of the ocean floor are unsuitable for paleoclimatic reconstruction. This is illustrated in Fig. 6.2 for foraminiferal studies (Ruddiman, 1977a) though it should be noted that in some areas unsuitable for foraminiferal preservation the remains of other organisms, such as diatoms or radiolarians, may provide a useful record (Sancetta, 1979; Pichon et al„ 1992; Pisias et al„ 1997).

Biogenic oozes are made up primarily of the calcareous or siliceous skeletons (tests) of marine organisms. These may have been planktic (passively floating organisms living near the surface [0-200 m]) or benthic (bottom-dwelling). For paleoclimatic purposes, the most important calcareous materials are the tests of foraminifera (a form of zooplankton) and the much smaller tests, or test fragments, of coccolithophores (unicellular algae) known informally as coccoliths (Figs. 6.3 and 6.4). These are sometimes grouped with other minute forms of calcareous fossils and referred to as calcareous nannoplankton (nanno - dwarf) or simply nanno-liths (Haq, 1978). Organic-walled dinoflagellate cysts are another important paleoceanographic indicator — of SSTs and sea-ice extent in high-latitude regions (de Vernal et al., 1993, 1994, 1998). The most important siliceous materials are the remains of radiolarians (zooplankton), silicoflagellates, and diatoms (algae) (Haq and Boersma, 1978; Fig. 6.5). By studying the morphology of the tests, individuals can be identified to species level and their ocean-floor distribution can then be related to environmental conditions (generally temperature and salinity) in the overlying water column (Fig. 6.6). However, it should be noted that the species assemblage in the sediment is a composite of all the species living at different depths in the water column as well as species with only a seasonal distribution in that particular area. The depth habitats of many zooplankton species are still not well known, and it is believed that some species live at different water depths at different times in their life cycles. Phytoplankton, and zooplankton that possess symbiotic algae, are restricted to the euphotic zone, at least during the productive phase of their life cycles. Depth habitats are of particular significance to isotopic studies of tests

PALEOCLIMATIC RESOLUTION OPTIMAL □] □ ""SUITABLE

FIGURE 6.2 Regions considered to be optimal, marginally suitable, and unsuitable for accurate and detailed Quaternary paleoclimato- ^

logical studies based on foraminifera (Ruddiman, 1977a). Recent cruises of the IMAGES project have targeted sites with high deposition rates n on the continental shelves. Cores recovered from many of these sites provide very detailed records of past conditions that are unattainable >

from the deep ocean. o

Calcareous Foram Test

FIGURE 6.3 Two calcareous tests (200x magnification) commonly used in paleo-oceanographic studies. Bottom: the foraminifera Neogloboquadrina pachyderma (left coiling) (ventral view, 200x), from the North Atlantic (Irminger Basin). Top: Globigerina bulloides (ventral view, 200x) from the Labrador Sea (photographs kindly provided by Laurence Candon, GEOTOP, Université du Québec a Montréal).

FIGURE 6.3 Two calcareous tests (200x magnification) commonly used in paleo-oceanographic studies. Bottom: the foraminifera Neogloboquadrina pachyderma (left coiling) (ventral view, 200x), from the North Atlantic (Irminger Basin). Top: Globigerina bulloides (ventral view, 200x) from the Labrador Sea (photographs kindly provided by Laurence Candon, GEOTOP, Université du Québec a Montréal).

KINGDOM

Protista

Protista

PHYLUM

Thallophyta

Protozoa

Thallophyta

Protozoa

CLASS Chrysophyceae

Bacillariophyceae Actinopoda Rhizopoda (diatoms)*

SUBCLASS

Silicoflagellatophycidae (silicoflagellates)*

Radiolaria Foraminiferida (Radiolarians)* (forams)t

ORDER Coccolithophorates (coccoliths)t

FIGURE 6.4 Taxonomic relationships of the main marine organisms used in paleoclimatic reconstructions. Asterisks indicate siliceous tests; the dagger indicates calcareous tests.

because oxygen isotope composition is a function of the water temperature (and to some extent salinity) at which the carbonate is secreted (see Section 6.3). If test walls are secreted at varying depths through the lifespan of an individual, simple correlations with surface water temperatures and salinities may not be meaningful (Duplessy et al., 1981).

Paleoclimatic inferences from the remains of calcareous and siliceous organisms have resulted from basically three types of analysis:

(a) the oxygen isotopic composition of calcium carbonate in foram tests (Hecht, 1976; Mix, 1987);

(b) quantitative interpretations of species assemblages and their spatial variations through time (Imbrie and Kipp, 1971; Molfino et al., 1982) and

(c) (of far less importance) morphological variations in a particular species resulting from environmental factors (ecophenotypic variations; Kennett,

Most work along these lines has concentrated on the Foraminiferida. In the following sections, therefore, the focus will be on foraminiferal studies. Paleoclimatic studies of coccoliths, radiolarians, diatoms, and silicoflagellates have mainly been in terms of changes in the relative abundance of assemblages (Pichon et al., 1992) though isotopic studies of diatoms have also been carried out (Juillet-Leclerc and Labeyrie, 1987; Shemesh et al., 1992). Oxygen isotope variations in coccoliths provide useful paleotemperature estimates and may provide even more reliable data than that based on forams alone (Margolis et al., 1975; Dudley and Goodney, 1979; Anderson and Steinmetz, 1981). However, there are problems in isolating sufficiently pure samples of very small microfossils such as diatoms or coccoliths.

Dinoflagellate Cyst

top right: dinoflagellate cyst Spiniferites mirabilis dorsal surface (scale bar = 20 |im) from the Gulf of Mexico (see de Vernal et al., 1992). Bottom left: the centric diatom Thalassiosira cf. nordenskioeldii (lOOx) from the Gulf of St. Lawrence; bottom right: the pennate diatom Cymbella proximo ( 1300x) from the estuary of the Gulf of St. Lawrence (photographs kindly provided by Johanne Turgeon and Anne de Vernal, GEOTOP, Université du Québec a Montréal).

top right: dinoflagellate cyst Spiniferites mirabilis dorsal surface (scale bar = 20 |im) from the Gulf of Mexico (see de Vernal et al., 1992). Bottom left: the centric diatom Thalassiosira cf. nordenskioeldii (lOOx) from the Gulf of St. Lawrence; bottom right: the pennate diatom Cymbella proximo ( 1300x) from the estuary of the Gulf of St. Lawrence (photographs kindly provided by Johanne Turgeon and Anne de Vernal, GEOTOP, Université du Québec a Montréal).

Globoquadrina paehyderma

Globigerina quinqueloba

Globigerina bulloides

Globigerinita uvula Globorotalia scitula Globorotalia inflata Globigerinita glutinata

Globorotalia truncatulinoides Globoquadrina dutertrei Globorotalia crassaformis Orbulina universa Globigerinell aaequilateralis Hastigerina pelagica Globorotalia hirsuta Globigerinoides ruber

Globigerinoides conglobatus Globigerina rubescens Hastigerinella digitata Globorotalia menardii Globorotalia túmida Pulleniatina obliquiloculata Globigerinoides sacculifer Sphaeroidinella dehiscens Candeina nitida Globoquadrina conglomerata

Globoquadrina hexagona

Globigerinella adamsi *lndo-Pacific only

Globorotalia

FIGURE 6.6 Sea-surface temperature ranges of some contemporary planktonic foraminifera, illustrating their temperature dependence.Width of lines indicates relative abundance (Boersma, 1978).

FIGURE 6.6 Sea-surface temperature ranges of some contemporary planktonic foraminifera, illustrating their temperature dependence.Width of lines indicates relative abundance (Boersma, 1978).

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