Heterozoan carbonates in subtropical to tropical settings in the present and past

International Journal of Earth Sciences, Oct 2010

Water temperature has received considerable attention as steering factor for the genesis of different types of marine carbonate sediments. However, parameters other than temperature also strongly influence ecosystems and, consequently, the carbonate grain associations in the resulting carbonate rock. Among those factors are biological evolution, water energy, substrate, water chemistry, light penetration, trophic conditions, CO2 concentrations, and Mg/Ca ratios in the seawater. Increased nutrient levels in warm-water settings, for example, lead to heterotrophic-dominated associations that are characteristic of temperate to cool-water carbonates. Failure to recognize the influence of such environmental factors that shift the grain associations towards heterotrophic communities in low latitudes can lead to misinterpretation of climatic conditions in the past. Modern analogues of low-latitude heterozoan carbonates help to recognize and understand past occurrences of heterozoan warm-water carbonates. Careful analysis of such sediments therefore is required in order to achieve robust reconstructions of past climate.

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Heterozoan carbonates in subtropical to tropical settings in the present and past

Hildegard Westphal 0 1 2 Jochen Halfar 0 1 2 Andre Freiwald 0 1 2 0 A. Freiwald Senckenberg Meeresforschung, Sudstrand 40, 26382 Wilhelmshaven, Germany 1 J. Halfar Department of Chemical and Physical Sciences, University of Toronto at Mississauga , 3359 Mississauga Rd. N, Mississauga, ON L5L 1C6, Canada 2 H. Westphal (&) MARUM Center of Marine Environmental Sciences and Department of Geosciences, Universitat Bremen , Leobener Strae, 28359 Bremen, Germany Water temperature has received considerable attention as steering factor for the genesis of different types of marine carbonate sediments. However, parameters other than temperature also strongly influence ecosystems and, consequently, the carbonate grain associations in the resulting carbonate rock. Among those factors are biological evolution, water energy, substrate, water chemistry, light penetration, trophic conditions, CO2 concentrations, and Mg/Ca ratios in the seawater. Increased nutrient levels in warm-water settings, for example, lead to heterotrophicdominated associations that are characteristic of temperate to cool-water carbonates. Failure to recognize the influence of such environmental factors that shift the grain associations towards heterotrophic communities in low latitudes can lead to misinterpretation of climatic conditions in the past. Modern analogues of low-latitude heterozoan carbonates help to recognize and understand past occurrences of heterozoan warm-water carbonates. Careful analysis of such sediments therefore is required in order to achieve robust reconstructions of past climate. - The increasing public focus on climatic and global changes makes reliable environmental reconstructions of the past more crucial than ever. The record of the past is an important basis for understanding the processes, interactions and dynamics of changing environmental conditions. The short-term changes we observe today are but snap shots. Hence, for extrapolating current trends, for assessing causes, dynamics, and reaction strategies, a deep-time perspective is extremely valuable. Sedimentary rocks are the most important archives of environmental conditions during Earth history. Among sedimentary rocks, carbonates are particularly valuable, because they are mostly of biogenic origin and thus record environmental conditions with a wealth of different facets. Until the end of the 1960s, it was generally accepted that the formation of volumetrically significant carbonate deposits is restricted to the tropical-subtropical climate belt. In fact, the approach to study modern systems as analogues for ancient deposits was developed in warmwater settings (Ginsburg 1956, 1957; Purdy 1961, 1963). It was only in the late 1960s when it was recognized that significant carbonate production also takes place outside the tropics in settings where terrigenous influx is restricted (Chave 1967). Generally, the region of tropical carbonate sedimentation is separated from extra-tropical regions of carbonate formation by the 20 C winter isotherm (e.g., Betzler et al. 1997); however, the distribution of modern coral reefs is constrained by winter minimum temperatures above 18 C (Newell 1971; Belasky 1996). In the 1980s, numerous studies have dealt with modern extra-tropical carbonates, in particular in the southern hemisphere (Nelson et al. 1988; James and Bone 1989; James et al. 1992; James 1997). During the 1990s, numerous studies focused on carbonate settings of polar regions (Henrich et al. 1992, 1997; Andruleit et al. 1996; Freiwald 1998; Rao et al. 1998). Modern deep-water carbonates came into focus with improved marine technology, and since the late 1990s, intensive research of benthic deep-water carbonates such as coral mounds takes place (see reviews of Roberts et al. 2006, 2009). The approach to study modern analogues has greatly improved the interpretation of ancient carbonate rocks (e.g., Grammer et al. 2004). Studies of modern carbonate depositional systems establish relationships between external parameters that, in contrast to the geological past, can be directly measured (cf. Westphal et al. 2010). This includes oceanographic parameters such as seasonality, trophic conditions, temperature, and salinity (Lees and Buller 1972; Carannante et al. 1988). In contrast, studies of ancient carbonate systems have traditionally focused on the interpretation of temperature and relative sea-level position (e.g., Kendall and Schlager 1981; Handford and Loucks 1993). More recent studies of ancient carbonates emphasize the influence of trophic conditions and ocean chemistry among other factors (Pomar 2001a; Hallock 2001; Pomar et al. 2004). While the actualistic concept of the three large carbonate realms (warm, cold, deep) is by now well established (see Schlager 2003), the large and diverse group of carbonates that do not fit into this scheme is currently strongly under-represented in the literature. Wright and Burgess (2005) developed the concept of a carbonate p (...truncated)


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Hildegard Westphal, Jochen Halfar, André Freiwald. Heterozoan carbonates in subtropical to tropical settings in the present and past, International Journal of Earth Sciences, 2010, pp. 153-169, Volume 99, Issue 1, DOI: 10.1007/s00531-010-0563-9