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)