Microbioerosion - Trace Fossils as Indicators of Sedimentary Environments - page 221

Formation from the Mainz Basin, Germany. Only recently have two further case
studies tackled drill-core material, both targeting Holocene reefal development
in the Pacific. In the first study, Heindel et al. (2009) were able to trace the
deepening-upward trend during the last postglacial sea-level rise and to link
it to a possible onset of eutrophication based on a detailed ichnocoenosis anal-
ysis carried out on drill-core material from Tahitian reefs. In the second study,
Chazottes et al. (2009) inversely investigated a series of Holocene shallowing-up
sequences during reef growth recorded in coral cores from New Caledonia.
Caution for the application of microborings as a paleobathymetrical
indicator concerns the following issues: (1) This method always provides only
relative bathymetrical evidence in the form of an assignation to a specific photic
zone, since other factors controlling the absolute depth (mainly the turbidity)
are not known and may vary on a small scale ( Perry and Macdonald, 2002 ).
(2) A significant local variance in ichnocoenosis composition may further be
encountered in spatially highly structured paleoenvironments such as a reef
setting comprising illuminated, shaded, and even cryptic subhabitats (“crypto-
photic” sensu Heindel et al., 2009 ) potentially supporting the whole range of
index ichnocoenoses even at very shallow depth ( Gektidis et al., 2007; Perry
and Macdonald, 2002 ). (3) The investigated substrate needs to be evaluated
with respect to potential transport from a shallower photic zone to its final site
of deposition, making sessile substrates such as corals more reliable and calling
for a reasonable amount of samples to be investigated in order to achieve a solid
interpretation.
Table 2 summarizes the current state of knowledge about the bathymetrical
distribution of relevant ichnotaxa and their known or inferred producers. While
a wealth of studies has been carried out in intertidal to aphotic waters, the supra-
tidal zone is understudied with respect to ichnotaxonomical treatment of the
inhabiting euendolithic cyanobacteria, and the respective index ichnocoenosis
is yet to be defined.
4.2 Paleothermometry
The evaluation of microborings as a paleotemperature and/or paleolatitude indi-
cator is still in its infancy, even though a respective potential has long been
pointed out (e.g., Golubic et al., 1975 ). In modern seas, many euendoliths
are eurythermal and have a cosmopolitan distribution (e.g., O. quekettii , P. ter-
ebrans , Dodgella priscus , Conchyliastrum enderi ), but certain taxa are steno-
therm and occur within specific temperature limits resulting in a restricted
biogeographic distribution. This applies, for instance, to many chlorophytes
and cyanobacteria, which share a similar upper temperature limit of about
35 C but show considerable differences with respect to their lower tolerance
limit ( L¨ning, 1985 ). This factor acts in concert with the general trend of deple-
tion in the phototrophic borer spectrum with higher latitude as a result of
the seasonal fluctuation of light availability ( Schmidt and Freiwald, 1993 ).