| Abstract: | Since the Late Triassic, when the nannoplankton began to
produce calcareous platelets thereby causing a major shift
in carbonate deposition from the shelves into the deep
oceans, this diverse and abundant group of unicellular,
planktonic marine algae have been intimately linked to
global change. In modern oceans, it has been demonstrated
that the distributions of certain sensitive nannoplankton
taxa mirror discrete water-masses, the existence of which is
a function of ocean circulation and climate. Thus, global
climate exerts, and must have exerted in the past, a major
influence over calcareous nannoplankton and their
distributions.
Broad nannofossil palaeobiogeographic zones (PBZs) have been
defined for the Late Cretaceous Indian and Pacific Oceans,
at 5my intervals, based on a combination of semiquantitative
and quantitative data from nine Indian Ocean and 16 Pacific
Ocean DSDP/ODP sites. The taxa used in these definitions
either displayed high relative abundances, or were not
highly-abundant but did appear to be geographically
restricted. Since modern nannoplankton distributions are
determined by temperature and/or nutrient concentrations, so
these latitudinally-distributed PBZs are interpreted as
indicating discrete water-masses, possessing differing
temperature and nutrient properties. Grossly, temperature is
intrinsically linked with nutrient concentration, with
warmer waters tending towards oligotrophy, whilst cooler
waters are generally more eutrophic. Thus, movements of the
fronts separating these PBZs through time can be used as
proxies to primarily indicate warming or cooling trends.
Data from the Indian Ocean shows a trend of Late
Albian-Cenomanian cooling, Cenomanian-Turonian warming, and
Turonian to Maastrichtian cooling, with possible warm
phases in the mid-Campanian and Late Maastrichtian, and a
Late Campanian-Early Maastrichtian cooling pulse. Data from
the Pacific Ocean is currently being interpreted, and the
results of this will also be presented.
Comparison of available oxygen isotope sea-surface
temperatures (SSTs) with the PBZ-derived warming and cooling
trends shows a good correlation between the two proxies,
underlining the utility of nannofossils as proxies for
Mesozoic climate change. However, it has been noted that
correlation can be imprecise or offset because SST data is
often generated without regard to a precise stratigraphic framework. |