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In-Depth Information
note that these measurements were taken from
fast-growing corals, not from stacked peritidal
depositional cycles.
In either case (pure Milankovitch or mixed
Milankovitch and sub-Milankovitch), both mod-
els rely on an astroclimatic forcing mechanism as
a cyclic driver for carbonate sedimentation. The
signs of this forcing are recognized via statistical
analysis of Latemar cycles and cross-applied to the
Mendola succession. Comparative sedimentology
indicates that Holocene carbonate successions
formed at rates commensurate with multimillen-
nial processes, providing further evidence for a
link between an allocyclic, Milankovitch cycle
driven process and the sedimentary record.
Periodic drivers for Triassic millennial
shallowing-upward cycles are diffi cult to identify
because no such drivers have yet been linked to
the deposition of stacked shallowing-upward
cycles in the Holocene or Pleistocene (see previ-
ous discussion). Kent
et al
. (2004) argued for tidal
forcing operating at around 1.8 kyr, citing a theo-
retical tide at the periodicity identifi ed by Munk
et al
. (2002). However, Munk
et al
. (2002, p. 382)
stated that 'the equivalent tidal amplitude of the
millennial term is estimated at 0.04 mm', which
is clearly not enough to generate the metre-scale
Latemar cycles. Similarly, there is no known con-
nection between millennial-period Dansgaard-
Oeschger climate cycles and Heinrich-type
warming events cited by Emmerich
et al
. (2005)
as possible cycle drivers. No link has yet been
established between these cycles and formational
periodicities of shallowing-upward carbonate
successions of the Holocene and Pleistocene.
Recently, Roth & Reijmer (2005) identifi ed oxy-
gen isotope excursions in a 30-m-long core from
the leeward margin of the Great Bahama Bank that
have both centennial and millennial (1.7 kyr) peri-
odicities. While this discovery establishes a link
between centennial and millennial cyclic processes
and carbonate sedimentation, the cycling has yet to
be linked to the depositional periods of platform-
interior shallowing-upward depositional cycles.
The identifi cation of periodic megacycle drivers
within the millennial model is likewise problematic.
Studies from the Holocene and Pleistocene have
yet to identify a lower-frequency (5-10 kyr) cyclic
process by which millennial tides, Bond cycles,
or other climatic processes will produce bundled
shallowing-upward carbonate megacycles.
While the rates of eustatic rise and fall required
for millennial cyclic carbonate deposition are
rapid, rates for subsidence and sediment accu-
mulation required for the development of deposi-
tional cycles within this context are likewise very
high. The subsidence required to generate 700 m
of section in 1 Myr is approximately 0.7 m kyr
1
.
As both the Latemar and Mendola sections pre-
serve cyclic successions of similar stacking and
bundling trends, subsidence would have operated
at near-equal rates at both localities during the
study interval in question. In addition, accumula-
tion rates (including time for sedimentation and
subaerial exposure) of platform interior and tidal-
fl at sediments would need to be extraordinarily
high in the millennial framework, requiring a sus-
tained accumulation rate in excess of 0.7 m kyr
1
approaching the growth rates of Holocene corals,
The successions of carbonate depositional
cycles at Latemar and Mendola Pass sections
record periodic, allogenic processes operating at
millennial periodicities
In reference to the question of cyclicity in sedi-
mentation in Latemar, Emmerich
et al
. (2005)
wrote 'radiometric age dating on detrital zircons
in air-borne tuff layers intercalated within the
cyclic succession of the Latemar solved the con-
troversy' (Emmerich
et al
., 2005, p. 11). However,
this statement is by no means universally
accepted. Indeed, the mean ages in their correla-
tion scheme (see Fig. 7b in Emmerich
et al
., 2005)
indicates two age-reversals (i.e. older-upward
rather than younger-upward), the fi rst occurring
from 'Tc' (241.2 + 0.8/
0.6 Ma) to LAT-31 (242.6
± 0.7 Ma) and the second occurring between
the platform-interior ash beds LAT-30 (241.2
+ 0.7/
0.7 Ma).
Moreover, the ±0.6 kyr and larger uncertainties
may preclude the direct use at these ages to assign
any specifi c millennial timing to the Latemar cycle.
Nonetheless, the uncertainties point towards a
millennial rhythm, and here existing proposals
for specifi c millennial timings from experiments
are simply chosen.
Emmerich
et al
. (2005), and independently,
Kent
et al
. (2004), suggested that both cycles
and megacycles in the Latemar succession
operate at sub-Milankovitch cycle frequencies,
i.e. 0.9-1.97 kyr per cycle and 3.5-10 kyr per
megacycle. As a result, the accumulation of
the entire Latemar buildup was calculated to
have occurred over approximately 1 Myr. Indeed,
the prevalent 5:1 megacycle bundling originally
identifi ed by Goldhammer
et al
. (1987) was not
recognized by Kent
et al
. (2004).
0.6 Ma) and LAT-32 (241.7 + 1.5/
