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Fig. 1 Photograph of Latemar from southwest. [6]

Latemar Controversy

Latemar is a carbonate platform with a radius of around 3 km located in northern Italy. It was formed during Middle Triassic around westernmost area of Triassic Tethys under marine environment. The focus of this essay is on the formation mechanism of the top 470 m of the Latemar platform succession, which contains around 600 cycles in a group of 5 thinning-upward megacycles. The Latemar controversy is over the mechanism of Latemar formation. Intense debates exist among researchers on whether the strong cyclic character of Latemar was mainly allocyclic, meaning driven by external forcings, or autocyclic, driven by more local forcings.

Fig. 2 Locality of Latemar during Triassic [2].

Latemar formation requires an accumulation mechanism that occured repetitively throughout the platform formation. As the cyclic carbonate stratigraphy is observed from Precambrian to present, this mechanism has to be independent of geological age. Therefore, Goldhammer and Hinnov[2] proposed that the cyclic patterns at Latemar is allocyclic and was formed due to Milankovitch cycles. Under this hypothesis, the 600 cycles of Latemar was formed through 9-12 million years (Myr) due to the 20 thousand-year (kyr) duration time of Milankovitch cycle. Goldhammer and Hinnov conducted time seris analysis and found 20 kyr cycles superimposed on 100 kyr megacycles at Latemar, showing long-term subsidence superimposed by rapidly changing sea levels under Milankovitch control. They concluded that this 5:1 bundling was resulted from a combination of weak eccentricity and dominant precession forcing - glacioeustasy changes. However, Brack and Kent[3] argued that the formation time was much shorter and is autocyclic dominated. As the Middle Triassic is about 10 Myr, they doubted that the formation of Latemar could take more than the whole middle Triassic. In this article, three different dating methods used in studies on Latemar formation are introduced and hypotheses from both autocyclic and allocyclic camps are discussed.

One approach to date Latemar succession is to connect the layers in Latemar to somewhere else where the age is better dated and use that as the age of latemar formation. The red line in the right figure crosses a series of carbonate platforms and basinal beds. In Fig. 3, a vertical section along this line is illustrated, where the blue parts represent the pelagic basinal part of Buchenstein Beds with (semi-)pelagic sediments. The green parts represent carbonate platforms, for example, Seceda and Bagolino, which are both very important locations for dating the Latemar succesion. If layers can be found in Latemar corresponding to certain layers in the Buchenstein beds, then the period for Latemar formation is just the same as the time taken to form the corresponding Buchenstein beds. This leads us to the first dating method, biostratigraphy.

Fig. 3 Vertical sections of Seceda ad Bagolino Buchenstein Beds (source:Peter Brack)

Biostratigraphy

With biostratigraphic approach, fossils of Ammonites and Daonella were sampled. Dark ammonites were found in basinal Buchenstein Beds and they were formed in pelagic environment. Brighter ammonites were found in Latemar, deposited in platform interior. These ammonites found in both basinal bed and the Latemar are of the same type and are coeval. Faunas L1–L4 are Ammonoids and Daonella. At the beginning, ammonites were only found in very low altitude in Latemer (L1), therefore only the lower age boundary was certain. L3 and L4 were first used for the upper age threshold, found on the slopes near latemar, which were interpreted to be coeval to upper cyclic Latemar succession Later in 2003, ammonites (L5) were also found on top of Latemar, which finally enabled defining the upper age boundary. These fossils were connected to the same type of ammonites found in the Buchenstein beds of Bagolino and Seceda.

Fig. 5a Sampling profile of faunas on Latemar [3].

Fig. 5b Different colours of Ammonite fossils.

These Ammonoids lived around the periods of Anisian and Ladinian. The Ammonite species sequence in basinal succession corresponds nicely to that found in the Latemar. Based on the coeval fossils, the maximal equivalent of the 470 m Latemar cyclic succession in the basinal succession was found to be around 6-10 m. Since the formation time of this 6-10 m succession is equivalent to that of the 470 m succession at Latemar, The 9-12 Myr formation time suggested by Hinnov and Goldhammer [2] therefore seems rather unreasonable.

Fig. 4a Sketch of Latemar seen from Viezzena to southeast. From bottom to top: Contrin Formation, lower platform facies, lower cyclic facies, tepee facies, upper cyclic facies [1].

Fig. 4b Red line represents continuously growing beds of carbonate platform. Orange lines represent slopes, dipping beds of rubble.

A simplified sketch is drawn to illustrate the positions of Latemar (in the middle) and other carbonate platforms. The carbonate platform (600-800 m thick platform interior succession) in the middle was formed under the influence of sea surface fluctuations and the basinal deposits are about 60-70 m.

Fig. 6a Biostratigraphy at Latemar, Seceda and Bagolino. Different Ammonite types indicated by different colour bars. (source: Peter Brack)

Fig. 6b Comparison of the 470 m at Latemar and the coeval 10 m Buchenstein sequences (source: Peter Brack).

In addition to the biostratigraphic features in Fig. 6a, another interesting feature to be noticed is the tuff layers (green). This can be used for a more precise dating method: radiometric dating.

Radiometric Dating

This daing method is based on the decay of Uranium (U) to lead (Pb) and the time-dependent enrichment of Pb within zircon. Newly-formed zircon deposits contain no Pb, meaning any Pb found in the mineral is radiogenic. The presence of volcanoclastic layers can help in finding the age thresholds of the Latemar section as volcanic ashfalls contain zircons, which can be used for isotope dating and deliver more precise, absolute ages comparing to the biostratigraphic approach.

High-resolution, single-grain U-Pb age determinations has been performed on zircons from several individual volcaniclastic intercalations at Seceda, which was found to be closely correlated with sections at Bagolino and Monte San Giorgio. This 75 m thick succession of pelagic sediments is interrupted by multiple layers of acidic volcanoclastic material - "Pietra Verde" bed (Fig. 7). The slope deposits in the background are interfingering with the Buchenstein formation. A lateral connection exists between these platform deposits (such as Latemar) and the Buchenstein beds. In addition, as both the Latemar and the Buchenstein beds lie directly above the Contrin Formation, a common lower boundary of these two sequences can be identified.

Fig. 7 Volcanic ash layers in green, Contrin Formation in yellow. (source: Peter Brack)

Ages of crystallisation of the zircon grains are taken as estimations for the deposition time of the volcaniclastic layers. An age of 241.2 +0.8/-0.6 Myr (95% confidence level) has been obtained from zircon grains in the lower part of the Buchenstein Beds. These tuff beds correspond to the lowermost part of the cyclic succession at Latemar. For the uppermost Buchenstein Beds, an age of 238.0+0.4/-0.7 Myr was obtained from zircon. This corresponds to a level younger than the top of the cyclic succession at Latemar.

Magnetostratigraphy

The third dating method is magnetostratigraphy. Normally time interval between two adjacent magnetic polarity change is around 0.5 Myr. A collection of 73 samples was taken from the base to the top of latemor successions. The initial samples of virtual geomagnetic pole (VGP) latitude versus stratigraphic sample level are indicated by the white dots. With the contamination on magnetization from lightnings removed, the black dots show the calibrated VGP latitude versus altitude. As a result, we can see that there is no polarity change, or at most one potential polarity change, observed within the cyclic succession of Latemar. This suggests that we cannot be anywhere close to 9-12 Ma of deposition time, but that this 470 m of Latemar was formed in less than 1 Myr.

Fig. 8 VGP latitude versus pre-calibration (white) and calibrated (black) samples. Sampling profile on Latemar on left [3].

With dating methods developing over time, a better time constraint on Latemar formation period is being achieved. The measurement error has been reduced from around 1 Myr in the 90s to around 50 kyr now. While earlier dating was taken on Bagolino, later on the dating was taken directly on Latemar. A latest study in 2018 [5] also agreed with the 1 Myr formation time. This reserach paper mentioned a thin layer of less than 1 meter in that region with precession feature.

Summary

Results from different dating methods seem to lead us to a formation time of 1 Myr. However, with 600 cycles, this results in a cycle duration of 1 kyr. Considering the amount of 470 m of sediment thickness, a constant sedimentation rate of around 1 m/kyr is calculated. Seemingly surprising, this is not entirely impossible. As an example, sedimentation rate at Lake Zurich reached more than 1 m/kyr.

Even though disagreeing with the allocyclic hypothesis from Hinnov and Goldhammer, Brack et al. [1] also concluded that the existence of precession forced parts in the Buchenstein Formation. However, Brack et al. proved the bedding frequencies to be much higher at Latemar, suggesting the existence of a much faster formation mechanism. Hence, all evidences seem to lead to autocyclic cycles at Latemar.

Hardie and Hinnov [4] commented on these hypotheses in 1997. Based on thickness hierarchies, which are exposure-capped cycles, the cycles cannot be generated by single-periodicity sea-level signal. Furthermore, they argue that Hinnov in 1994 found that Latemar’s upper cyclic facies exhibit all major frequency components of climatic precession. They also questioned if the sampled volcanoclastic rocks are primarily ashfalls. With respect to other studies that reported “reworked” acid volcanic material in the Buchenstein beds in the Dolomites, Hardie and Hinnov [4] argued that Brack et al. [1] might have used reworked and resedimented late Anisian-earliest Ladinian volcanic debris. Hence they doubted that the isotope ages may appear to be younger than the actual age due to reworking. They criticised from a methodology point of view, the use of transmitted light instead of cathodo-luminescence (CL) and back-scattered electrons (BSE). But Kent and Brack argued back that results from the most convining method, which is U-Pb zircon dating, were ignored.

On the other hand, regular cyclic bedding patterns are found at locations that are over 200 km apart from each other. The same pattern occurs in Buchenstein beds at different facies and basins, such as Seceda and Andraz in the Dolomites, as well as Bagolino and S. Valentino in the Lombardy. This wide-spread distribution speaks for an external forcing of the deposition cycles. Meanwhile, other localities have much thinner sequences, suggesting local differences in subsidence.

Brack et al. [1] proposed three explainations to the cycle duration at Latemar: (1) Cycles have identical periods and represent sea level oscillations with longer than 8 kyr duration (sub-Milankovitch cycles). This leads the question of a possible influence of tectonic component, since normally sea level does not decrease at this ratey on itself. Therefore, there is more than just sea level fluctuations controlling the pattern. (2) Latemar stack is composed of intervals with Milankovitch characteristics as well as other non-periodic signals or signals of much shorter duration, which can explain the local occurrence of 5:1 bundles. (3) None of these Latemar “cycles” are periodic, neither allocyclic or autocyclic. Other possible explanations include tidal forcing and climatic variations. Climatic variations with cycles of about 1.5 kyr can be observed in the late Pleistocene and Holocene with stronger impacts than solar fluctuations [3].

What do you think about the two theories, allocyclicity or autocyclicity, what makes more sense to you?

Ocean, Climate, Paleoclimate and Geoscience

Latemar Controversy

Biostratigraphy

Radiometric Dating

Magnetostratigraphy

Summary

References