Stratigraphy and facies differences of the Middle Darriwilian Isotopic Carbon Excursion (MDICE) in Baltoscandia

. The Middle Darriwilian Isotopic Carbon Excursion (MDICE) is a global isotopic event described in sections from different palaeocontinents. Here we present new stable carbon isotopic data from carbonates of ten sections in different parts of the Baltoscandian Palaeobasin (Estonia, Latvia, Lithuania, Sweden, NW Russia). The definition of the MDICE as a chemostratigraphic unit is discussed, as well as the subdivision of its peak into two distinct peaks. The MDICE is one of the longest carbon isotopic events in the Palaeozoic. It was preceded by the L­chondritic cosmic dust flow event, which may have been responsible for cooling through the Darriwilian and the initiation of the Great Ordovician Biodiversification Event. High­resolution chemostratigraphic analyses show that the time interval between these environmental events and the base of the MDICE is up to one million years. Due to the long duration of the MDICE the modelling of this excursion should address more complex scenarios than a simple response of the carbon cycle to rapid climatic perturbations.


INTRODUCTION AND GEOLOGICAL SETTING
The Middle Darriwilian Isotopic Carbon Excursion (MDICE) was first described by Ainsaar et al. (2004) in the Baltoscandian sections. Recorded in several palaeo continents like Baltica (Ainsaar et al. 2007(Ainsaar et al. , 2010, Laurentia (Leslie et al. 2011;Bergström et al. 2018), Argentine Precordillera (Albanesi et al. 2013), Siberia (Ainsaar et al. 2015), South China (Schmitz et al. 2010;Munnecke et al. 2011), North China (Bang & Lee 2020) and Tarim Basin (Zhang & Munnecke 2016), it is now known as one of the three major global Ordovician carbon isotopic events (together with the Guttenberg and Hirnantian carbon isotopic excursions). The raised δ 13 C values of the MDICE are covering the middle or upper parts of the Darriwilian Stage, but the amplitude of variation in carbon isotopic values is within the limits of 2‰ in most cases.
The chemostratigraphy of the Darriwilian Stage is relatively well studied in the sections of the Baltoscandian Palaeobasin compared to other areas of the world. Carbon isotope data from 18 outcrop and drillcore sections of Estonia, Latvia, Sweden and NW Russia have been published since 2004 (Ainsaar et al. 2010;Wu et al. 2017, and references therein). However, the stratigraphic range of the MDICE as a chemostratigraphic unit has been dealt differently (only the peak values or the limbs of the curve included) and the facies variations of the isotopic values have not been studied systematically. In this study, we present new data from ten sections in northern and southern Estonia, western Latvia, Lithuania, NW Russia (Pskov and Leningrad regions) and Sweden (Gotland and Jämtland). Data from Lithuania, Gotland and the Pskov region are first reported from these areas. Comparative analysis of all these isotopic curves helps to evaluate the chemostratigraphic potential of the characteristic intervals of the MDICE, including previous work on the subdivision of the sedimentary succession to isotopic zones (Ainsaar et al. 2010) or different peaks (Lehnert et al. 2014;Wu et al. 2015). This work allows us to track and discuss more precisely secular palaeoenvironmental changes and lateral (facies) variability of the carbon isotopic signal during the Darriwilian in the Baltoscandian Palaeobasin.
The Middle Ordovician strata in Baltoscandia comprise a nearly continuous succession of limestones and marls (Nestor & Einasto 1997). The general distribution of the Ordovician fauna and lithofacies delineates largescale facies zones within the Balto scandian Palaeobasin (Jaanusson 1976;Harris et al. 2004; Fig. 1). The Early Ordovician Tremadocian silici clastic ramp environment of the Baltoscandian Palaeobasin was replaced by a coolwater carbonate ramp setting in the Floian and evolved into a temperate carbonate ramp in the Darriwilian Age (Nestor & Einasto 1997;Dronov & Rozhnov 2007). Carbonate sediments, wackestonespackstones, formed in the inner to middle ramp settings, mainly below the fairweather wave base in northern Estonia (North Estonian Confacies by Jaanusson 1976; Estonian Shelf facies by Harris et al. 2004). This area is characterized by numerous sedi mentary gaps, some of which are related to smallscale stratigraphical uncon formities marking boundaries of depositional sequences (Nestor & Einasto 1997;Dronov et al. 2011). The Scandinavian Basin, including its broad embayment, the Livonian Basin in southern Estonia, western Latvia and Lithuania, was characterized by argillaceous carbonate sedimentation (mainly wacke stones-mud stones) in distal ramp/basinal settings. Some limestone units are of reddish or mixed greyreddish colour (the Kriukai, Baldone, Segerstad, Stirnas, Holen and Lanna formations) within the generally greenishgrey Ordovician succession (Fig. 2). The biostratigraphic correlation of the units across the facies belts is mainly based on microfossils, such as chitinozoans (Nõlvak & Grahn 1993) and conodonts (Viira & Männik 1997;Viira et al. 2001).

MATERIAL AND METHODS
Altogether 446 wholerock samples, taken with a mean interval of 0.3 m from six outcrop and four drillcore sections, were analysed (see supplementary online data at https://doi.org/10.23679/502). The sections represent a wide area and different facies zones of the Baltoscandian Palaeobasin (Fig. 1). The samples for iso topic analysis were collected from carbonate rocks avoiding obvious veins or burrows. The powdered material from the Kurtuvėnai166 (Lithuania), Aizpute41 (Latvia) and Värska6 (southern Estonia) drillcore sections, and the Osmussaar outcrop section (northern Estonia) was analysed for carbonate carbon stable isotopic composition using a Thermo Fisher Scientific Delta V Advantage mass spectrometer in the Department of Geology, University of Tartu. Material from the När drillcore section (Gotland, Sweden), and the Lunne (Jämtland, Sweden), Mishina Gora (Pskov District, Russia), Volkhov and Lynna (Leningrad District, Russia) and KundaAru (northern Estonia) outcrop sections was analysed by the Thermo Fisher Scientific Delta V Advantage mass spectrometer in the Department of Geology, Tallinn University of Technology. Carbon isotopic results are reported, using the usual δnotation, as per mil deviation from the VPDB standard. The accuracy of the analyses is in the order of σ = 0.05‰.

DARRIWILIAN CARBON ISOTOPE STRATIGRAPHY
Geological sections from the distal ramp/basinal facies (Scandinavian Basin) Kurtuvėnai, Aizpute, När, Lunne, Värska and Mishina Gora represent the most complete sedimentary successions in the respective areas, with well expressed carbon isotopic curves (Figs 3, 4). The Dapingian part of the succession (Volkhov Regional Stage) is characterized by scattered δ 13 C values between 0.5‰ and 1‰ with no clear trend (Baltic Carbon Isotopic Zone BC1; Ainsaar et al. 2010). This segment ends with the rising limb of the MDICE. The curve low point in the lowermost part of the Darriwilian Stage has been named the Lower Darriwilian Negative Isotopic Carbon Excursion (LDNICE) by Lehnert et al. (2014). The rising limb of the MDICE, described as Zone BC2 by Ainsaar  2010), is covering stratigraphically most of the Kunda Regional Stage and is characterized by an increase of 1-1.5‰ in δ 13 C values in the sections of the distal ramp facies. The plateau of the MDICE (Zone BC3) has δ 13 C values between 1.5‰ and 2‰ and roughly corresponds to the Aseri Regional Stage in Baltoscandia (Ainsaar et al. 2007(Ainsaar et al. , 2010. This plateaulike peak of the MDICE is followed by a long falling limb (Zone BC4) which is terminated by a low in the curve (the Upper Kukruse Low by Kaljo et al. 2007; Lower Sandbian Negative Isotopic Carbon Excursion, LSNICE, by Bauert et al. 2014) with δ 13 C values around 0‰ in the uppermost part of the Kukruse Regional Stage.
The MDICE can be defined sensu lato as an interval between these two low points in the isotopic curves, the LDNICE and the LSNICE. Therefore, the MDICE is covering stratigraphically the Kunda, Aseri, Lasnamägi, Uhaku and Kukruse regional stages in Baltoscandia, corresponding to the Eoplacognathus pseudoplanus, E. suecicus, Pygodus serra and P. anserinus conodont biozones (Fig. 2). Considering the latest zircon U-Pb datings from sections in Sweden (Lindskog et al. 2017), the MDICE started at about 467 Ma, reached its maximum at 465-463 Ma in the E. suecicus conodont Biozone and gradually returned to the δ 13 C base values at about 457 Ma (earliest Sandbian; Ainsaar et al. 2010;Wu et al. 2017). With the duration of ~10 million years, it is apparently one of the longest carbon isotopic events in the Palaeozoic.
Some authors have tried to subdivide the MDICE peak into several peaks, which could have chemo stratigraphic value. The MDICE main peak plateau has also been named the Aseri peak, MDICEA (Ainsaar & Meidla 2016), corresponding to the E. suecicus conodont Biozone (Ainsaar et al. 2007;Wu et al. 2017). This peak is preceded in several Baltoscandian sections by a lower peak (MDICE peak 1 by Lehnert et al. 2014; the Kunda peak, MDICEK by Ainsaar & Meidla 2016) in the Kunda Regional Stage. In the studied sections, the MDICEK peak is best visible in the interval of the Baldone Formation of the Aizpute and Värska core sections (Figs 3,4). It has also been well recognized in the Tartu   The MDICEA peak is positioned in all studied sections of the Scandinavian Basin (distal ramp/basinal facies) in the Segerstad Formation. Still, the peak has been described higher than the Segerstad Formation in some earlier studies, e.g. in the Skarlöv Formation (Tingskullen, Wu et al. 2017;Kargärde, Ainsaar et al. 2010) or Stirnas Formation (Mehikoorma, Ainsaar et al. 2010). This confirms the diachronous character of these Ordovician local stratigraphic units, defined mainly by rock colour (e.g., reddishbrown limestone of the Segerstad Formation defined by Jaanusson & Mutvei 1953 and later also introduced in the Baltic states -see Männil & Meidla 1994). The MDICEA peak interval of the curves has been described by Lehnert et al. (2014) as separate MDICE peaks 2 and 3 in several sections of Sweden. However, these peaks cannot be followed in other parts of Baltoscandia and can be regarded here as parts of the main MDICEA peak.
The Darriwilian sections representing the middle and inner ramp facies (the Estonian Shelf) of Baltoscandian Palaeobasin can be correlated by carbon isotope chemostratigraphy with the sections from the deeper marine facies, but not always without additional biostratigraphic control (Ainsaar et al. 2010). The MDICEA peak and curve lows below and above the MDICE can be better recognized in the easternmost sections (KundaAru, Volkhov River) than in NW Estonia (Osmussaar; Fig. 4), although the δ 13 C values and amplitude of the excursion are significantly lower within the Estonian Shelf than in the sections in the deeper part of the basin (Fig. 3). The northwestern part of Estonia represents the shallowest part of the palaeobasin (Nestor & Einasto 1997) and therefore the relatively thin carbonate units there are separated by numerous stratigraphic gaps, making the isotopic curves discontinuous and the true peaks more difficult to observe. The δ 13 C values in some parts of the outcrop sections are also obviously diagenetically depleted, from -2‰ to -3‰ (Osmussaar, Mishina Gora; Fig. 4), which complicates the chemostratigraphic correlation. Anomalously negative δ 13 C values in the Segerstad Formation of the Mishina Gora section (Fig. 4) may be related to occasional recrystallization of fractured carbonate rocks followed by the dislocation and deformation of all the sedimentary strata in this Late Palaeozoic circular structure area (Dronov 2004;Komatsu et al. 2019). The MDICEK peak cannot be distinguished in the sections within the shallower part of the shelf.

FACIES DIFFERENCES IN THE STABLE CARBON ISOTOPIC COMPOSITION
The Baltoscandian isotopic data show clear differences in Darriwilian-Sandbian δ 13 C values between the major facies zones, defined by lithological and faunal dif ferences (Jaanusson 1976;Nestor & Einasto 1997). The MDICE preexcursion, peak/plateau and post excursion δ 13 C values in the Estonian Shelf facies (e.g., in KundaAru, Volkhov; Fig. 4) are all 0.5-1‰ lower and in northwestern Estonia (Osmussaar) even 1.5-2‰ lower than in the sections from deeper in the Scandinavian Basin. A similar depletion trend in δ 13 C carb values towards the shallower facies has been described in the Baltoscandian Basin by Saltzman & Edwards (2017) and Lindskog et al. (2019). The aquafacies differences reflected in δ 13 C carbonate bulk rock values in Ordovician carbonate basins have also been recorded in previous studies from North America (Holmden et al. 1998;Panchuk et al. 2006;Saltzman & Edwards 2017).
Several processes related to sedimentary facies differences might explain this phenomenon. Sections in the proximal nearshore parts of the palaeobasin are obviously stratigraphically incomplete, because of gaps caused by sealevel fluctuations and condensed de position due to limited accommodation space. These gaps may cut off some of the intervals of peak isotopic values. This may partly be the case in the Osmussaar section where the gaps are more significant, but the respective beds with the MDICEA interval (the Aseri Regional Stage and E. suecicus conodont Biozone; e.g., Viira et al. 2001) are apparently present within the rest of the Estonian Shelf.
The depletion of carbonates in 13 C in the sections is a common phenomenon close to the hardground surfaces and is commonly linked to subaerial exposure events (incl. meteoric diagenesis) or bacteriallymediated early subsurface cementation (e.g., Dickson et al. 2008). The Darriwilian sections of the shallowest part of the Baltoscandian Basin (e.g., Osmussaar) contain numerous welldeveloped pyritized hardgrounds and generally lower δ 13 C values of carbonates in this area could be explained by discontinuous sedimentation. Indeed, there are some samples with δ 13 C values of -2‰ to -3‰ close to the hardground surfaces in the Osmussaar section. However, the other moderately depleted (-1‰ to +1‰) parts of the carbonate succession in the shallowmarine facies areas (Osmussaar, KundaAru, Volkhov) show no direct relationship between lowered δ 13 C values and hardground surfaces in the sections, but the whole δ 13 C curves are slightly shifted towards negative values instead, depending on their position in the facies profile. The most probable explanation for these faciesdependent nearshore depletion trends could be an influence of local input of isotopically light carbon from various sources to the shallow restricted platform, due to the oxidation of organic matter in land or in water (Saltzman & Edwards 2017;Lindskog et al. 2019).

ORIGIN OF THE MDICE
The MDICE differs from the other Lower Palaeozoic global carbon isotopic excursions in several features. Firstly, based on the biostratigraphic correlations (e.g., Ainsaar et al. 2010) and the latest volcanogenic zircon datings (Lindskog et al. 2017), the timeequivalent of the excursion is altogether up to 10 million years, i.e. 3-5 times longer than the SPICE, GICE, HICE, Ireviken and other prominent carbon isotopic excursions. Secondly, with 2‰ relative δ 13 C change it represents a much smaller positive excursion than other global isotopic events. Thirdly, the MDICE covers several sedimentary se quences in Baltoscandia and shows no obvious correlation with shortterm global sealevel fluctuations (Ainsaar et al. 2007).
Several authors (Rasmussen et al. 2016;Wu et al. 2017) have pointed at the causal link between the MDICE event and the global biodiversity rise of marine benthic fauna (the Great Ordovician Biodiversification Event -GOBE, Webby et al. 2004). The GOBE has been at tributed to the global cooling (Trotter et al. 2008) and the MDICE has been suggested to be related to the increased primary productivity associated with this environmental event (Rasmussen et al. 2016;Wu et al. 2017). According to Schmitz et al. (2008Schmitz et al. ( , 2019, the midOrdovician (Darriwilian) cooling or glaciation together with sealevel fall was initiated by extraordinary Lchondritic cosmic fall and dust. Recently, on the basis of the nitrogen isotopic data from the SinoKorean Block, Bang & Lee (2020) suggested that the global MDICE event may be an environmental response to changing seawater circulation associated with global sealevel rise in the Middle Ordovician.
The interval of the cosmic dust and meteorite fall event occurs in Baltoscandia at or close to the Täljsten Bed in the lower part of the Holen Limestone, which is correlated with the Šakyna Formation in East Baltic sections. These units are biostratigraphically positioned near the boundary of the L. variabilis and Y. crassus co nodont biozones (Eriksson et al. 2012;Schmitz et al. 2019). The precise correlation of the base of the MDICE (the LDNICE) shows that this stratigraphic level is clearly situated 2-5 m higher than the Täljsten-Šakyna interval in the deeper part of the Baltoscandian Palaeobasin (Figs  2, 3). Therefore, considering the slow deposition rates of the Darriwilian carbonates in Baltoscandia (Lindskog et al. 2017), the MDICE isotopic excursion began up to one million years later than the extraterrestrial dust flow, the suggested Darriwilian glacial event and the GOBE. It could principally be possible that the isotopic composition of seawater dissolved inorganic carbon in the shelf seas responded to the perturbation of the global oceanatmosphere system with some delay and there is a causal relationship between the MDICE and the triggers of the GOBE (Rasmussen et al. 2016). Still, the modelling of the MDICE as an unusually longlasting isotopic event should address more complex scenarios than a simple delayed response of the carbon cycle to rapid climatic perturba tions and additional multiproxy geochemical, sedimento logical and palaeontological evidence should also be considered.

CONCLUSIONS
The global Middle Darriwilian Isotopic Carbon Excursion (MDICE) can be defined as an interval of raised δ 13 C values between the Lower Darriwilian Negative Isotopic Carbon Excursion (LDNICE) and Lower Sandbian Negative Isotopic Carbon Excursion (LSNICE). Two distinct peaks can be distinguished in sections all over the Baltoscandian Basin: the main peak MDICEA (Aseri) and the preceeding MDICEK (Kunda) peak. With the duration of ~10 million years the MDICE is one of the longest carbon isotopic events in the Palaeozoic. This event was preceded by the Lchondritic cosmic dust flow, which might be responsible for global Darriwilian cooling and the initiation of the Great Ordovician Biodiversification Event (GOBE). However, the time interval between these environmental events and the base of the MDICE is up to one million years. Therefore, the modelling of the longlasting MDICE isotopic change should address more complex scenarios than a simple response of the carbon cycle to rapid climatic pertur bations.