The DEEP site sediment sequence obtained during the ICDP SCOPSCO project at Lake Ohrid was dated using tephrostratigraphic information, cyclostratigraphy, and orbital tuning through the marine isotope stages (MIS) 15-1. Although this approach is suitable for the generation of a general chronological framework of the long succession, it is insufficient to resolve more detailed palaeoclimatological questions, such as leads and lags of climate events between marine and terrestrial records or between different regions. Here, we demonstrate how the use of different tie points can affect cyclostratigraphy and orbital tuning for the period between ca. 140 and 70 ka and how the results can be correlated with directly/indirectly radiometrically dated Mediterranean marine and continental proxy records. The alternative age model presented here shows consistent differences with that initially proposed by Francke et al. (2015) for the same interval, in particular at the level of the MIS6-5e transition. According to this new age model, different proxies from the DEEP site sediment record support an increase of temperatures between glacial to interglacial conditions, which is almost synchronous with a rapid increase in sea surface temperature observed in the western Mediterranean. The results show how a detailed study of independent chronological tie points is important to align different records and to highlight asynchronisms of climate events. Moreover, Francke et al. (2016) have incorporated the new chronology proposed for tephra OH-DP-0499 in the final DEEP age model. This has reduced substantially the chronological discrepancies between the DEEP site age model and the model proposed here for the last glacial-interglacial transition.
Since the demonstration of a strong astronomical control on the oxygen
isotope composition (
However, when marine records are used for tuning terrestrial archives there
is an implicit assumption of synchronicity between climatic events
recognized in marine proxies and those in terrestrial archives, often
identified using different proxies. Under scrutiny such a relationship may
not be sustainable, as terrestrial and marine proxies could indicate
different processes at local and global scales, with different responses to
climatic forcing. For instance, marine pollen studies indicate that broad
land–sea correlations and average ages of respective stages are generally
correct, but that there may be significant offsets in the precise timing of
terrestrial and marine stage boundaries (e.g. Shackleton et al., 2003;
Tzedakis et al., 2003) when, e.g., pollen and benthic foraminifera
The development of U
An increasing number of studies are now devoted to the use of tephra layers for correlation and synchronization of archives (see e.g. Lowe, 2011 for an extensive review). In the Mediterranean region, the use of tephra layers as chronological and stratigraphic markers (Wulf et al., 2004, 2008; Zanchetta et al., 2011, 2012a, b; Blockley et al., 2014; Albert et al., 2015; Giaccio et al., 2015) has largely improved our ability to synchronize archives and proxies, and to recognize leads and lags between different paleoclimate records (e.g. Regattieri et al., 2015). Therefore, the parsimonious use of tuning based on independently dated archives, along with the strong stratigraphic constraint afforded by tephra layers is perhaps the most rigorous way to provide a chronological reference for archives which lack an independent chronology (e.g. Regattieri et al., 2016). However, tephrostratigraphic and tephrochronological work also depends on the accuracy of existing data, and radiometric ages provided for proximal and distal deposition of the same tephra can vary by up to several thousand years. For example the Y-3 tephra is a widespread marker in the central Mediterranean (Zanchetta et al., 2008), for which an age range of ca. 31–30 ka has been proposed for the supposed proximal deposits (e.g. Zanchetta et al., 2008) but this age range has been recently challenged by Albert et al. (2015) who dated distal Y-3 deposits to be between 28.7–29.4 ka.
Here we attempt to compare different proxy series from MIS 5 (ca. 130–80 ka; cf. Railsback et al., 2015) from the “DEEP” core composite profile, drilled in Lake Ohrid (Fig. 1) within the framework of the ICDP-SCOPSCO project (Wagner et al., 2014a, b), with recent radiometrically dated continental records in the central Mediterranean, to further constrain the age model of the DEEP record for this period. The major aims are to understand (1) which proxies are most useful for correlating different archives during specific intervals of time; (2) which proxies can provide fundamental information on time-lag relationships between specific environments, and (3) which proxies can be confidently considered as an expression of local-to-regional climatic change. The approach employed here is different from that previously used to produce a chronology for the DEEP site composite long record, which is based on tephrostratigraphy, cyclostratigraphy and/or orbital tuning through the marine isotope record (Baumgarten et al., 2015; Francke et al., 2015, 2016). In contrast, our approach provides more detailed insights into the chronological framework of a discrete time period, and aims to contribute to the synchronization of paleoclimate records in the Mediterranean region.
Lake Ohrid originated in a tectonic graben and formed during the latest
phases of uplift of the Alps (Stankovic, 1960). It is located on the border
between Macedonia (FYROM) and Albania and covers an area of 358 km
The lake is thought to be the oldest lake in continuous existence in Europe, with current age estimates varying between ca. 1.2 and 5 million years from geological investigations and between 1.5 and 3.0 Ma from molecular clock analyses of endemic taxa (Trajanovski et al., 2010). Preliminary analyses from SCOPSCO DEEP core sediments confirm a limnological age for Lake Ohrid of > 1.2 Ma (Wagner et al., 2014a, b; Baumgarten et al., 2015). The peculiar hydrological conditions of the lake and the presence of > 300 endemic species make Lake Ohrid a hotspot of biodiversity and a site of global significance (Albrecht and Wilke, 2008; Föller et al., 2015).
The “DEEP” core was retrieved in the central basin of Lake Ohrid (41
DEEP site proxy series plotted on age models from Francke et al.,
2016 (left) and Francke et al. (2015; right). From top:
B-SiO
Proxy data used here comprise total inorganic carbon (TIC), total organic
carbon (TOC), and biogenic silica (B-SiO
Comparison of selected DEEP proxies (TIC
Figure 2 shows the correlation of selected proxy series from the DEEP site.
The general structure of the different proxies shows a relatively good
agreement, as already discussed in other contributions of this themed issue
(Francke et al., 2016; Lacey et al., 2016; Just et al., 2016). Interglacial
sediments are typically characterized by calcareous and slightly calcareous
silty clay, while clastic, silty clayey material dominates in the glacial
periods (Francke et al., 2016). However, although orbital-scale
sedimentological variability and sedimentation rates appear to remain fairly
constant, differences are apparent when the cores are examined at higher
resolution. The transition between MIS6 and the Last Interglacial (i.e.,
MIS5e) is of particular interest. In the original Biogeosciences Discussion
paper by Francke et al. (2015) the age model used for the DEEP site assumed
an age of 129
The comparison of DEEP proxy data during the MIS6-MIS5 transition with
regional records (Fig. 3) shows some interesting features, which highlight
the timing and evolution of the glacial/interglacial transition at Lake
Ohrid and may represent the starting point for tuning consideration. A
majority of Mediterranean
A similar two-stepped pattern for the MIS6-MIS5 transition is also observed
in
From bottom: TIC (% wt) and sedimentation rate of DEEP site
plotted on age models from Francke et al. (2015, Discussion version, grey);
Francke et al. (2016, blue); this study (red); Alkenone SST (
To strengthen the proposed correlation of events during the MIS6-5e
transition, we also consider the position of the tephra layer P-11 from
Pantelleria Island in different records (Fig. 3, red dots; Paterne et al.,
2008; Caron et al., 2010; Vogel et al., 2010), which is correlated with the
tephra layer OH-DP-0499 recognized in the DEEP core (Leicher et al., 2016;
Fig. 2). As shown in Fig. 3, this tephra layer occurs at the base of the
first small, but pronounced, increase of TIC in the Ohrid record. In the
ODP-963A record from the central Mediterranean (Fig. 3; Sprovieri et al.,
2006; Tamburrino et al., 2012) this tephra layer (here correlated with ODP3
layer) corresponds to the first increase in the abundance of
In the central Mediterranean, and specifically for Corchia and Tana che Urla
caves, speleothem calcite
The designation of additional tuning points during the interglacial appears
more complicated. During the first part of MIS5e some common patterns are
evident, like the prominent increase in TIC, TOC and B-SiO
Chronological tie points discussed in this study. DEEP core ages
and associated 2
Two robust target points for synchronization are represented by the tephra
layers OH-DP-0404 and OH-DP-0435 (Fig. 2), which were independently dated in
other records (Table 1). Particularly, both tephras occur in the POP section
from the Sulmona Basin (Regattieri et al., 2015) and thus their recalculated
ages can be obtained from this age model. Tephra OH-DP-0435 is also used in
Francke et al. (2015, 2016) as tie point, and the
From the above discussion, we suggest an alternative age model for the MIS 5 DEEP record (Fig. 4) using the tie points shown in Fig. 3 (green and purple arrows) and detailed in Table 1. This new age model was calculated using the Bacon software (Blaauw, 2011), using the same settings employed also for the construction of the DEEP site chronology by Francke at al. (2016). The simulation is limited to the chronological interval for which tie points are available (ca. 140–70 ka).
As noted before, the most significant differences are in the timing of the whole glacial/interglacial transition in the first age model of Francke et al. (2015). However, in the final version of the age model from Francke et al. (2016), incorporating the new age here proposed for the OH-DP-0499 tephra layer, the differences are less evident (Fig. 4). There is a good fit between ca. 115 and 108 and ca. 95–88 ka, whereas ages diverge again at the base of the record. Interestingly, the new model allows for comparison between the Ohrid record and with SST reconstructions from the Western Mediterranean (core ODP-975), which, as previously explained, is an indirectly, radiometrically dated record (Fig. 4). Despite a minor chronological offset, the pattern of TIC variability during the transition is consistent with that of SST.
From bottom: DEEP site pollen record (AP-
Figure 4 also illustrates the change in sedimentation rate in the different age models. It is possible to see that by increasing the number of aligning points the sedimentation rate becomes significantly different, suggesting a faster decrease at the time of the interglacial inception. Sedimentation rate increased again around 120 ka, and then remained stable since ca. 105 ka. We note that the Francke et al. (2016) age model (and most other age models too) are based on the assumption of gradually changing sedimentation rates. This might be true if studying long sequences at low resolution. However, changes in sedimentation rates become more important when examining a sequence at higher resolution. On the long-term scale, and using the chronological tie points of the 11 tephras from the orbital tuning used in the Francke et al. (2015, 2016) age model, relatively constant sedimentation rates are inferred for the DEEP core site record. On closer inspection, however, there might be significant changes, particularly at the MIS6-5e transition, as inferred from the new age model (see also Francke et al., 2016), as it is highly unlikely that a decrease in clastic input from the catchment (prevailing during glacials, even if partially compensated by a reduced input of organic matter and calcite, and indicated in lithofacies 3 of Francke et al., 2016) is completely, simultaneously and equally compensated by an increase in carbonate precipitation reaching > 80 % during the interglacial (MIS 5e peak, Fig. 4). This means that it is highly likely that there are significant changes in sedimentation rates, which can only be detected by high resolution studies and by a detailed comparison of different records, as indicated in this study.
From Fig. 4 it is also possible to note that the strong increase in SST
and TIC occurred slightly before the maximum of summer insolation at
65
With the new age model presented here it is also possible to attempt a more precise regional correlation of pollen records. In Fig. 5 pollen records from Tenaghi Philippon, (Fig. 1, Milner et al., 2012, 2013; Pross et al., 2015) and Monticchio (Fig. 1; Brauer et al., 2007) are plotted against the DEEP site pollen record (Sadori et al., 2016). The sharp increase in the AP percentages at ca. 130 ka is almost synchronous in all the mentioned records, and simultaneous to the highest rate of SST increase in the western Mediterranean (Fig. 4). A comparison of the chronology from different records after the end of the Eemian forest phase is more problematic, since the first clear forest opening coincides with the C24 cold event in the North Atlantic (Sánchez-Goñi et al., 1999). In the DEEP core, two tephra layers and a robust alignment point at the end of GI24 probably make this chronology the most reliable, even if in the younger part of the record there are no further alignment points.
The proposed correlation exercise described here can potentially be extended
in the future to other sections of the DEEP record. The
Regional proxy records that have been independently dated support the
development of a more detailed chronology for the Lake Ohrid DEEP site
record in the interval covering the MIS6/5 transition and the first part of
MIS5. The aligning with regional proxies indicates that the most prominent
rate of increase of B-SiO
During the MIS5 interglacial, different proxy records show generally similar patterns but with evident leads and lags, which can make the selection of the tuning points somewhat more complex. However, the presence of two regionally widespread tephra layers allows a relatively good anchoring of the chronology.
It is important to remark that the approach proposed here can be extended to relatively few intervals of the long DEEP record because independently radiometrically dated records in the Mediterranean region are rare for periods older than the MIS5 (e.g. Bar-Matthews et al., 2000; Drysdale et al., 2004; Giaccio et al., 2015; Regattieri et al., 2016). Therefore, the approach proposed by Baumgarten et al. (2015) and Francke et al. (2016) still appears the most suitable for the definition of general chronological framework of the long record.
The SCOPSCO Lake Ohrid drilling campaign was funded by ICDP, the German Ministry of Higher Education and Research, the German Research Foundation, the University of Cologne, the British Geological Survey, the INGV and CNR (both Italy), and the governments of the republics of Macedonia (FYROM) and Albania. Logistic support was provided by the Hydrobiological Institute in Ohrid. Drilling was carried out by Drilling, Observation and Sampling of the Earth's Continental Crust's (DOSECC) and using the Deep Lake Drilling System (DLDS). Special thanks are due to Beau Marshall and the drilling team. Ali Skinner and Martin Melles provided immense help and advice during logistic preparation and the drilling operation. We thank two anonymous reviews for the constructive comments and criticisms, which improved the quality of the manuscript. Edited by: F. Wagner-Cremer