|The manuscript “Exploring a link between the Middle Eocene Climatic Optimum and Neotethys continental arc flare-up” suggests the existence of a possible causal relationship between the Eocene arc volcanism associated with the subduction of the Neotethys ocean and the Middle Eocene Climatic Optimum (MECO). To test this hypothesis the authors provides new Ar-Ar ages of Middle Eocene volcanics exposed in NW Iran, compiles available Ar-Ar and Zircon U-Pb ages of Cenozoic intrusive and effusive rocks exposed all around Iran, and estimate the volume of CO2 that might have been released in the atmosphere during the flare up. This working hypothesis represents an intriguing idea, however, it has not been clearly demonstrated in this version of the manuscript. |
My main point is the same raised by Reviewer 1 “the successful link between the arc volcanism and climate change depends on how much carbon dioxide has been outputted” around the MECO. Furthermore, Reviewer 1 wrote: “Clearly, the authors have much overestimated the thickness of 40 Ma volcanic rocks. According to figure 2, we see volcanic events throughout the whole Eocene. Although there is an intense event at 40 Ma, still, the 40 Ma volcanic rocks are only a part of the Paleogene volcanic strata (3-9 km). You must be precise how thick is the 40 Ma rock.“ This is a key point that the authors did not address.
The authors replied that: “Berberian & King (1981) state that “Extensive volcanism, with a wide range of composition, started in the Eocene Period (50 Ma) and continued for the rest of the period with the climax in Middle Eocene time (about 47-42 Ma). Despite their great thickness (locally up to 6 and 12 km) and wide distribution, the volcanics and tuffs were formed within a relatively short time interval.” I have not found in the original paper any analytical data supporting this conclusion. Furthermore the authors replied that: “In the Alborz and Central Iran, middle Eocene extrusive volcanic formations are reported to be very thick, with estimates ranging from 3-5 km in the Alborz Mountains (Stöcklin, 1974), to 6-12 km locally throughout nearly all of Iran (Berberian and King, 1981). More recent estimates of the thickness are 3-9 kilometers (e.g. Morley et al., 2009; Verdel et al., 2011” This new text does not indicate the thickness of volcanics and volcaniclastics deposited around the MECO (or let’s say at 40±2 Ma).
In section 4 the authors calculate the volume of volcanic rocks that have been erupted during the flare up (3-9 km of thickness times 40.000 km2) and based on a “linear relationship of a lava volume” (see line 150) they estimate the amount of released CO2. It is not clear to me what is the age of the Middle age volcanics considered in the calculation. From figure 2 and from the text I have the impression that the authors considered all the magmatic rocks produced during the entire flare up, which lasted about 20 million years (from ca. 55 to 35 Ma), and not the thickness of the 40±2-My-old magmatic rocks. In the text, the authors recall also table 2, but I could not find it in the text. As specified above, the amount of magmatic rocks emplaced around the MECO is a crucial point. Without an idea of such a thickness, the released CO2 cannot be estimated, and the working hypothesis cannot be tested.
Curiously, if the authors look at the Zachos et al., curve (δ18O curve vs age), they will see that the flare up in Iran coincides with the progressive Eocene cooling that culminates with the sharp temperature drop at the Eocene-Oligocene boundary (actually I think that such a curve, which is the base of all paleoclimatic reconstructions, should be shown also in this manuscript). To me this lack of correlation suggests that, although voluminous, the entire magmatic flare up in Iran did not have a strong impact on global climate, or at least that did not produce a change in the long-term global cooling trend.
If the authors want to demonstrate a causal relationship between arc volcanism and MECO they need to document an increase in magmatic flux at 40±2 Ma. So far, nobody as really demonstrated it. To me, at least as first approach, they should look at few stratigraphic sections all around the country, measure their thickness, extract the depositional ages and then estimate changes in magmatic flux through time. I think that is the most direct way to test such a working hypothesis. After that, they may look at the areal distribution on Middle Eocene rocks, assuming that these are really Middle Eocene rocks that were deposited around the MECO and not during the entire flare up.
Indeed, the Peshtasar Formation is a good target because available ages from Vincent et al (2005), recalibrated by the authors, indicate deposition of an up to 1.4-km-thick sequence of volcanics and volcaniclastics between 41 and 39 Ma. Note that we are talking about 1.4 km of volcanics and volcaniclastics and not 3 to 9 km. Similar work should be done in other areas.
I understand that the compilation provided by the authors try to overcome the paucity of stratigraphic information available in literature, but that strategy is biased toward the sampled stratigraphic intervals. It may be correct for intrusive rocks because there might be a cluster of ages (assuming that the cooling recorded by the Ar-Ar system occurred within 1-2 Million years) indicating a specific episode of magma emplacement and possibly an overproduction of magma. However, according to available data (Verdel et al., 2011, is probably the best reference) effusive and pyroclastic rocks could have been deposited between 55 and 35 Ma at rather uniform rates . Except the work of Vincent et al., 2005, there are no studies pointing toward an increase in the magmatic flux around the MECO.
Finally, concerning the compilation and the genesis of magma (see comment 1 of Reviewer 2), I suggest looking at a recent publication of Rabiee et al., in Gondwana Research titled: "Long-lived, Eocene-Miocene stationary magmatism in NW Iran along a transform plate boundary"; there, new ages of intrusives and a similar compilation is provided.
Here are few minor points:
1) In the abstract, the authors suggest that magma emplacement in carbonaceous rocks may have increased the total amount of CO2 released. This is not really addressed in the text except in lines 159-161 where the authors write: “Indeed, the Eocene extrusive volcanism in Iran erupted through significant amounts of carbonate-rich rocks of Jurassic, Cretaceous, Paleogene age (e.g. Berberian and King, 1981)”. By looking at geologic maps in NW Iran, it seems to me that most of Eocene intrusions are found within Eocene volcanics and volcaniclastics (meaning that they intruded at shallow depth) rather than in Paleo-Mesozoic carbonates (while Paleogene carbonates are rather thin). I cannot see evidences of intrusions in carbonaceous lithology based on available geologic maps. It may be true, with erosion that has not brought yet these rocks at the surface, but currently there is not any evidence for that.
2) Reviewer 1 suggested also to look also at other regions as possible sources of arc volcanism around the MECO. Of course, the first region to look at would be the entire Middle East, which represents the upper plate of the Neothetys subduction system (Turkey, Armenia, Georgia, Azerbaijan). The authors replied ”Unfortunately, the lack of shapefiles of Eocene volcanic and intrusive rocks in Armenia and Azerbaijan, along the Lesser Caucasus Mountains (e.g. Allen and Armstrong, 2008), and plutons and volcanic rocks in Armenia (e.g. Moritz et al., 2016; Sahakyan et al., 2016), hampers calculations on additional CO2 emissions within these regions”
I do not think that the lack of shape files hampers the calculations. The lack of a geologic map with clear ages of volcanics hampers the calculations. If you have a geologic map with good age information you can easily digitize the contours of the Middle Eocene volcanics and create a shape file. Note, however, that there is still a large volume of volcanic and volcaniclastic rocks buried below late Cenozoic sediments that are difficult to estimate. This is particularly true in Central Iran where depositional processes are dominant and localized exhumation hasn’t yet exposed the Eocene volcanics. This means that any estimates based on outcrops will be always a very minimum value.
Finally, I am not a geochemist, so I cannot comment on the calculations for estimating the released CO2, but I guess that the starting point is a reliable estimate of the volume of volcanics and volcaniclastics ejected around the MECO.
I hope these comments will help. Good luck.