Dear Amy Chew,

Should we derive a title for this special issue of CoP, it might be become “The bane of stratigraphy in the early Paleogene”.

I have read through your manuscript, as well as the reviews and responses. The deep problem in the original submission lay in presenting the stratigraphy correctly.

The original manuscript needs significant modification (major revisions) before publication. However, it appears that much of this has already been done with your suggested changes to the text and figure 2 (although the latter still needs work, below).

The tentative decision is to accept a revised version with major revisions.I will then examine to see whether it should be sent for further review.

Sincerely,

Gerald (Jerry) Dickens

Referee Commentary

Clyde gives a very good and lengthy review. I agree very much with his commentary, and think all his points are valid and need to be addressed in a good final paper.

Some of these points were addressed in the response, but issues remain. I elaborate here.
(WC1) The carbon isotope signature in conjunction with polarity chron determination provides an incredibly powerful tool for stratigraphic correlation across Earth’s surface. It is incorrect to suggest “their comparison is of a terrestrial record with marine records, for which there is no particular expectation of congruence.” There absolutely should be congruence of terrestrial and marine records in terms of carbon isotope variations and polarity chrons. More specifically, in the present case, there should be two prominent negative carbon excursions near the top of C24r, the younger one perhaps within the base of C23n. This was predicted before the work of Abels et al. (2012).
With this as background, I do not fully understand Figure 1/revised Figure 2. There is something either wrong in logic or wrong in presentation. I think it is the latter, and that the red rectangles in “panel a” are not supposed to be the same as the red lines in “panel b”. This has to be amended.

(WC2) The EECO remains poorly defined for multiple reasons. These include evolving absolute time scales, and more vexingly, how does one set bounds on a time interval based on temperature for any particular location on Earth’s surface, especially if temperature is fluctuating rapidly during EECO. Several papers have noted this problem, and there may never be a right solution. In fact to even have “optimum” in the definition of EECO is sort of funny. Nonetheless, as best as can sort of be agreed upon ca. 2015, and on the GPTS 2012 time scale, was that EECO loosely occurred within about 53-49 Ma and the core of EECO occurred at nominally 51 Ma. The way to realize this is that the K/X carbon isotope excursion marks the start of the EECO as defined in stable isotope records and this event happened at about 53 Ma on the GPTS time scale.

However, on the 2012 GPTS time scale H-1 occurred at about 54.1 Ma and the PETM occurred at about 55.9 Ma.

In any case, H-1/ETM2 and H-2 both DEFINITELY occurred well before (at least >1.0 Myr) the start of EECO but DEFINITELY within the general long-term warming trend from the late Paleocene through the early Eocene. The point here is to get all written correctly and within the framework of current knowledge, all the while realizing there have been some serious miscommunication and stratigraphy problems.

(WC3) The link between changes across hyperthermal events and the near future has to be couched very carefully. Not only are there the issues of additional anthropogenic influence as stressed by Clyde, there is the issue of rate. I will be up front: I think the idea of the PETM being initiated in 13 years (Schaller et al., PNAS, 2014) is beyond complete bollocks (Zeebe et al., PNAS, 2014); most of these events likely began over thousands of years. The point here is that rate of change is a crucial component, presumably especially so for mammals that might migrate.

Abels and Gingerich give a lengthy review that also highlights potential stratigraphic problems. They raise an intriguing point about sedimentation rates, namely that if the two locations had proportional sedimentation, the H-1 and H-2 events should be about 10 m apart at Fifteenmile Creek rather than the 20 m indicated.

There are, of course, two possibilities: the purported bioevents do not correspond to H-1 and H-2, or the sedimentation rates were not proportionally constant at the two locations in the early Eocene. Certainly, the latter case is possible, given recent papers discussing changes in hydrology during the PETM and other suggested hyperthermals, including H-1.

Of course, as noted by everyone, all could be clarified with a good carbon isotope record at Fifteenmile Creek.

Gunnel gives a short review that makes similar points to those above.

Huber makes some very good points concerning background temperatures for the Early Eocene and rates of temperature change during this time interval. While the PETM and other hyperthermals may be our best analogs for future climate change, they were definitely imperfect analogs. The initial boundary conditions were different; the rates of change in multiple parameters, though very fast from a geological perspective, were very slow compared to those in the modern.

As noted above, any linkages between the past and the future need to be written very carefully.

Editor Commentary

Basic Comments:

(1) Place references in chronological order as per CoP final papers.

(2) Some of the writing and referencing is not precise. The referees have given several examples. I give some more below for the first two pages. The manuscript needs a good cleaning before final publication. Please go through the manuscript and make sure all is correct.

Specific Comments:

-- Line 1373 --

Lines 4-5: First sentence not needed. Remove.

Lines 6-18: Should be reworded something like: “The latest Paleocene and earliest Eocene (ca. 58–51 Ma) is relevant to understanding modern anthropogenic warming because the time interval was characterized by global warming and massive inputs of carbon to the ocean and atmosphere (Zachos et al., 2008). Changes in temperature and carbon cycling happened on both long and short time scales. In general, Earth’s surface warmed from the late Paleocene through the Early Eocene, culminating in the Early Eocene Climatic Optimum (EECO), the hottest sustained period of the Cenozoic and centered at about 51Ma on the GPTS 2012 time scale (REF). Superimposed on this long-term change were several short-term (<100 ka) warming events, now coined “hyperthermals” (REF). Each of these events is marked by a large decrease in the d13C composition of carbon bearing phases in sedimentary strata, which are referred to as carbon isotope excursions (CIE), and carbonate dissolution in deep-sea sediment. The latter is important because it tells us that the hyperthermals were related to massive inputs of 13C-depleted carbon to the ocean and atmosphere (REF).

The most prominent and best known of the hyperthermals is the Paleocene-Eocene Thermal Maximum (PETM) (Zachos et al., 2008; McInerney and Wing, 2011), the onset which now defines the base of the Eocene at 55.9 Ma (REF). Various examinations of records across the PETM implicate release of several thousand petagrams of reduced carbon into the ocean–atmosphere system within < 20 ka (review in McInerney and Wing, 2011). The carbon addition was somehow related to a ~100 ka period of elevated global temperature (5–7°C warmer), and perturbations in earth surface systems, including changes in biota (Bowen et al., 2006; Gingerich, 2006; McInerney and Wing, 2011). On land …”

Lines 26-27: Now should be “A major advantage of examining records across the late Paleocene-early Eocene interval is the potential to characterize faunal responses to a range of climatic perturbations that occurred over both long and short time scales.”

The above changes would clean up all sorts of little problems, some which have been noted by the referees.

-- Page 1374 --
Lines 2-3: The Sexton et al. (2011) paper does not relate to this time interval. However, a paper that makes the very point in this sentence and the next sentence is: Nicolo et al. (Geology, 2007).

Line 10: Should be “planktonic assemblages … were somewhat similar …”