New Estimation of the Post Little Ice Age Relative Sea Level Rise

Round 1

Reviewer 1 Report

Pascucci et al., “New Estimation of the Post Little Ice Age Sea Level Rise”.

 

Summary: Pascucci et al., present a nice record of rising relative sea levels for sites in Sardinia based on the elegant premise of time-specific variations in requirements for the building blocks quarried from the local sandstone quarries that are now below sea level. The manuscript, in general, reads well with excellent use of figures and photographs. I would recommend some minor revisions, mostly tightening up of terminology, inclusion of errors and a slightly expanded conclusion section (currently, this is a little jarring as many of the ideas are presented here for the first time). I would therefore recommend the acceptance of the manuscript for publication in Geosciences subject to the aforementioned changes.

 

Recommendation: Accept with minor corrections.

 

Major Comments:

 

1.     Greater precision in use of terminology:

a.     Last Interglacial: (Line 143, “last Interglacial highstand deposits (132 to 75 ka) are…”). The age range here is for ~ MIS 5, rather than the Last Interglacial sea level highstand (i.e., when global mean sea level was above present,~130 to 116 ka, e.g., Dutton and Lambeck, 2012^ref.1, Stirling et al 1998^ref.2). Please reword, perhaps “MIS 5 deposits are …”?

 

Line 236: reference to the entirety of MIS 5 as the Last Interglacial is incorrect. Interglacials of the last 800 ka are warm, with small volumes of land ice an therefore high sea level intervals. Recent definitions use such sea level thresholds (Tzedakis et al., 2017^ref.3) or the  absence of substantial Northern hemisphere ice (apart from Greenland) (Past Interglacials Working Group of PAGES, 2016^ref.4) to facilitate the distinction between interglacials and interstadials. Using these definitions, the latter substages  of MIS5 (i.e., MIS 5a and 5c) would be instadials rather than an interglacial. This is important not just from a sea level perspective but also climatically, given the initiation of northern hemisphere glaciation at the end of MIS 5e (i.e., at the end of the interglacial). Perhaps reword this to “latest stages of MIS5…”?

 

b.     Little Ice Age: (Line 389) Whilst the term Little Ice Age is widely used in the palaeoclimate literature, there is considerable temporal and geographic variation in expression of climate (e.g., Jones and Mann, 2004^ref.5, Matthews and Briffa, 2005^ref.6, Ljungqvist et al 2012^ref.7, PAGES 2k Consortium, 2013^ref.8). Rather than saying the city reached its peak during the Little Ice Age, rather use the date range and add that this is consistent with a local expression of the Little Ice Age (with appropriate references). Similarly, line 445 – use active during the 13^th to mid 16^th century, rather than the Little Ice Age.

 

c.      Relative sea level (RSL) and global mean sea level (GMSL):  sea level is not distributed uniformly in time or space due to gravitational and isostatic processes. You have a relative sea level record. To compare this to the Kemp et al., 2011 study (in your conclusions) or any other GMSL determination, or other sites, you will need to correct your data for GIA effects. This confusion/lack of precision in your writing is most apparent in lines 461 to 467. Please be careful using just sea level – be specific, is it relative sea level or GMSL. Please check the manuscript throughout (e.g., in figure 4, are you referring to GMSL or RSL?).

 

2.     Elevations and uncertainty of the units:

It would be useful/interesting to know the elevations of the various units described. This is not key to the paper but would provide additional valuable information to the sea level/tectonics communities. For example, what is the maximum elevation of the Last Interglacial deposits? Is this consistent for all sites. Whilst this is not essential to the manuscript, what is missing is a clear statement in the methodology of the elevation errors associated with your methods (“rulers and optical”, line 160)? Please add the uncertainty in you elevation estimates. This become important later in your study.

 

3.     Clearer statement of assumptions, uncertainties and calculations required: Within the manuscript (methods) a clearer statement of the assumptions of your study are needed in order for the reader to assess the validity of your conclusions. In order for the quarry floors to act as sea level markers, the following assumptions must hold: (i) that the quarry floor was worked down to the lowest possible level prior to abandonment; (ii) that the ‘abandonment level’ was above the lowest elevation of the material (i.e., that the quarry was abandoned prior to the exhaustion of the material); (iii) that no subsequent later reworking occurred; (iv) that the ‘abandonment level’ was 10 to 20 cm above (mean or mean low water springs?) sea level at the time of abandonment so as to facilitate dry working/loading of boats (no reference for this is provided, please state where this comes from – this is crucial to your estimates for the magnitude and rate of sea level rise); (v) that ~ 0.5 to 1 cm of tectonic subsidence from the time of abandonment (i.e., if the quarry was abandoned in the 13^th century (oldest possible, although unlikely),  ~700 years at 0.02 mm/yr, but if abandoned in the mid 16^th century, this gives ~470 years at 0.01 to 0.02 mm or ~0.5 to ~1 cm of subsidence); and (vi) that there has been negligible glacio-isostatic adjustment, which is likely not the case as the predicted present rate of sea level rise for the region is ~0.2 to 0.3 mm/yr (VM2 Earth model in ICE5G, accounting for unloading and hydro-isostatic effects, Peltier et al., 2004) which would equate to ~9 to 14 cm since the mid 16^th century. Given the following assumptions, you can then go on to calculate the difference in mean sea level (or mean low water springs) since the abandonment of the quarries.

 

Assuming mean low water springs (MLWS) is 17.5 cm below mean sea level (MSL) (given that the tidal range is ~875px), the quarry floors are 20 to 30 cm below MLWS (or 37.5 to 47.5 cm below MSL) at present. At the time of abandonment (t_A), (let’s assume mid-16^th century, i.e., ~470 years before present), MSL (or MLWS, depending on what the 10 to 20 cm for dry working/loading is benchmarked to) was ~10 to 20 cm below the quarry floor. The minimum/maximum MSL difference to present MSL is 47.5/67.5 cm, respectively. However, this must be corrected (small amounts of) tectonic subsidence, ~0.5 to 1 cm if abandoned in the mid 16^th century. This gives a max/min rate of rise of 1.4/ to 0.99 mm/yr, assuming the quarries were abandoned at time t_A. This does not include any corrections for ongoing hydro-glacio-isostatic adjustments (which would give max/min rates of 1.2 to 0.69 mm/yr). These calculations should be laid out in the results/discussion section (currently the first mention of the rates of rise at the site is in the conclusions).

 

4.     Conclusions: This section ‘feels’ a little rushed and a lot of ideas are thrown at the reader very quickly. I am unclear why the Maunder Minimum (MM) is mentioned, is this just because the two time intervals overlap? There is a very weak association between low solar output during the Maunder Minimum and climate and multiple other factors may have been responsible for the cooler climate (e.g., Owens et al., 2017^ref.9). Also, how does the MM relate to sea level?

 

The increased storminess in the Mediterranean was only mentioned in the conclusions – why was this not discussed in the discussion section? This seems like one plausible hydrographic mechanism (for raising sea levels) that would lead to the abandonment of the quarries. What else could have caused sea level to increase during this interval? Perhaps enhanced mass loss from Greenland (Long et al., 2012^ref.10), oceanographic/wind forcing (i.e., storminess mentioned in the conclusions) (e.g., Saher et al., 2015^ref.11), temperature (Kopp et al., 2016^ref.12, Kemp et al., 2011^ref.13) or ongoing hydro-glacio-isostatic adjustment (e.g., Lambeck et al., 2004^ref.14)…?

 

This is a nice piece of work but you need to lead the reader into your conclusions. Currently many of the ideas in the conclusions are presented for the first time rather than providing a summary of your work.

 

Minor comments

The manuscript is well written but it would benefit from proof reading by a native English speaker (to remove some minor quirks in the prose, e.g., line 330 “merge’ rather than emerges).

 

Inconsistency in formatting:

1.     Inconsistent formatting of the manuscript - some double spaced, some single spaced etc.;

2.     The occasional typo, e.g., line 143 “highstad” rather than highstand, missing bracket line 265, “Mounder” rather than Maunder minimum, line 448;

3.     Where material is quoted directly both “” (e.g., lines 65 to 66) and italics (lines 68 to 71) have been used, and;

4.     Missing information in the references, e.g., ref 56 is missing the title.

Check for consistency throughout the manuscript.

 

 

Figures: The figures are mostly good with a lovely use of photographs. However, the section logs in figures 5 and 6 are too small and therefore hard to read (ages in figure 5 are especially small) and interpret.

 

References cited:

1.    Dutton, A. & Lambeck, K. Ice volume and sea level during the last interglacial. Science. 337, 216–219 (2012).

2.    Stirling, C. H., Esat, T. M., Lambeck, K. & McCulloch, M. T. Timing and duration of the Last Interglacial: evidence for a restricted interval of widespread coral reef growth. Earth Planet. Sci. Lett. 160, 745–762 (1998).

3.    Tzedakis, P. C., Crucifix, M., Mitsui, T. & Wolff, E. W. A simple rule to determine which insolation cycles lead to interglacials. Nature 542, 427–432 (2017).

4.    Past Interglacials Working Group of PAGES. Interglacials of the last 800,000 years. Rev. Geophys. 54, 162–219 (2016).

5.    Jones, P. D. & Mann, M. E. Climate over past millennia. Rev. Geophys. 42, RG2002 (2004).

6.    Matthews, J. A. & Briffa, K. R. The ‘little ice age’: re‐evaluation of an evolving concept. Geogr. Ann. Ser. A, Phys. Geogr. 87, 17–36 (2005).

7.    Ljungqvist, F. C., Krusic, P. J., Brattström, G. & Sundqvist, H. S. Northern Hemisphere temperature patterns in the last 12 centuries. Clim. Past 8, 227–249 (2012).

8.    PAGES 2k Consortium. Continental-scale temperature variability during the past two millennia. Nat. Geosci. 6, 339 (2013).

9.    Owens, M. J. et al. The Maunder minimum and the Little Ice Age: an update from recent reconstructions and climate simulations. J. Sp. Weather Sp. Clim. 7, A33 (2017).

10.   Long, A. J. et al. Relative sea-level change in Greenland during the last 700yrs and ice sheet response to the Little Ice Age. Earth Planet. Sci. Lett. 315–316, 76–85 (2012).

11.   Saher, M. H. et al. Sea-level changes in Iceland and the influence of the North Atlantic Oscillation during the last half millennium. Quat. Sci. Rev. 108, 23–36 (2015).

12.   Kopp, R. E. et al. Temperature-driven global sea-level variability in the Common Era. Proc. Natl. Acad. Sci. 113, E1434–E1441 (2016).

13.   Kemp, A. C. et al. Climate related sea-level variations over the past two millennia. Proc. Natl. Acad. Sci. U. S. A. 108, 11017–11022 (2011).

14.   Lambeck, K., Anzidei, M., Antonioli, F., Benini, A. & Esposito, A. Sea level in Roman time in the Central Mediterranean and implications for recent change. Earth Planet. Sci. Lett. 224, 563–575 (2004).

Author Response

Dear Reviewer,

I would like to thank you for the very constructive comments.

Here below my answer to your comments.

The terminology, as suggested, as been more precisely cited. The term Last interglacial (when used) has been referred to the stadial MIS5e. For the others we have left or just the time or referred to them as interstadials. The same correction has been done for the term highstand. Little Ice Age. The ms is focussed on the "New Estimation of the Post Little Ice Age Relative Sea Level Rise", we therefore cannot avoid the use of this term. However, we ave in many partes referred just to the time interval comprised bteween 1350 and 1750 AD as suggested by the Reviewer. Relative Sea Level Rise. We agree with this comments and changed in the text (and also title) sea level rise with relative sea level rise. Elevation and uncertainty. We have explained and errors the methods Clearer statement...We have explained in the method as suggested by Reviewer. Conclusions. Revised just a little bit. The ms wants just to carry a little contribution to the post LIA relative sea level rise using archaeological evidences and not deals with climatic or other features that would be speculative and not supported by any evidence. Figures. Figures 5, 11, 13, 15 have been modified to improve their quality. References have been revised, as well as English 

Reviewer 2 Report

The study use sandstone quarries as archaeological relative sea level (RSL) indicator. Dating the quarries is based on the quarried stones ((building blocks) dimensions compared to well-dated blocks in buildings like Cathedrals their building and functioning time is known. They also assume that originally, the base of the quarry was above RSL and therefore, the fact that the bases are at present few decimeters below sea level is an indication to RSL rise (see below my specific remark about it). The claim it on lines 417-418 as an axiom but this assumption is not always in agreement and it has to be mentioned at least in the Discussion. These two essential conclusions have to be presented more cautiously, using “we assume” “probably”, etc.

But, I do recommend on publishing the paper in Geoscience following the general and specific remarks I made because: the field work carried out is very good, figures presentation is high quality and the most important argument is that so far there is very few archaeological RSL data from relatively stable areas in the Mediterranean, that obtained index points from the last Era, and this record can contribute to the Mediterranean RSL archives.  

In general

1.     The authors do not use “RSL” which is significant for their results: the sea level information obtained in the study area is relevant for restricted area since every coast has it’s on GIA, tectonics, etc.

2.     In the Introduction, the authors use terminology of “Medieval Warm Period (MWP)”, “ Little Ice Age (LIA)”, etc. and especially the time frame of these periods in too generalized way: the climate conditions (warmer or colder) and the time frame was not the same everywhere. Most data indicates that in the Mediterranean the main changes were between dry and wet conditions and less between cold and warm conditions. It also seems that seesaw pattern existed in the Mediterranean with Medieval dry conditions in the west and wetter conditions in the east. The data regarding “global” or north European climate in these periods is actually lees relevant.  On Lines 74-77 they mention very shortly the Italian climate in the LIA with references to text that I cannot read (no. 13) and two papers by Canuffo (17 and 18).  Regional and local climate data is the most relevant and has to be mentioned and discussed later in the Discussion. .

3.     To my opinion, the results present too much irrelevant information about the stratigraphy and chrono-stratigraphy of the outcrops.  It is a very good field work but less relevant for the research question since at the end, they dated the quarries based on the block dimensions. I suggest shortening 4.1.

4.     The GIA rates are hardly mentioned (see remarks 1 and 10 below) and it is essential to mention with references, if possible to add new, updated, GIA rates for the area and to discuss the relative stability of the studied area which is extremely important for the reliability of the data.  

5.      There are too many references that are not in English. Not acceptable.  

6.     The paper needs professional English review!! It is essential.

7.     The figures are all in very good quality and well-explained by the figure captions.

 

Specific remarks 

1.     L. 130-131; low rate of subsidence is mentioned: is it the GIA factor? References are needed for this rate since personal communication in this case is not enough. It is highly important for RSL data. The paragraph mixes between “old” geology and new, which is the most important. Re-organization of the paragraph is needed.

2.     Materials and methods: L. 154: what detailed analyses have been carried out? It is important for the results. Did the study rely only on the blocks dimensions or also on any kind of sedimentological analysis (e.g. thin sections or mineralogical analysis)?

3.     In L. 171 it is mentioned that MIS 7 outcrops are missing in the study area but this phase appears in Figure 4. How do you explain it?

4.     265-269. Not clear – needs English review. But in any case, this paragraph comparing the sandstones to the Balearic Islands belongs to the Discussion, not to the results part.

5.     L. 280-282. The authors use publication no. 39 as a reference for their assumption that the quarries were originally excavated up to the sea level and not below. This document is not in English and I cannot read it. This point is extremely significant for the functional height interpretation and cannot be relied on publications that are not in English. And as mentioned in other places in the letter, this assumption is not agreed by all the community and it has to be mentioned with references.

6.     Discussion: L. 376-388: see my comments above about the validity of all European dating and the climatic characteristics of sub periods in the last millennium.

7.     L. 405: “marine quarries”? do you mean “coastal”? Since it is assumed in the paper that quarries were not cut under the water.

8.     L. 417-418: see my comment above.

9.     L. 419-423: this argument is weak.

10.  L. 427-431. This discussion is highly important and has to be mentioned before (see my remark regarding  lines 130-131).  If possible, add new GIA modern rates evaluations.  The discussion must refer to Vacchi et al., 2018 in QSR, 201 pp. 396-408 with references therein, and especially to the publications by Stocchi and by Spada.

Author Response

Dear Reviewer,

I would like to thank you for the constructive comments to the ms.

Here below my answer to your suggestions.

RSL and GIA, they have been added in the text both in the chapter Methods and recalled in the conclusions; There is little (or no) literature on the LIA climate in Italy. I am sorry for that. The only data available are on the Alpine glacial and Venice lagoon. In this sense the ms could represent a good starting point for the study of climate occurred in this part of the Mediterranean during LIA. The ms aim is far from any climatic conclusion. It just want undeline that archaeological data may provide info on RSL rise occurred post 1750AD. Stratigraphy. Rev1 asked to improve it. We have decided to rephrase it a little be, but substantilly we do not have changed it too much. For us, it is important to underline which are the different sedimentological character of the quarried sandstones and their age. This last is also function of the good quality of the material. GIA has been mentioned The lack of references in English is mainly due to the lack of any study (in the recent time) of the Alghero quarries. Most of the papers in Italian we cited come from the historical archive of the city of Alghero. They are in Italian with many part in Catalan and Latin inside. I have checked for English references but there are not (neither translations). English has been revised Figures have been in part changed to be more readable.

Specific remarks

GIA factor: considered; Materal and method has been rivised We have added one figure (11D) where is visible the today underwater dock used to load the boats. This may be compared with that of the 30th Century 20 cm above the present day sea level. This would clarify what the Reviewer has asked. 376-378 we have left as it was. We mostly agree with literature data. 405 marine quarry -->coastal quarry 417-418 --> see above 419-423 --> we do not agree with this comment and left this part as it was 427-431. We have not added the suggested reference because referred to Canada and not Mediterranean Sea. We added Stocchi e Spada 2009