RC1: 'Comment on nhess-2022-262', Anonymous Referee #1, 29 Mar 2023

Thank you for all your helpful comments. You'll find our answers in bold below. 

Abstract

line 17: the sedimentary record of a hurricane can be related to the storm surge, but also related to heavy rainfall / runoff that washes onshore material into the lagoon – specify the type of deposit here and potentially in other parts of the text.

L17: “We investigated the geomorphological impacts and the sedimentological record of related to the storm surge of this hurricane in Barbuda’s Codrington Lagoon.”

line21: deposits – add s.

Done.

line 28 (and 655-660): sediment thickness and storm intensity are not directly related. The amount of sediment available for erosion / transport is important. For example, if there was not spit or only a low barrier at some time in the history of the lagoon, another cat-5 hurricane that directly hit the island would not be resembled by an event unit as thick as the one deposited during Irma. Please add to the discussion lines 655-660.

Other parameters like the storm track and distance of the storm to the field site, play an important role, too.

This was added to the discussion:

L683-688: “Moreover, the lagoon and barrier morphology has likely changed during the last 3700 years, and we do not have any information about those changes prior to the oldest marine map (1750-1785 CE, Fig. 3). If the spit morphology changed during the last millenaries, it would have great consequences on the Codrington Lagoon sensitivity to hurricane sediment record. Other parameters like the storm track and distance of the storm to the field site, play an important role too (Biguenet et al., 2021). These limitations do not allow us to firmly conclude whether the Irma Hurricane was unique at the scale of the last 3700 years.

Introduction

Line 32: ‘hurricane storms’ – a hurricane is a type of storm in Central to Northern America. Therefore, delete ‘hurricane’ or ‘storm’ – depending on if you want to include all storms worldwide or just refer to those in Central and Northern America.

The word “storms” was removed.

Line 33, 61: tropical rainfall would only refer to tropical storms. Again, decide if you use the term storm (then delete tropical) or hurricane (tropical is then correct).

We kept the word “tropical” as we chose to keep the word “hurricane”.

Lines 36 – 40, 700-704: yes, studies show an increase in intensity and frequency in the last centuries / decades. But the studies also show that intensity and frequency changed several times in the last few thousands of years. This needs to be added.

This was added in the “Past hurricanes in Barbuda” section:

L119-125: “Paleoclimate studies have shown that SSTs, storm intensity and frequency have varied through time in Barbuda (Brasier and Donahue, 1985; Khan et al., 2017; Knowles, 2008). Knowles (2008) identified three distinct periods: two active regimes of high frequency storminess from 0-1500 and 3500-5000 year BP, bracketing an inactive regime from 1500-3500 year BP. The changes in the storminess frequencies are also influenced by many parameters such as regional wind shear which vary in response to the interannual migration of the Intertropical Convergence Zone (ITCZ), the Atlantic Multidecadal Oscillation (AMO) and the North Atlantic Oscillation (NAO) (Wallace et al., 2019)”.

Lines 44, 45: submersion is not the correct term here. Use inundation instead.

The word “submersion” was changed to “inundation”.

Chapter 2.2

Line 117: you mention the study of Knowles 2008. Your study would highly benefit from a comparison with the results published in 2008. Did Irma erode the underlying strata? How do the sediment characteristics (thickness, grain size, composition, …) differ?

The 10 cores discussed in Knowles (2008) were collected in 2006. No core exceeded one meter in length, but they recorded a 5000-year history. The studied pond is located at the southeastern corner of the island, is less than one meter deep and hypersaline. The sandy barrier is low (less than one meter high). The cores are composed of marls interspersed with sandy layers (maximum 14). There are no detailed grain-size analyses. The marls are dark in the core bottom and light gray and dark in the core top. The dark marls show a slightly higher organic content than the light marls. The author described the cores with “significant noise”, and the lack of stratigraphy makes it difficult to cross correlate each sand layer. Only one sand layer was identified with 137Cs dating: Hurricane Donna (1960). The dating was obtained in only one core, where the Hurricane Donna deposit was 15 cm thick. It is the thickest deposit in all the cores and in all the other cores (if the correlation is right).

There is no deposit of this age in our cores from Codrington lagoon. The deposit may have been eroded by Irma, or it is also possible that no sandy layers were deposited in Codrington lagoon during Hurricane Donna. It is also very delicate to compare sandy layer thicknesses from different lagoons, as the sand sources are different. Thus, the comparison with Knowles (2008) only allows us to say that Hurricane Donna was a strong event recorded in one pond in Barbuda and perhaps also in the Codrington Lagoon, but we have no trace of it.

Thus, we added two phrases to the manuscript to compare with the results from Knowles (2008):

L131-132: “Hurricane Donna overwash deposit was probably recorded in Salt Pond (Knowles, 2008).”

L675-680: “Nevertheless, one major limitation of our sedimentological study is related to the absence of an accurate age model and to 14 C age inversions. Thus, we cannot rule out that the sedimentation within the lagoon was not continuous and that some hurricanes were not recorded and/or were eroded. For example, the overwash deposit of Hurricane Donna (1960) was probably recorded in Salt Pond, on the eastern part of the island (Knowles, 2008), but no deposit of this age has been identified in the studied cores from Codrington Lagoon. It can be assumed that a deposit of this age has been eroded by Irma.”

Lines 129-137 repeat large parts of lines 65-72.

The lines 129-137 were rewritten:

L136-143: “The Category 5 Hurricane Irma, which occurred in September 2017, was the latest and strongest hurricane that struck Barbuda (Table 1, Fig. 2). The hurricane reached Barbuda at its maximum intensity. The island was struck for more than three hours by winds of 155 kt (287 km/h) with maximum recorded gusts of wind speed of 360 km/h, which caused the direct death of three people and the almost entire destruction of the structures on the island (Cangialosi et al., 2018). The minimum pressure record was of 914 mb (Table 1). Hurricane Irma also induced a significant storm surge with a peak water level at 2.4 m mean higher high water (MHHW) recorded at a tide gauge located on Barbuda (Cangialosi et al., 2018). This storm surge induced marine flooding with runup reported to reach some 600 meters inland (United Nations Development Programme, 2019).”

Chapter 2.3

Line 140: wrong reasoning --> the Lesser Antilles Arc marks the eastern boundary of the CP. It does not form the eastern boundary.

The word “forms” was changed to “marks”.

Chapter 3.5

Line 219: shells larger 2 mm were remove. What is the % of removed material >2mm in regard to the total sample?

Samples collected for particle size analysis were small (less than 5 mm radius). Shell debris larger than 2 mm was very rare. The few samples contained fewer than 3 shell fragments larger than 2 mm.

Lines 219-220: I do not understand what this sentence means: Because of their large content in shells debris, the mean grain sizes of sampled sediment were not coherent with the visual observations. Rewrite.

The phrase was rewritten:

L225: “Using the mean grain size to describe the samples was not appropriate here due to the high amount of shell debris, which tended to considerably increase the mean grain size.”

Grain size documentation: the documentation of modes/peaks is complicated and confusing to read. Is there a need to name them EM1, EM2,…  for each sample. The grain size for EM1 is different, e.g., 20 and 4 µm. Why not name the mean grain size, e.g. medium-sand, medium to fine sand and put the value e.g. 269 µm in brackets. A table with the grain size parameters may be helpful, too.

A phrase was added to help the comprehension:

L237-239: “To simplify the grain-size descriptions for both cores in the manuscript, the different endmembers will be designed as followed: EM1 = clay to silt (4 µm to 50 µm), EM2 = fine sand (127 µm), EM3 = medium to coarse sand (269 µm to 993 µm).”

All the “EM1”, “EM2” or “EM3” in the manuscript were changed for the corresponding grain-size mode “clay to silt”, “fine sand” or “medium to coarse sand”: L369-373, L401-406, L414-417, L433-438.

Lines 225-230: do you think the 4 peaks represent 4 different sources?

We tried to correlate the main grain-size modes with the three observed facies to understand the sources of each facies. However, we cannot say for sure that one grain-size mode corresponds to one sediment source, as it is possible that some sources were mixed in the sediment. The 127 µm grain-size mode (fine sand) is nevertheless probably related to the sand that arrived in the lagoon from the ocean.

Chapter 4.1

Lines 323-327: it is not clear where you describe the topmost Irma-deposits. Write this more clearly.

At this point in the manuscript, we simply describe the different units without interpretations.

We can only say in the discussion that a thick sandy unit was deposited after Hurricane Irma but we cannot say if the entire unit was deposited during Irma or if it was thickened with time:

L644-646: “In contrast, the northern inlet became larger and was still open in September 2021. Seismic profiling within this inlet in June 2021 shows that a large and thick ebb tidal delta and a 1-m-thick sand sheet developed within the main channel.”

Chapter 4.2.2

lines 360-364: this entire thing about endmembers is confusing. Why not name the dominant gran size modes / peaks in each facies.

All the “EM1”, “EM2” or “EM3” in the manuscript were changed for the corresponding grain-size mode “clay to silt”, “fine sand” or “medium to coarse sand”: L369-373, L401-406, L414-417, L433-438.

% are confusing, too – as they do not sum up to 100%.

For every facies, the grain-size endmembers sum up to 100% per core.

Line 365: ‘carbonated ‘is no correct English term. It´s carbonate sand. Delete d.

This mistake was corrected in the entire manuscript: lines 375, 378, 408, 410, 421, 447, 468, 468, 526, 538, 558, 610.

Lines 403-405: same - % do not sum up to 100%.

For every facies, the grain-size endmembers sum up to 100% per core.

Line 417: is it a transition or a boundary? A boundary would be an abrupt change.

“Transition” was changed for “boundary” in all the manuscript:

L430-431: “The boundary between Facies 3 and Facies 2 in cores BARB1-18-01 and BARB1-18-02 is correlated with the strong amplitude reflector in UI3 (in red, seismic profile 13, Fig. 6B) in unit Ul3.”

L510-511: “The bases of cores BARB1-18-01 and 02 were easily correlated with two similar thin F1 layers intercalated in F3 and a sharp boundary between F3 and F2 (Fig 8).”

L512-513: “The ages near the F3-F2 boundary in cores BARB1-18-05 and 06 are different: 1059 – 1255 cal. BP and 2057 – 2306 cal. BP, respectively.”

L516-518: “Consequently, it appears that the F3-F2 boundary was probably not synchronous in all the lagoon, with a later boundary in the central part compared to the southern part of the lagoon.

L564-565: “The boundary from the F3 to F2 environment seems to be related to relative sea level rise that led to a wider lagoon with a larger fetch allowing wind waves to develop (Fig. 1B).

L575: “The deepening recorded by the boundary from F3 to F2 could also be explained by subsidence.”

L598-600: “The boundary between uplift and subsidence operates over a short distance likely because Barbuda is located just above the downdip limit of the seismogenic part (Weil‐Accardo et al., 2022). The abrupt boundary between F3 and F2 in the Codrington Lagoon could be the consequence of a sudden subsidence during a major earthquake.”

L603-605: “Given this age is close to the transition boundary between F3 and F2 in our cores (Fig.8), this earthquake may have been responsible for the subsidence of Codrington Lagoon, as recorded by a deepening in the lagoon environment.”

Lines 420-423, 427: same - % do not sum up to 100%.

For every facies, the grain-size endmembers sum up to 100% per core.

Chapter 4.2.3

Line 453: shown – not showed.

It was corrected.

Line 455: ‘carbonated ‘is no correct English term. It´s carbonate sand. Delete d.

Done.

Chapter 4.2.4

Which local reservoir age/delta R values were used for radiocarbon dating.

Radiocarbon dating was established from organic macroremains (wood, leaves, and roots), which does not require the use of reservoir age. The 14C ages were calibrated at two sigma using the Intcal20 calibration curve (Reimer et al., 2020).

Chapter 5.1

Line 535: are all silicates windblown dust or are any biogenic siliciclastic components, e.g. sponge needle or diatoms, present? Again, calling it siliciclastic endmember is strange.

Ten slabs were observed with a scanning electron microscope (SEM), and the elementary composition at the grain scale was highlighted through chemical maps. The chemical maps did not reveal any siliceous debris, such as sponge needles or diatoms. The environment is almost exclusively carbonate, and the presence of silica in this region is related to long-range-transported (LRT) aeolian particles brought via summer easterly trade winds from Africa to the islands (Kumar et al., 2018). Furthermore, the facies with the highest values of silicates in the observed facies from the Codrington lagoon is facies 3, which displays the finest grain size (clay to silt) of all the observed sedimentary facies.

Line 544: ‘carbonated ‘is no correct English term. It´s carbonate sand. Delete d.

Done.

Subsidence: is it a tectonic (coseismic) subsidence or a drowning due to rising sea level? Is there any strong sedimentary evidence for the subduction, e.g., droned mangroves, a change in diatoms, forams or ostracods that imply a facies shift?

There is no evidence of subsidence in the sedimentary cores from the Codrington lagoon. The hypothesis was nevertheless discussed because of the abrupt transition between F3 and F2 in the Codrington Lagoon. This could be the consequence of sudden subsidence during a major earthquake that is known from turbidite deposits in the Barbuda Basin (Seibert et al., 2019).

Line 551, 586: again, is it a transition or a boundary? It is a transition if it is related to a slow sea level rise.

“Transition” was changed for “boundary” in all the manuscript.

Line 597: ‘carbonated ‘is no correct English term. It´s carbonate sand. Delete d.

Done.

Line 597: skeletal material – be more precise (e.g. sea urchin spines, calcareous algae, …).

The phrase was corrected L609-611: “F1 mainly consists of carbonated sand, derived from eroded fragments of marine skeletal material (e.g., corals, mollusks, bivalves, and foraminifers) and of shell fragments (100 to 1000 μm).”

Line 600: do you refer to a marine inundation event like a storm or tsunami (several hours) or a marine incursion (rising and falling of sea level).

The marine inundation refers to event like tropical or tsunami:

L612-616: “However, the presence of rare allochthonous marine species indicates a transport from the shoreface into the lagoon during marine incursions. Given that the microtidal regime of Barbuda favors the development of an isolated lagoon (e.g., Boyd et al., 1992), as shown by historical maps and satellite images before Hurricane Irma, the presence of allochthonous species suggests an overwash of the sandy barrier. Consequently, F1 layers are interpreted as overwash deposits related to either hurricanes or tsunamis.”

Lines 605-605: add a discussion on storm versus tsunami evidence.

This is already discussed in the Discussion section: L616-628.

“Consequently, F1 layers are interpreted as overwash deposits related to either hurricanes or tsunamis. The thin F1 layers intercalated within F3 do not present any visible tsunamigenic criteria, such as intraclasts of underlying sediment or mud caps. Given that hurricanes occur much more frequently than tsunamis in Barbuda, we assume that F1 layers intercalated with F3 could record hurricane events. However, it is also possible that one of the F1 layer intercalated in F3 corresponds to the potential 3500 cal. BP tsunami event recorded by turbidite/homogenite (Seibert et al., 2019). The F1 layer at the top of cores BARB1-18-01 and BARB1-18-05 display rather homogeneous 210Pbex values, indicating deposition of the F1 layer triggered by a single rapid event reworking previously deposited material (Fig. 10, Bruel and Sabatier, 2020). Moreover, both 210Pbex profiles exhibit an intermediate value at the base of F1, which may indicate the mixing of sediment from different ages and thus erosion at the base of the F1 deposit. From these observations, we assume that the thick F1 sand layer deposited at the top of the six cores corresponds to a unique high-energy and instantaneous marine flooding event that occurred recently, before core sampling in March 2018 and after the maximum bomb test radionuclide fallout (1963). As no tsunami occurred in Barbuda during this period, the best candidate for this deposit is a hurricane (Fig. 2).”

Line 625: washover deposits – add deposits.

It has been added.

Line 631: ‘ebb tidal’ is no correct term. It is a tidal delta or tidal channel?

The phrase was corrected.

L644-646: “Seismic profiling within this inlet in June 2021 shows that a large and thick ebb tidal delta and a 1-m-thick sand sheet developed within the main channel.”

Fig. 5 The thickness of the Irma unit is extremely large (>1 m). This is very unusual and related to the fact that the transect was measured just east of the breach. It would have been interesting to see a transect further N or S where no breach is present to compare the characteristics (thickness, subunits, internal structures).

Unfortunately, this transect is the only one that was possible to do to show Irma unit. This was the only transect were the wind wave and depth conditions enabled us to do measurements. 

Fig. 8, 9, 10, 11: Core photos are too small to see any details. Add detailed high-resolution phots in the appendix.

Detailed high-resolution photos will be added in the appendix.

RC2: 'Comment on nhess-2022-262', Anonymous Referee #2, 14 Apr 2023

This manuscript is well written and very interesting to read. Most of the changes that need to be accounted for are grammar and spelling related. However, there are several issues with capitalization in the references that need to be addressed.

Thank you for all your helpful comments, the references were also corrected. You'll find our answers in bold below.

Line 46: I do not believe the quotation marks are necessary.

The quotation marks were removed.

Line 60: The order of words in this sentence is the same as in your second sentence in the first paragraph. Consider rearranging the order of words.

The phrase was corrected:

L60-61: “Hurricane Irma brought many coastal hazards such as wind exposure, wind waves, storm surge and tropical rainfall.”

Line 107: Thalassia testudinum should be italicized.

It was italicized.

Line 142: Superscript needed on yr-1.

It was changed to “2 cm.yr⁻¹”.

Line 237: This does not make sense. Are you saying the software included those images (if so, change "including" to "included")? Should the sentence have a comma instead of the word "and"?

The phrase was corrected L244-246: “Vertical cross-sections of 63 µm resolution (voxel size) were reconstructed using ImageJ software with the Fiji distribution extension (Schindelin et al., 2012).”