Provenance of Bengal Shelf Sediments: 2. Petrology and Geochemistry of Sand

Round 1

Reviewer 1 Report

Dear Editor the article entitled “Provenance of Bengal Shelf Sediments 2. Petrology and Geochemistry of Sand” is very interesting and well written and deserves publication.

Some minor issues I detected are listed below:

Better change to Petrological and Geochemical characteristics/features etc.. of Sand.

We will not say the “physics” of the something.. so we cant say geochemistry of sand.

Section 4.3., page 10, line 358: “…the presence of organic matter”. I don’t think the authors present any data of organic matter, and their assumption is not based anywhere. The affiliation of Cs, Cu to LOI is not a clear indication. LOI can represent a variety of minerals and not OM (since LOI = too high temp). I suggest to remove any mention to organics both here and also in line 430.

Line 583: further instead of farther

Line 631: the mineralogical and geochemical characteristics of.. instead of mineralogy and geochemistry (see above note).

I look forward seeing a published version of this MS.

Sincerely

Author Response

REVIEWER 1

Title of Article: in general the terms “petrology” or “geochemistry” refer to the respective scientific fields and I recommend not to be used in the context the authors used. Better change to Petrological and Geochemical characteristics/features etc.. of Sand. We will not say the “physics” of the something.. so we cant say geochemistry of sand.

We chose not to follow the Reviewer’s suggestion here. A research in Google Scholar shows that very many scientific articles contain “petrology of sand/stone” or “geochemistry of sand/stone” even in the title, and thus we assume that this is rather common use. Moreover, it makes a pair with the title of the companion paper.

Page 1 Lines 14 to 16: please add comma after the word Detritus and after the word estuary (line 16).

We do not feel that this stylistic suggestion does increase readability. We prefer not to add those commas, that only break the continuity (and thus the readability) of the sentence.

Section 4.3., page 10, line 358: “…the presence of organic matter”. I don’t think the authors present any data of organic matter, and their assumption is not based anywhere. The affiliation of Cs, Cu to LOI is not a clear indication. LOI can represent a variety of minerals and not OM (since LOI = too high temp). I suggest to remove any mention to organics both here and also in line 430.

Agreed and removed.

Line 583: further instead of farther

Several times English-speaking reviewers have pointed out to us that farther is the correct form where distance is concerned. We assume that farther is correct English in this case.

Line 631: the mineralogical and geochemical characteristics of.. instead of mineralogy and geochemistry (see above note).

Done.

Author Response File: Author Response.docx

Reviewer 2 Report

This paper examines distinct (fluvial, estuarine/deltaic, shelfal) segments of the Ganga-Brahmaputra system, and aims at assessing the relative contribution of these two rivers and of their drainage basins to coastal and shelfal depositional environments, with a look into the composition of the coeval deep-water turbidites. Despite the relative paucity of samples analyzed compared to the vastness of this sediment system, the comprehensive (petrographic, heavy-mineral, and geochemical) approach adopted for this study and its combination with the vast existing database available for the study area make the conclusions of this study well supported by the data presented.

Sediment provenance and all physical processes that may control sediment composition are investigated. The analysis of all size classes, from clay to sand, enables a thorough reconstruction of the routing system. Changes in sediment composition as a function of transport as bed load or suspended load are also addressed.

The traditional chemical indices of weathering used for the interpretation of the ancient stratigraphic record are discussed critically, and the authors are able to demonstrate that these indices can be strongly influenced by factors other than weathering.

In general, this is a well-documented example of source-to-sink analysis from one of the largest river systems in the world. The paper is well written and easy to read, and I recommend publication following minor revisions only.

I have a few remarks on the manuscript.

Given the very high quality of the data presented (and of the interpretations), it’s a pity that only a vague facies characterization (delta, estuary…) is offered to the study samples. Lacking precise facies interpretations, sedimentary processes can hardly be assessed based on petrographic, mineralogical and geochemical criteria alone, and in my opinion a few interpretations should be weakened a bit.   The manuscript could probably benefit from a slight re-organization in terms of data presentation and interpretation. The title of section 5 (“The Bengal sediment system”) is misleading. This section follows the “Results” section (4) and precedes the “Provenance” section (6). Several headings and sub-headings in section 5 point to geographic names (see sub-sections 5.1, 5.2, and 5.3), which makes this part of the manuscript difficult to follow. In my view, this section defines, instead, key concepts that are, to a certain extent, complementary to other concepts discussed in Section 6. Accordingly, the title of section 5 should stand like a bullet point (for example, “Grain-size and hydraulic sorting”). In sub-section 4.3, the geochemical composition of the Meghna Estuary and Bengal Shelf sand is presented uniquely in terms of absolute element concentrations, which appear poorly significant. Additional geochemical data about Zr and other elements from the Meghna Estuary, Bengal Delta and Bengal shelf are then reported in sub-sections 5.2 and 5.3, where data are mixed with interpretations. Element ratios are just presented in sub-section 5.4.1.

In summary, this paper will be certainly of interest to a wide spectrum of geologists working both on the modern and the ancient record, and I look forward to seeing the material in print.

Author Response

REVIEWER 2

This paper examines distinct (fluvial, estuarine/deltaic, shelfal) segments of the Ganga-Brahmaputra system, and aims at assessing the relative contribution of these two rivers and of their drainage basins to coastal and shelfal depositional environments, with a look into the composition of the coeval deep-water turbidites. Despite the relative paucity of samples analyzed compared to the vastness of this sediment system, the comprehensive (petrographic, heavy-mineral, and geochemical) approach adopted for this study and its combination with the vast existing database available for the study area make the conclusions of this study well supported by the data presented.

Sediment provenance and all physical processes that may control sediment composition are investigated. The analysis of all size classes, from clay to sand, enables a thorough reconstruction of the routing system. Changes in sediment composition as a function of transport as bed load or suspended load are also addressed.

The traditional chemical indices of weathering used for the interpretation of the ancient stratigraphic record are discussed critically, and the authors are able to demonstrate that these indices can be strongly influenced by factors other than weathering.

In general, this is a well-documented example of source-to-sink analysis from one of the largest river systems in the world. The paper is well written and easy to read, and I recommend publication following minor revisions only.

I have a few remarks on the manuscript.

Given the very high quality of the data presented (and of the interpretations), it’s a pity that only a vague facies characterization (delta, estuary…) is offered to the study samples. Lacking precise facies interpretations, sedimentary processes can hardly be assessed based on petrographic, mineralogical and geochemical criteria alone, and in my opinion a few interpretations should be weakened a bit.

Samples BR446, BR8230, and BR1051 are bedload samples collected in the active estuary channel at water depths between 10 and 17 m, as better specified in the revised text (section 3). Sediment samples of the subaqueous delta range in facies from sandy topsets to silty forsets and bottomsets, and to relict outer shelf sand, as fully illustrated and discussed in the article.

The manuscript could probably benefit from a slight re-organization in terms of data presentation and interpretation. The title of section 5 (“The Bengal sediment system”) is misleading.

The title of section 5 has been changed into: The Bengal Sediment System: Processes and Products, hoping that it better clarifies the aims and content of this section.

 This section follows the “Results” section (4) and precedes the “Provenance” section (6). Several headings and sub-headings in section 5 point to geographic names (see sub-sections 5.1, 5.2, and 5.3), which makes this part of the manuscript difficult to follow.

We do not very much see how can we do differently. Section 5 discusses sedimentary processes and products in order, starting with the Ganga and Brahmaputra Rivers and hydraulic sorting in river channels (5.1), passing next to the Meghna estuary and subaerial Bengal Delta (5.2), and to the Bengal Shelf (5.3). Finally it discusses chemical weathering in the Bengal sedimentary system (5.4).

 In my view, this section defines, instead, key concepts that are, to a certain extent, complementary to other concepts discussed in Section 6. Accordingly, the title of section 5 should stand like a bullet point (for example, “Grain-size and hydraulic sorting”). In sub-section 4.3, the geochemical composition of the Meghna Estuary and Bengal Shelf sand is presented uniquely in terms of absolute element concentrations, which appear poorly significant.

Section 4 is in fact where we present the raw database, with only a few inferences.

Additional geochemical data about Zr and other elements from the Meghna Estuary, Bengal Delta and Bengal shelf are then reported in sub-sections 5.2 and 5.3, where data are mixed with interpretations.

Yes, because in section 5 we discuss the effect of sedimentary processes and the data reported there are strictly pertinent to that discussion.  Zr concentration (as well as other elements cited there) is one excellent way to highlight hydraulic concentration of zircon and other very-high-density heavy minerals.

 Element ratios are just presented in sub-section 5.4.1.

Yes, because subsection 5.4 is where we discuss weathering processes, as revealed by the use of different sets of parameters, including element ratios.

In summary, this paper will be certainly of interest to a wide spectrum of geologists working both on the modern and the ancient record, and I look forward to seeing the material in print.

Thank you very much for the careful reviews and critical comments received, even though in several instances we chose not to follow the Reviewers’ requests, principally because of different subjective views concerning style and organization.

Author Response File: Author Response.docx