Highstand Fans paper - Comment and Reply

One of the primary purposes of publishing papers is that it gives the community - other experts in your field - an opportunity to read, digest, and comment on your work. Most of the time, discussion and interaction among researchers is conducted at conferences or via one-on-one correspondence (e-mail/phone). Every once in a while, however, researchers will 'officially' comment through the journal in which the original paper came out.

In September 2007, we published a paper in Geology about the occurrence and significance of submarine fan growth and activity during sea level highstands. If you don't have access to *Geology *or you want to read a shorter summary with less jargon, I've blogged about it before in this post.

Doug Inman, professor emeritus from Scripps Institution of Oceanography and a giant in the field of oceanography submitted a comment to our paper a couple months ago (click here for full-text html; click here for pdf). Inman conducted some of the pioneering work regarding the connection of sedimentary processes in coastal areas with what is happening in the deep sea offshore of southern California.

Inman's primary comment on our work has to do with implications to sediment budgets within the context of our results:

We wish to compare this “natural” flux of sediment into the Ocean-side littoral cell with that measured by Covault et al. as accumulating in the La Jolla fan at a rate of 2.9 km³/k.y. = 2.9 × 10⁶ m³/yr over the period 13 ka to the present.

After laying out some of the nitty-gritty of sediment bulk density and calculations of sediment flux into tons per year, Inman says:

[Submarine] fan deposition rates over the past 13 ka of 2.9 × 10⁶ to 3.8 × 10⁶ ton/yr are in remarkably close agreement with the estimated natural sediment flux of 2.2 ×10⁶ ton/yr into the Oceanside littoral cell during historic times, considering the gross approximations of the calculations and the vast difference in time scales between the two estimates, and that sediment flux into the cell was likely to have been much greater during parts of the Holocene.

So, Inman isn't commenting on any error or misunderstanding in our work, but wanting to apply our results to something he has worked on for decades - historic sediment budgets in coastal southern California. Implicit in this official comment is that perhaps this is something we should have addressed in our paper.

In this process, the authors of the paper that is the subject of a submitted comment have the opportunity to reply (you can read it in full here). In terms of addressing the main point about comparing our sediment accumulation rates (at the 10,000 yr scale) to sediment accumulation rates from the historical record (10-100 yr scale), we state:

...we avoided directly comparing rates of historical sediment flux into the Oceanside littoral cell and Quaternary La Jolla fan growth, which span three orders of magnitude with respect to duration.

This ties in to a post I wrote about the Sadler (1981) paper (and a lot of subsequent confirmation) regarding comparing sediment accumulation rates from different measured time intervals. Read that post and then the original Sadler paper to get into the details. But, essentially, there is an inherent bias as a result of unsteady processes (in this case, the nature of deposition and hiatus, but this applies to many of the processes on the Earth's surface) such that rates of accumulation are lower for longer measured time intervals. This is why we avoided making the comparison Inman did in his comment.

That said, I think Inman makes a good point that deserves further investigation. Reconciling how to compare the rates we've measured in the last century or so with calculations from the geologic record goes beyond our particular paper and study.

This was the first official Comment and Reply I have been involved in and it was kind of fun in a way. I enjoyed it.

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