Looking at chart 1, it seems to me that the distribution of the chromium-53 ratio in today's seawater is a reasonable match to the ratios seen in today's sediments, and not to that seen in ancient rocks, while the distribution for today's rivers and estuaries is not a good match for today's sediments, and, if anything, is a better match to the ancient rocks.
Absent any other evidence, this seems to suggest that the fractionation seen in today's sediments may be the result of processes occurring in seawater rather than in rivers, and if so, that would in turn suggest that what happens in rivers and estuaries is not a good guide to the fractionation we should see in ancient rocks, even if we assume ancient rivers were mostly like the Rio Tinto - unless the ancient seawaters were acidic enough to prevent fractionation occurring there.
Absent any other evidence, this seems to suggest that the fractionation seen in today's sediments may be the result of processes occurring in seawater rather than in rivers, and if so, that would in turn suggest that what happens in rivers and estuaries is not a good guide to the fractionation we should see in ancient rocks, even if we assume ancient rivers were mostly like the Rio Tinto - unless the ancient seawaters were acidic enough to prevent fractionation occurring there.