An excellent page that explains one kind of strong scientific support for an old earth — Isotopic Sorting and the Noah's Flood Model — is moderately long (27 k, plus the end-of-page links & references) and scientifically sophisticated, combining ideas from the diverse fields of biology, paleontology, geology, and oceanography, plus nuclear isotopes.  It's well written, but (due to its length and technical difficulty) readers who are not comfortable with science may not feel confident that they can understand it.  Therefore, to help you focus on the main ideas I've quoted only the introduction and conclusion, plus one small excerpt from the middle sections.  Highlighting with bold is in the original, but I've added the colors.

Isotopic Sorting and the Noah's Flood Model

Last edited: 03/14/2002

Stable isotope stratigraphy is a method of correlating sedimentary deposits in time and space based upon ratios of various stable isotopes.  This is possible because the isotopic composition of global seawater evolves over time, as a result of various biological and geological processes.  Variations in the isotopic composition of seawater over time are recorded by various proxies — for instance low-MG calcite, or inorganic minerals such as barite.  Boggs (1987, p. 688) writes:

Variations in the relative abundance of certain stable, nonradioactive isotopes in marine sediments and fossils can be used as a tool for chronostratigraphic correlations of marine sediments.  Geochemical evidence shows that the isotopic composition of oxygen, carbon and sulpher in the ocean has undergone large fluctuations, or excursions, in the geological past — fluctuations that have been recorded in marine sediments.  Because the mixing time in the oceans is about 1000 years or less, marine isotopic excursions are considered to be essentially isochronous throughout the world.  Variations in isotopic compositions of sediments or fossils allow geochemists to construct isotopic composition curves that can be used as stratigraphic markers for correlation purposes.

The interesting point is that global isotopic changes occur throughout the geologic record, both in the shells of individual organisms and in inorganic marine precipitates, such as barite.  Isotopic curves have been reconstructed for the entire Phanerozoic, documenting changes in seawater isotopic ratios during the past 500+ million years (e.g. Veizier et al., 1999; McArthur et al., 2001).

Fairly abrupt isotopic changes can often be correlated across the entire earth, for example a large del 13C excursion at the base of a specific conodont zone correlated with an extinction event.  This implies that deposition of the shells and sediments in the geologic record occured at a rate that is fairly slow compared to the rate at which oceanic mixing occurs, about ≈10^3 years or so (Holser, Magaritz, and Wright 1986; Kump 1991).  This is inconsistent with models in which a substantial portion of the geologic record is deposited by a single catastrophe lasting only months.  On the diluvial model, all of the shelly fossils in the geologic record are the remains of animals that lived in a preflood ocean prior to the flood.  In order to reconcile this hypothesis with the global isotopic patterns seen in the geologic record, the flood would have to transport and sort brachiopod shells, conodonts, forams and other calcerous fossils, and even inorganic minerals such as barite, by very subtle but consistent differences in C, O, S, and Sr isotopic ratios!  And it would have to work virtually in real time across the entire earth.  The difficulties entailed by trying to fit these data into a flood-model are obvious.

note:  I'll jump directly from this introduction to the conclusion, except for the quotation below.  It's from a section about "The Permo-Triassic Boundary," and the page-author emphasizes a conclusion in bold:
Erwin (1993) notes that "the isotopic signatures are so similar from sections ranging from restricted basins to open marine that the only reasonable conclusion is that the major shifts are globally synchronous events" (p. 198).

Conclusion

A simple explanation for these isotopic variations and their concordance from basin to basin is simply that the isotopic composition of the oceans and atmosphere have varied over time, and that the isotopic composition of shells, organic matter and so on in the geologic record reflect the isotopic composition which prevailed in their environment at the time they lived.

Flood geology, on the other hand, will hardly be able to explain this data in terms of a single catastrophe lasting only months.  For instance, why would these variations even exist in the first place, if all the marine organisms in the fossil record lived immediately prior to the flood?  And even if all these marine organisms did NOT live immediately prior to the flood, instead accumulating in the 1600 or so years before the flood, there would still remain the immense problem of how the flood could possibly sort forams, brachiopod shells, conodonts, as well as ‘organic matter,’ by *tiny* but consistent differences in O, C, S, and Sr isotopic composition, in virtually identical stratigraphic sequences, in basins seperated by thousands of miles.

This page is cited in the Selected Topics for AGE-SCIENCE.