Rosa Rubicondior: Refuting Creationism - Why Nature Creates Fossils Of Different Ages In The Same Deposit

Collection of creatures that can disturb sediment and relocate shells.

Florida Museum.

Paleontologists spent 20 years carbon-dating thousands of marine fossils, then used them to decode a process fundamental to Earth’s history – Research News

A fossil bed is not necessarily a collection of animals that lived and died together. Two shells lying side by side can belong to creatures whose lives were separated by hundreds or thousands of years. For creationists who interpret fossil-bearing deposits as the products of a single, year-long biblical Flood, that raises an awkward question: how much history is actually represented within a supposedly catastrophic snapshot?

An international study, published in PNAS, brings substantial evidence to bear on that question. Drawing on two decades of research, the team assembled dates for more than 7,500 marine fossils, obtained using radiocarbon dating and other methods. They investigated “time averaging”: the mixing of remains from different generations within the same assemblage.

Their principal finding was that sediment accumulation rate is the strongest predictor of this mixing. Where sediment accumulates slowly, remains from successive generations can collect before permanent burial. Faster accumulation generally limits that opportunity. Comparisons between observations and computer simulations showed that sedimentation exerted a stronger influence than the other factors examined, including mixing by burrowing animals and the destruction of skeletal remains. These findings concern the relatively recent fossil record; their application to much deeper time still requires testing.

The creationist response cannot therefore consist merely of declaring that the fossils were all buried by Noah’s Flood. Burial and death are different events. A sudden event could gather older shells and redeposit them, but that would concede that the resulting assemblage contains a history predating the catastrophe. It would still be necessary to explain the age differences, the source of the older material and the surrounding sedimentary evidence.

Nor does a vague appeal to “errors in carbon dating” solve the problem. An identical age offset applied to every specimen would leave their age differences intact. Other proposed errors would need to be specified, quantified and shown to account for the observed pattern. Simply assuming that inconvenient measurements must be wrong is not an explanation; it is a refusal to accept evidence.

The significance of this research is that the history preserved within fossil assemblages can be investigated and measured. Scientists can ask how much time a collection represents, identify the processes responsible and test their explanations against thousands of specimens. For Flood geology, the challenge is to produce an equally convincing account of those observations. A biblical narrative, however confidently repeated, supplies neither the measurements nor the explanation.

Three Different Ages^ The Organism, the Fossil Assemblage and the Sediment. Finding fossils together does not necessarily mean that the organisms lived together, died together or were buried immediately after death. To interpret a fossil bed, palaeontologists distinguish three aspects of its history:

1. When each organism lived and died

A shell’s age concerns the organism that produced it. Dating the shell does not, by itself, establish when it arrived at its present burial site. It may have remained near the seabed surface or been moved after its owner died.

2. How much time the assemblage represents

A collection of fossils can include remains from many generations. The age differences between its oldest and youngest members may span centuries or millennia, even when the specimens lie beside one another. This mixing of different ages is called time averaging.

3. When the enclosing sediment was deposited

Older shells can be incorporated into younger sediment. Consequently, the age of an individual fossil and the date of its final burial need not coincide. A rapidly deposited layer can contain material with a much longer history.

How does time become mixed?

Where sediment accumulates slowly, shells from successive generations can collect before permanent burial. Burrowing animals disturb the sediment, while erosion and redeposition can introduce older remains into younger deposits. The research discussed here identifies sediment accumulation rate as the strongest predictor of the extent of time averaging.

A drawer full of old coins

Imagine putting coins minted over two centuries into a drawer today. The drawer’s contents represent 200 years of minting, although filling it took only minutes. The dates on the coins do not establish how long the drawer took to fill; equally, filling it today does not erase the coins’ earlier history.

Why this matters for Flood geology

Different fossil ages alone do not prove that the enclosing layer took thousands of years to form: a rapid event could redeposit older remains. However, that explanation must account for the remains’ earlier history, their measured ages and the sedimentary evidence. Simply calling the layer a “Flood deposit” answers none of those questions.

Nor would an identical dating offset make all the fossils contemporaneous: adding or subtracting the same number of years leaves their age differences unchanged. Errors that affect specimens differently require separate evaluation; they cannot simply be assumed because the results are inconvenient.

Glossary

Assemblage

A collection of fossils considered together from a particular sample or deposit.

Time averaging

The inclusion of remains from different times within one assemblage.

Bioturbation

Disturbance and mixing of sediment by living organisms, such as burrowing worms and crustaceans.

Reworking

The removal of previously deposited material and its incorporation into a later deposit.

Further reading: Florida Museum’s account of the research.

The paper in PNAS was accompanied by a Florida Museum news release:

Paleontologists spent 20 years carbon-dating thousands of marine fossils, then used them to decode a process fundamental to Earth’s history
Key points

Over the course of 20 years, an international team of scientists collected and carbon-dated more than 7,500 marine fossils from ocean beds around the world.
Scientists used the dataset to determine which of several environmental factors contributes most to time averaging, a phenomenon in which organisms that lived in different time periods are mixed and preserved together in the same fossil bed.
The results indicate that sedimentation is more important than other factors, such as the number of burrowing animals in a given area or the durability of fossils.
The study, published in the journal PNAS, establishes guidelines regarding the types of research questions paleontologists can investigate using fossil samples.

In productive marine environments, a square meter of seafloor can be perforated by hundreds to thousands of isolated and interconnected tunnels through which crawl and writhe a cornucopia of clams, shrimp, sea stars, sand dollars, snails, worms and other animals. All that excavation mixes up the sediment, along with any shells and other skeletal remains that happen to be there. This temporal smearing is a problem for paleontologists, because when that piece of seafloor is buried and becomes part of the fossil record, it’s difficult and expensive to figure out how much mixing took place.

What continually amazes me is just how much time a bunch of fossils collected from a single sediment layer can represent. In some cases, well-preserved fossil organisms that are found next to each other might have lived hundreds or thousands of years apart.

Rafal Nawrot, co-author
Department of Palaeontology
University of Vienna
Vienna, Austria.

In addition to burrowing animals, several other factors influence time averaging, including:

The durability of organic material is important. Most organisms decompose or get picked apart by scavengers before they become fossilized. For this reason, both the marine and terrestrial fossil records are primarily composed of hard skeletal material, like shells and bones. But even these break and disintegrate if they aren’t preserved quickly enough.
Another important factor is the rate of sedimentation, which occurs unevenly in different parts of the ocean and changes through time. Deltas, for example, have high rates of sedimentation, whereas other areas might only receive a fine dusting over long periods of time. If sedimentation is slow, skeletal remains of organisms accumulate over long periods of time, but if sedimentation is fast, the remains are buried quicker, and age mixing is reduced.
Biological productivity is also crucial. The number of fossils paleontologists can expect to find while digging in one spot is strongly correlated with the number of organisms that were previously around to be fossilized in the first place.

Through a project that was 20 years in the making, members of an international consortium of scientists say they have determined which of these factors is the most important for time averaging and thus primarily controls the temporal resolution of paleontological data.

Selection of creatures that disturb the sediment.

Top L to R: Diopatra ornata; A bootlace worm in the genus Lineus; Axianassa ngochoae
Center Left Top: Glycera brevicirris; bottom: The Atlantic jackknife clam (Ensis directus); right: Chaetopterus djiboutiensis
Bottom L to R: Sea urchins, like this Brissus latecarinatus; A lugworm in the genus Arenicola

Photos: Florida Museum invertebrate zoology

Our results demonstrate that if we know how quickly sediment accumulates — which can be deduced from the environmental context — we can determine how much time is captured by a given fossil assemblage: The faster individual shells or bones are buried below the sediment surface, the less likely it is that remains from multiple generations of organisms will accumulate and be preserved together.

Rafal Nawrot

Sedimentation rates have long been anticipated to be an important component of time averaging, but gathering data needed to rigorously and comprehensively assess this issue is difficult, time-consuming and very expensive.

Rapid sedimentation can sometimes preserve the outlines of soft tissue, as is the case in this fossil seagrass specimen with the imprint of a brittle star on one of the leaves.
Florida Museum photo by Kristen Grace

Moderate to low rates of sedimentation usually result in only hard, durable fossils being preserved.
Florida Museum photo by Kristen Grace

By integrating multiple projects, the authors analyzed more than 7,500 fossils, which were dated using radiocarbon and other methods and collected from a variety of oceanic environments around the world, from shallow coastal settings to the edges of continental shelves.

The various research groups involved in the project — which includes scientists based in Australia, Austria, the Bahamas, Brazil, Italy, Germany, Slovakia and the United States — separately collected, studied and published papers on the fossils over a period of two decades. When they learned of each other’s work, they decided to join forces and share data.

Nothing of this scale has ever been attempted before because it’s simply not feasible to do so, but thanks to the fact that we had a whole bunch of teams that worked on similar topics and used similar methods, we were able to compile it.

Professor Michal Kowalewski, lead author.
Florida Museum of Natural History
University of Florida, Gainesville, FL, USA.

Radiometric dating, one of the primary methods the authors used, takes advantage of the fact that radioactive atoms always decay into more stable, non-radioactive atoms at a steady, predictable rate. This allows scientists to estimate the age of minerals and fossils.

Many animals have skeletons that contain a type of radioactive isotope called carbon-14. Plants absorb carbon-14 during photosynthesis and use it to make more of themselves. Herbivores get carbon-14 secondhand by eating plants, carnivores get it from herbivores, and decomposers get it from all of the above. This list includes humans. Any part of your body that contains carbon — which is every part of your body — is radioactive. Fortunately, carbon-14 emits radiation in the form of electrons, which for us is kind of like receiving a constant but imperceptibly low-level electric shock — not at all like the cell-shredding gamma rays emitted by uranium.

Carbon-14 has a half-life – the amount of time it takes for half of any given number of radioactive atoms to decay — of around 5,730 years. That meant the authors were restricted to the most recent fossil record, up to 55,000 years old, which is about the cutoff when any remaining carbon-14 in a fossil can be reliably measured.

The researchers also used a technique known as amino-acid racemization, which uses ratios of amino acids. As in the case of carbon isotopes, the ratio of different forms of a given amino acid also changes through time in a predictable way.

Photograph of a fossil bed several feed high, packed with almost nothing but fossils with a pickaxe for scale. Limestone beds like this one in Cuba demonstrate just how rich a fossil bed can be when conditions influencing their burial and preservation are just right.

Florida Museum photo by Roger Portell

The reason no one has attempted dating on such a grand scale before is primarily due to the high cost of radiocarbon and amino-acid dating. Most research groups can afford to obtain data for only a few dozen specimens, but thousands of specimens are needed to fully evaluate the scale and drivers of time averaging. Distributing the cost across multiple labs over two decades helped significantly reduce this barrier, as did recent technological advances in radiometric dating that lowered the cost and made it possible to use much smaller samples than was previously possible.

Through this unique collaboration, Kowalewski and his colleagues have what is possibly the largest collection of fossil carbon dates ever compiled, which can now be used on a variety of research topics that would have been intractable otherwise.

The dataset is incredibly powerful. We’re now working on multiple follow-up projects that explore various aspects of time averaging and related processes. You can use it to answer a lot of questions, but of course, we started with the big one.

Professor Michal Kowalewski.

After compiling the carbon dates from their fossil specimens, the authors simulated age distributions by varying the rates of bioturbation (mixing caused by burrowing animals), sedimentation and fossil destruction. Then they compared the real age distribution of carbon-dated fossils with the different simulated distributions to see which of the models most closely matched the actual patterns observed in the data.

The results were unambiguous.

Sometimes life turns out to be more exciting than you thought,” Kowalewski said. “In this case, the outcome is beyond any dreams we may have had when we started.

Professor Michal Kowalewski.

Knowing that the rate of sedimentation is the single most important factor in determining the extent to which fossils of different ages become mixed will unlock research avenues that were previously restricted. And assuming the same pattern holds true for oceans further back in time, the results can be extended to fossils that are much older than the ones that still contain residual amounts of carbon-14.

Publication:
M. Kowalewski, A. Tomašových, R. Nawrot, D. Scarponi, M. Zuschin, P.G. Albano, Q. Hua, D.S. Kaufman, S.M. Kidwell, M. Ritter, M.G. Simões, L. Torres, B. Bachmann, M. Berensmeier, T.A. Dexter, A. Haselmair, I. Gallmetzer, C. Pellegrini, L. Schweigl, J. Steger, & M.A. Kosnik
Sediment accumulation rate predicts the temporal resolution of marine fossil assemblages
Proc. Natl. Acad. Sci. U.S.A. 123(31) e2615368123, https://doi.org/10.1073/pnas.2615368123 (2026).

Significance
Even within a single sediment layer, fossil assemblages contain the remains of organisms that lived at different times, like cemeteries where humans who lived centuries apart are buried together. This temporal mixing, or time averaging (TA), determines which biological and geological processes are meaningfully archived in the fossil record. To understand the processes that control TA, we analyzed 384 samples from marine sediments deposited since the last ice age. TA ranged from decades to millennia and scaled predictably with the sediment accumulation time, inversely related to sediment accumulation rate (SAR). Despite many processes that could drive temporal mixing, our results demonstrate a powerful paleontological principle: SARs predict the temporal resolution of fossil assemblages.

Abstract
Time averaging (TA), the mixing of noncontemporaneous organisms in fossil assemblages, governs the resolution of paleontological data and determines which biological and environmental processes are recorded in the geological record. To assess variation in TA and its dependence on external drivers, we used amino acid and radiocarbon methods to individually date 7,593 specimens across 384 samples from late Quaternary marine assemblages of calcifying organisms. Bivalve mollusks dominated the data, but 65 samples represented other taxa. Estimates of TA, measured by dispersion of specimen ages within samples, ranged from decades to millennia and scaled proportionally (r² = 0.77) with sediment accumulation time (SAT), an inverse of sediment accumulation rate (SAR) that accounts for the stratigraphic span of samples. In agreement with numerical modeling, the observed TA estimates were an order-of-magnitude higher than SAT and modulated by SAT-dependent effects of vertical mixing and skeletal disintegration. When SAT is short, effective disintegration rates are too slow to suppress the amplifying role of mixing but increase in importance when SAT is long. The strong dependency of TA on SAT points to the overriding role of SAR in controlling the temporal resolution of fossil assemblages, notwithstanding other interacting drivers known to influence TA. These results demonstrate a long-suspected paleontological rule: The temporal resolution of fossil assemblages scales predictably with SAR. This straightforward relationship provides a quantitative guideline for determining the temporal adequacy of sedimentary records as archives of Earth system processes.

M. Kowalewski, A. Tomašových, R. Nawrot, D. Scarponi, M. Zuschin, P.G. Albano, Q. Hua, D.S. Kaufman, S.M. Kidwell, M. Ritter, M.G. Simões, L. Torres, B. Bachmann, M. Berensmeier, T.A. Dexter, A. Haselmair, I. Gallmetzer, C. Pellegrini, L. Schweigl, J. Steger, & M.A. Kosnik
Sediment accumulation rate predicts the temporal resolution of marine fossil assemblages
Proc. Natl. Acad. Sci. U.S.A. 123(31) e2615368123, https://doi.org/10.1073/pnas.2615368123 (2026).

Copyright: © 2026 The authors.
Published by National Academy of Sciences of the USA. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

For creationists, the difficulty is that a fossil bed can preserve a history far longer and more complicated than the single catastrophe they invoke to explain it. Shells found together need not represent animals that lived together, and rapid final burial does not erase the centuries or millennia separating their deaths. Appealing to a Flood that gathered older remains merely shifts the question: where did those remains come from, and what history had they already accumulated?

Nor can that history be dismissed by declaring radiocarbon dating unreliable. A common age offset would leave the differences between specimens intact; any proposed errors that affected specimens differently would require evidence and quantitative evaluation. An alternative explanation must account for the observed patterns, including their relationship with sediment accumulation. Rejecting measurements because they conflict with Genesis supplies no such explanation.

The research also illustrates how science tackles the limitations of its own evidence. Time averaging complicates the interpretation of fossil assemblages, so scientists investigate its extent, identify its causes and establish which questions the material can reliably answer. Recognising those limitations strengthens our understanding of the fossil record.

There was no need to invoke a supernatural catastrophe to explain these findings. Ordinary sedimentary processes provide a testable account of how remains from successive generations become preserved together. Flood geology’s task is to match that explanatory power with evidence. Insisting that the answer must fit a year-long biblical narrative does nothing to explain the much longer histories preserved within the deposits themselves.

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