Hi Etude.
I once asked an archaeologist I served on jury duty with how accurate C14 dating was....Her answer was: "I'm sure they calibrate that somehow." Well needless to say, that was not a very satisfying answer.
Individual archaeologists may not necessarily be familiar with the particulars of radiocarbon dating. The discipline is very diverse and filled with specialists who are experts in their own fields. Archaeometry is itself a subspecialty and the 14C analysis is carried out in designated laboratories.
I mentioned my concern about how the carbon got into the specimen in the first place, especially in formerly living organisms (not just layers of rock). I asked how we were able to determine if the C14 being measured was the C14 absorbed while the tree was alive or if the C14 seeped into the tree years later after it fell and was inundated with water and C14-laden air or was deposited along with other minerals in some dinosaur bone before either was petrified.
My understanding is that 14C enters into organic tissue through metabolic processes, first in phototrophic organisms and then secondarily through the food chain. The exchange of 14C occurs throughout an organism's life until metabolism ceases, at which point absorption stops. In general there is no statistically significant uptake after death through simple exposure to background radiation since 14C in background radiation is several orders of magnitude lower than the levels it is found in living tissue; organic processes concentrate radioactive isotopes in tissue at levels far higher than what exists in the environment (which is why there is a higher risk of long-term radiation poisoning from eating food grown and raised in places contaminated with radiation). In samples older than 57,000 years, only a tenth of one percent of the original amount of C14 remains in organic matter, which approaches the levels found in background radiation. So for the vast majority of samples, such as wood or plant remains within the past five thousand years, exposure to background radiation has no statistically significant effect on 14C levels in organic materials. The main exception to this are samples contaminated either by more recent organic matter or those buried in "14C-producing environments" where higher levels of 14C (higher than normal background radiation) occur naturally through geological processes. Contamination is minimized through careful selection of samples (as potential for contamination can be recognized from the archaeological or geological context), cleaning of samples at the laboratory, and the identification of outliers through sample replication from multiple sample points, cross-dating, and calibration, among other things. The high level of consistency of radiocarbon dating makes the identification of outliers rather trivial in many cases. For instance, here are some real uncal BP dates from Akhenaten's reign (ca. 1353–1336 BC) via samples obtained from Tell el-Amarna....can you find the outlier??
uncal BP dates for Akhenaten: 3051±27 BP, 3064±28 BP, 3082±29 BP, 2862±26 BP, 3096±38 BP, 3092±27 BP, 3094±37 BP, 3070±37 BP
Everything here except for 2862±26 BP agrees within a narrow range, between 3051 and 3096, which is remarkable considering that Akhenaten had a 17-year reign. Note also that the outlier is much younger than the other ages. Contamination with newer organic material would usually not produce older ages (which is what anti-science denialists wish to explain away through contamination), but rather make samples seem too young. Then the other samples can be averaged together to produce a more accurate uncalibrated date. But even going by the date at the middle of this range (3082±29 BP), we can see that the date gives us almost exactly the right age when calibrated: 1361±38 calBP (using the 2007 CalPal calibration curve). The outlier date gives a calibrated age (1041±43 calBP) that is completely impossible with our knowledge of Egyptian history and which also goes against the very consistent pattern of radiocarbon dates not just for Akhenaten but for the whole sequence of the New Kingdom. That's the key point here. Contamination is haphazard and willy-nilly; it produces outliers, not beautifully patterned, consistent chronologies that correspond closely to other independently derived sequences. Appealing to contamination as an attempt to discredit the whole science of radiocarbon dating (which is what is done in creationist pseudoscience) is to lose the forest for the trees.
The only thing that is for sure about C14 is that it has a specific rate of decay or half life.
This is not true at all. Read the article I linked above. Calibration has made radiocarbon dating into a much more finely tuned tool, and one that continues to improve as datasets grow. It isn't simply a matter of half lives and hasn't been since the early 1970s.
What I've gathered from the discussion so far is that, while C14 dating is not meant to be completely accurate (given the percentage variances in the way it's calibrated) and serves only as a ballpark marker
Of course it's not "completely accurate" (what is?), but it is astonishingly accurate when properly conducted. Coupled with Bayesian statistical modelings, its even been used recently to generate approximate dates for the pharaohs of the third and second millennium BC: http://www.sciencemag.org/content/328/5985/1554.full.pdf. But my point in ninja's other thread is despite the small expected range of error, it is impossible to move around Egyptian chronology on the order of many hundreds of years which lacks justification and which would disrupt the interlocking consistency between independent lines of evidence.
we do make assumptions about how it's calibrated (that the C14 absorbed must have been constant or at least the same as in other sites tested; that the sample tested is uniform compared to the rest of the object being tested; that the percentage variance in different objects tested or the average of different readings is preferable than a single reading that can be accurately established. (http://www.nytimes.com/1990/05/31/us/errors-are-feared-in-carbon-dating.html?pagewanted=all&src=pm)
I notice that article was from 1990. And the article primarily discussed samples on the order of 9,000, 20,000, 30,000 years old, where the error range is quite large, a very different situation than that concerning samples from the second millennium BC. And in the twenty years since that article was published, methods have continued to improve even for handling very old specimens: http://www.sciencedaily.com/releases/2010/02/100211111549.htm.
I'm not saying that we (especially scientists) don't have the leniency to make those assumptions. However, if we do, we need to be willing to concede that the reading is less than 100% accurate (even if it's just close enough)
No scientist has ever claimed that radiocarbon dating is 100% accurate. The real issue here is with those who claim that it is so unreliable that all dates (even those from samples within the last 5,000 years) are off by many hundreds or even thousands of years (as required by Young Earth Creationism, which holds that human civilization and the earth is no older than 6,000 years).
Leolaia, I think your citation of ninja_matty69's reference to the global flood might be a bit misplaced. The key word is "IF". I hope ninja_matty69 is not asserting to the Biblical flood as an actual occurrence
No, the source that ninja was quoting was most certainly a Young Earth Creationist interested in discrediting science that indicates that humans were on earth longer than 6,000 years and that existing monuments are older than the biblical date for the Flood. "If" is rhetorical. Think about it....why else would a mythological story be used in a discussion about problems in radiometric dating?
And no, what the person said has no relevance to small local floods. The arguments depend on global conditions governing 14C availability. Hence, a global Flood.
The point is that if there was a significant "cloud cover" in our atmosphere (something that is also scientifically suggested at different times in our geological history), the C14 rate would have been different and would have affected samples greatly and the flooding might well have contaminated a lot of specimens.
Biblically-derived notions like a cloud canopy covering the antediluvian planet and a global Flood were not scientifically credible to begin with, but now there is now such a wealth of information on palaeoclimatology thanks to dendrochronology, varves, ice cores, etc. that scientists have a very accurate knowledge of what the climate of the earth was like at various times in the past.
One recent investigation depicts Israel as an "intellectual construct". It also points to other "cigarette-sized" scrolls found (beside the Dead Sea scrolls), which indicate sufficient differences in "old testament" accounts to signal the lore and poetic origins of the Old Testament. (http://www.pbs.org/wgbh/nova/ancient/bibles-buried-secrets.html)
That's the Ketef Hinnom amulets containing the Priestly Benediction. They were not found among the Dead Sea Scrolls but in a pre-exilic cemetary in the Hinnom Valley. I am not aware of substantial differences between the text of the Priestly Benediction in the amulets and what is in the OT. But even if they are, the amulets quote what was then oral liturgy and not necessarily a biblical text.
