Foraminifera

Planktonic foraminifera (Figure 2'], nicknamed forams, are one-celled, amoebae-like protozoa that float at various depths in the oceans, eating the still smaller photosynthetic algae and secreting calcareous shells that survive the foram's demise. They evolve rapidly

Calcareous Foraminifera
figure 2 i Foraminifera. [Photo courtesy of Brian Huber and National Museum of Natural History. ]

and spread out over wide areas, making them handy for indexing rock formations. Their sensitivity to temperature and salinity also make them useful in interpreting ancient environments. Foramini-fera occur in the hundreds of thousands, even in a single hand-sized specimen.

At the time the Alvarez theory appeared, the paleontologists who had been studying fossil plants believed they had largely survived the K-T boundary; those who had been studying ammonites believed they had gone extinct well before it. A return to the field for more collecting showed that both interpretations were wrong: The ammonites and the plants each suffered a massive extinction right at the K-T boundary. In contrast, the intensive study of the forams in the years following the appearance of the Alvarez theory led some paleontologists to just the opposite conclusion: What appeared to be a mass extinction was, they claimed, something else.

In 1980 few doubted that the foram extinction had been nearly complete and had been timed exactly to the K-T boundary; indeed, so many foram species disappeared that their level of departure almost defined the boundary. At Snowbird I, Hans Thierstein of the Scripps Institute of Oceanography showed that over 97 percent of foram species and 92 percent of the genera became extinct at the K-T boundary,26 and Jan Smit reported that the K-T foram extinction was so thorough that only one species, Guembelitria cretacea, survived, with all the subsequent foram species having evolved from it.27

The puzzle for students of earth history is how creatures that made their living floating in the sea all could be killed at once. A clue comes from another group of forams—the benthic variety that live on the seafloor—which suffered a much lower rate of extinction at the K-T boundary. Some believe that the difference in survival rate stems from the dependence of the floating plankton on "primary productivity"—that is, they ate the even smaller plantlike phytoplankton and therefore would die if those organisms were not available. The benthic forams, on the other hand, lived down in the detritus of the seafloor where they could feed on the accumulated organic debris, which would have been abundant after so many other creatures, including their floating cousins, had died in the K-T extinction. Thus if the upper layers of the oceans became sufficiently poisoned to kill the phytoplankton, the floating forams would die out but the deeper benthic variety would live on.

The simple picture of nearly complete foram extinction right at the K-T boundary did not go long unchallenged. Gerta Keller, who emerged as Smit's leading opponent in the interpretation of K-T

microfossils, studied forams at the K-T sections at El Kef, Tunisia, and along the Brazos River in Texas. At Snowbird II, Keller reported that "planktonic foraminifera show 30-45% of the species disappearing during the 300,000 to 400,000 years prior to the K-T boundary. . . . [They] show an extended K-T boundary extinction pattern beginning below and ending well above the boundary."28 If confirmed, this would falsify both predictions 1 and 2 for the fo-rams. But Smit, in contrast to Keller, did not find that any forams disappeared before the boundary.

Here we have an impasse. Two reputable scientists, each examining the fossils from the same section of rock, come to entirely different conclusions. What to do? The answer was to conduct a blind test in which samples are carefully collected under the supervision of a neutral party and then distributed to other experts, who identify the fossils without knowing from where in the section they come. New samples were collected at El Kef and distributed by Robert Ginsburg of the University of Miami to four independent specialists, not including either Keller or Smit.

Ginsburg was to present the results of the blind test on the El Kef samples at Snowbird III. Keller had departed the night before, and Ginsburg, returning from the excursion to Mimbral, fell down an escalator. He prevailed upon Fischer to present the results and flew home.29 Richard Kerr reported that, after Snowbird III, "both sides claimed victory."30 Keller argued that each of the four investigators had found that at least some fraction—ranging from 2 percent to 21 percent—of the Cretaceous forams had gone extinct before the boundary, which essentially confirmed that the extinctions were gradual. But Smit disagreed, telling Kerr that this was a typical Signor-Lipps effect. Smit then lumped together the results from all four investigators, but using only species that at least two of them had found. Each species that Keller said had disappeared before the boundary, Smit's technique located in the last sample immediately below it. Smit summed up: "Taken together, they found them all. This eliminates any evidence for preimpact extinction in the [open ocean] realm."31

Keller responded that if some of the investigators had been mistaken in their identifications and had lumped together species that looked similar but were not, then what was actually a series of gradual extinctions would appear to have been sudden. But as Kerr reports, Keller's own taxonomy came into question. Brian Huber of the U.S. National Museum of Natural History had studied and written about the forams in a deep-sea sediment core (from Ocean Drilling Program Site 738), on which Keller subsequently published.

"None of her taxonomy or quantitative studies can be reproduced," said Huber; "the gradual side of the debate doesn't hold water because of her inconsistencies."32 Keller responded in a letter to Science in which she cited 13 errors or misstatements in Kerr's article.33 For one: "It was I who could not confirm Brian Huber's . . . study rather than the reverse. . . . Huber's comments are therefore not likely to have been objective."34 Kerr responded: "By combining the efforts of all four blind testers, Smit intensified the search until all of Keller's gradually disappearing species were found to persist up to but not beyond the impact."35

The results of the El Kef blind test were finally published in 1 99 7.36 Not surprisingly, Keller and Smit continued to disagree. Thus the notion that a blind test can resolve disputes of this kind seems not to be borne out in practice. Even when fossils are as abundant as the forams, uncertainty remains.

The key point in the dispute between Keller and Huber was the identification in the core from Ocean Drilling Program (ODP] Site 738 of specimens of a particular foram species, Parvularugoglobige-rina eugubina. Though Keller said that P. eugubina was common in the core from ODP Site 7 3 8 , 37 Huber could not find it.38'3' To resolve the dispute, Huber asked Keller for permission to visit her laboratory so that she could point out P. eugubina to him in her samples. She agreed and Huber set off from his home base in Washington, D.C., to Keller's lab at Princeton, where he was joined by paleontologist Chengjie Liu of Rutgers. Even under Keller's supervision, however, they could not find P. eugubina in Keller's slides, and, according to Huber, she refused to show them the most critical samples.40 Thus with regard to P. eugubina, it was Huber who could not confirm Keller's taxonomy, not the other way round, as she had claimed in her response to Kerr.

After Huber returned from "the worst scientific experience of his life," he learned that Keller and Norman MacLeod, who was also present at the Princeton meeting, had resorted to an unusual course of action.41 Taking their disagreement with Huber outside the pages of journals, they went to the top, writing to none other than the secretary of the Smithsonian Institution, Robert McCormick Adams, to complain of Huber's behavior and to ask for the loan of certain specimens, requesting that the loan be handled by some other Smithsonian paleontologist than Huber.42 Adams replied that he preferred to see such differences resolved "through the normal channels of scholarly discourse."43

MacLeod and Keller went on to co-edit Cretaceous-Tertiary Mass Extinctions: Biotic and Environmental Changes, which contains

20 individual scientific papers.44 None of Keller's critics contributed an article; half of the chapters are authored or co-authored by prominent critics of the Alvarez theory—Keller, MacLeod, Stinnesbeck. In the book, MacLeod and Keller sum up the foram evidence by repeating their claim that two-thirds of the species went extinct before the K-T boundary and the other one-third survived it. This hearkens back to the original argument of Officer and Drake that the change from the Cretaceous to the Tertiary was not instantaneous, but took place over an interval of time.

Most of the "Cretaceous" foram fossils that Huber studied from ODP Site 738 he found to persist above the K-T boundary. Did these species survive into the Tertiary or were they reworked? That is the question. If large numbers made it through the K-T event to live on in the Tertiary, they were not killed by a meteorite impact at K-T time but died later from other causes, falsifying prediction 2 for the forams. Thus the key question is whether the "Cretaceous" foram species found above the K-T boundary had already died out and were reworked into the Tertiary, or whether they actually survived the boundary event to die thousands of years later. Keller and MacLeod have addressed this question in a series of papers based on studies of K-T sections from around the world, testing for survivorship using techniques from paleontology, biogeography, and geochemistry. ODP Site 738 was among their most thoroughly studied cases. They concluded that there is no causal link between mass extinction event and direct effects of K-T boundary impact.45

As a youngster growing up on a farm in northern Ohio, Brian Huber could never have imagined himself on a research vessel in a spot so remote as to be called Desolation Island. Better known as Ker-guelen, this tiny dot just off Antarctica, deep in the southern Indian Ocean, was long an important stop for whalers and seal hunters. (For a superb fictional account, read Patrick O'Brian's Desolation Island.") Huber was there as a paleontologist on Leg 119 of the Ocean Drilling Program, which sailed from Mauritius in December 1987. During Leg 119, hole 738 was drilled in the seafloor off Kerguelen, giving Huber an enduring interest in this site. When the voyage began, like most paleontologists, he was dubious about the Alvarez theory. As he tells it, however, when the section of the core that traversed the K-T boundary was drawn to the surface and laid out on the deck, his doubts vanished. There, as at Gubbio, was the dramatic color contrast between the white, foram-rich sediment below and the fossil-poor, reddish section above. You could lay a knife blade right on the boundary, exactly where an iridium spike of 18,000 ppt, one of the highest ever measured, was later found. The boundary clay displays fine laminations that would have been destroyed by bioturbation, allowing that effect to be ruled out in this case.

Over the years, Huber and his colleagues have conducted detailed studies of the core from ODP Site 7 3 8.4' They have found that specimens of some foram species that are known to have gone extinct at the K-T boundary, and of specimens of inoceramids, a group of clamlike creatures that became extinct well below the boundary, occur in the core far above the boundary, where they could have gotten only by reworking. Thus, in this core, contrary to the claims of MacLeod and Keller, reworking is prominent. This point is further supported by studies of strontium and carbon isotope ratios within the core. Huber concludes that "the high occurrences of Cretaceous species in lower Paleocene [lowest Tertiary] sediments are likely the result of extensive reworking."48 He believes that only two of the Cretaceous species in the core from ODP Site 738 survived the K-T boundary and that the rest are reworked. But if MacLeod and Keller's methods for detecting reworking can err in this most studied of deep-sea cores, how do we know they are not in error in the other cores and geologic sections that they have studied and pronounced free of reworking? The two of them appear to be almost alone among micropaleontologists in denying that the foram evidence corroborates the impact-extinction link. But let us remind ourselves that we are not trying to prove that link, we are asking whether the foram evidence falsifies prediction 2, that species that became extinct at the K-T boundary will not be found above the iridium horizon, except where reworked. The majority of opinion is that the evidence does not.

What of prediction 1, that species will not have begun to go extinct prior to the boundary and will have disappeared suddenly? Keller, with MacLeod, continues the claim she made at Snowbird II and repeatedly since: The foram extinctions started as early as 300,000 years before the K-T boundary. But once again we find ourselves confronting the dreaded Signor-Lipps effect, which inexorably causes a sharp extinction to appear gradual, even at a modern tidal flat, where all the "extinctions" took place in an instant. How can it be ruled out in the case of the forams? Prediction 1 is not falsified by the foram evidence.

t HE f OSSILS s PEAK

The history of understanding the ammonite and fossil plant extinctions at the K-T boundary shows how absent or negative evidence—

at first interpreted as falsifying the Alvarez theory—dissipated as more and more data were collected. Time and again, when paleontologists returned to the outcrop, they found what had eluded them before. In contrast, as more data were collected, the foram evidence seemed only to grow more complicated. But today, with a few noted exceptions, most specialists believe that the foram evidence is also compatible with an impact-induced extinction. I conclude that the evidence from all three groups—ammonites, plants, and forams—cor-roborates predictions ' and 2: (') Prior to the K-T boundary, most species were not already going extinct; rather their disappearance was sudden and right at the boundary. (2) Except where reworking has occurred, species of taxa that did not survive the K-T extinction are not found above the iridium horizon. Now it is time to turn to the dinosaurs, the creatures that got us interested in the K-T boundary in the first place.

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  • MILLY MOORE
    How do foraminiferans die?
    3 years ago
  • Abaalom
    How much foraminifera was extinct before the kt boundary?
    10 months ago

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