Все, что я могу сказать по этому поводу- легче сделать анализ заново, чем найти точный ответ. Там этих статей - миллион, и в какой они описывают сам метод анализа, пойди угадай. По названию иногда невозможно, а все прочитать... может кто-нибудь другой изъявит желание Все что пока нашла - останки нашли в аллювиальных отложениях, которые сохранились между двумя слоями вулканических пород. Возраст этих пород: нижней 4.419 ± 0.068 Ma, верхней 4.416 ± 0.031 Ma. если я правильно понимаю, плюс минус и есть точность. 

Сами отложения представляют собой красно-бурый ил, глину и песок толщиной от 3 до 6 метров, которые образовались за период от 100 до 10 000 лет.

ПС ура, все-таки разобралась. 40Ar/39Ar это и есть метод. Аргон-аргон называется. Argon-argon dating is a radiometric dating method invented to supersede potassium-argon (K-Ar) dating in accuracy.

***** Выдержки из оригинала статьи в Science ****

Sediments containing Ardipithecus ramidus were deposited 4.4 million years ago on an alluvial floodplain in Ethiopia’s western Afar rift. The Lower Aramis Member hominid-bearing unit, now exposed across a >9-kilometer structural arc, is sandwiched between two volcanic tuffs that have nearly identical 40Ar/39Ar ages.
Geology. The Aramis Member directly overlies the Gàala ("Camel") Tuff Complex (GATC), which has a 40Ar/39Ar age of 4.419 ± 0.068 Ma (2, 3). This vitric tuff is 0.5 to 2 m thick and is rich in pumice and crystals. The Aramis Member includes light salmon (hue 5YR) to deep red-brown silt, clay, and sand of variable thickness and induration, deposited on a floodplain. These strata show a general increase in thickness toward the east, and they range from an average of 3 m up to 6 m. They are overlain by the Daam Aatu ("Baboon") Basaltic Tuff (DABT), which has a 40Ar/39Ar age of 4.416 ± 0.031 Ma (1). We define the Lower Aramis Member as the entirety of both tuffs and all sediments between them.

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A patchwork of variably fossiliferous localities along the outcrop arc has yielded a combined total of more than 6000 individually cataloged vertebrate fossils between the widespread volcanic marker horizons. The vertebrate assemblages are in close association with sedimentological and structural information, botanical and invertebrate fossils, and oxygen and carbon isotopic data on pedogenic carbonates in soil horizons. Integration of these data allows reconstruction of the physical and biological aspects of the depositional setting.

The Middle Awash was a persistent sedimentary basin during the Pliocene (4). The basin axis during deposition of the entire Aramis Member was southeast of the Ardipithecus localities, as evidenced by deltaic and lake margin deposits generally disposed to the southeast, in the direction of paleocurrent orientations and erosional features. Active volcanic centers of the CAC were located to the south (1). Paleoenvironmental data and structural reconstructions suggest that the overall elevation may have been greater than today’s ~600 m, although kinematic models are equivocal (5, 6).

The lower tuff (GATC) is underlain by a widespread cobble conglomerate. In the central part of the exposure arc, fossiliferous Lower Aramis Member sediments comprise predominantly massive and bioturbated silty clays deposited primarily on a low-relief floodplain far from the main river channel(s). Reworked GATC pumices and glass are present locally, but evidence of channels is limited to rare sandstone lenses generally situated below the fossiliferous strata. Massive (<1.5 m thick), predominantly micritic carbonate horizons and nodules representing groundwater and pedogenic deposits pinch out laterally within clayey silts. These are also locally fossiliferous.

Carbonate deposits in some localities contain characteristic features of tufas (7), such as fossil gastropods and other invertebrates, abundant and uncrushed calcite-replaced vegetation, vertebrate remains, and eggshells (guinea-fowl size). These suggest that the carbonate horizons generally formed at or near the landscape surface. Evidence of spring activity includes several 1-m-wide banded travertine deposits associated with faults. A porous microcrystalline carbonate with dense concentrations of calcite isomorphs of plant parts forms a broad, low dome just north of ARA-VP-6. However, in almost all sections excavated for isotopic, phytolith, and pollen analysis, the carbonates lack diagnostic features of tufas. Their micritic textures and the presence of terrestrial soil invertebrate faunal activity (such as dung beetle brood burrows) suggest that the carbonate horizons are derived from groundwater carbonate that generally formed at or near the landscape surface in seasonally saturated soils near springs (fig. S1).

Paleosols in the Lower Aramis Member are primarily protosols and calcisols (8) and are most strongly developed just below the DABT. Protosols are 10 to 200 cm thick, with massive to single-grain structure and abundant root structures. Calcisols are 50 to 100 cm thick and contain subsurface horizons (Bk) displaying massive to angular blocky ped structures, weakly developed argillic cutans, and small (<5 mm across) calcareous nodules and tubules. The best-developed calcisols are present directly below the DABT. These nodules are most abundant to the north and may imply that it was slightly drier there. Sections at the southeast end of the exposure arc show weaker soil development, perhaps implying deposition in a predominantly wetter, more axial environment.

The DABT is poorly consolidated. The lower third (~15 to 20 cm) of the unit is composed of bedded and laminated gray basaltic glass lapilli and scoria. Given the geometry of its basal contact and lack of underlying incision, we interpret the DABT to have fallen across a dominantly low-relief landscape. Reworked clasts of the DABT are seen in only a few small shallow channels. The presence of paleosols and minor channels before deposition of the DABT indicates that sedimentation was intermittent and that streams only slightly incised this part of the basin. Sediments immediately overlying the DABT near the ARA-VP-1 (TS), ARA-VP-7, and ARA-VP-10 localities contain massive to weak, blocky, root-marked protosols overlain by alternating massive clayey silt and fine, well-sorted sand with tubular, subvertical 1- to 2-cm burrows. These features indicate that the water table was high and that the floodplain was aggrading more rapidly after deposition of the tuff, in accordance with faunal evidence that more aquatic and water-dependent mammals were more abundant above the DABT.

The time span represented by the fossiliferous sediments of the Lower Aramis Member (between the two tuffs) is difficult to ascertain. The dates for the two tephras are statistically indistinguishable, the difference between them only 0.003 ± 0.075 million years at 68% confidence. Thus, the dates suggest that, most probably, this interval represents a few thousand to perhaps at most 100,000 years. Paleosols in similar aggrading distal floodplain environments often indicate geologically short time spans. The fossil assemblages collected from between the two tuffs are consistent with environmental stability during the interval, and no evolutionary trends are evident (9). On the basis of analogous settings, these sediments probably represent deposition within 100 to 10,000 years (10) and their paleontological contents would qualify as a "within-habitat time-averaged assemblage" (11).

The depositional environment of the Ar. ramidus fossils in the CAC differs somewhat from that of penecontemporaneous Gona fossils ~70 km to the northwest. The Gona conspecifics were recovered in mixed-habitat faunas along the western basin margin where lake deposits interfinger with small fluvial channels or lap onto active basaltic cones and flows (12).