We present orbital‐scale resolution (~10 kyr) benthic foraminiferal δ18O and δ13C records from the Kerguelen Plateau (Ocean Drilling Program Sites 751 and 747) from 14.5 to 20.0 Ma spanning the Miocene climate optimum (15–17 Ma). Our records fill a critical gap from ~17 to 18 Ma, a time when many other deep‐sea records are affected by dissolution. We tested the fidelity of published magnetobiostratigraphic age models for these sites by astronomically tuning to the 405 kyr eccentricity cycle. A comparison of spectral estimates between the untuned and tuned records, as well as coherency with Laskar's (2004) eccentricity solution, revealed quasi‐100 kyr cyclicity in δ18O and δ13C. There is only a weak signal associated with the 41 kyr obliquity cycle, likely due to the 10 kyr sampling limiting resolution. The δ18O variations point to persistent 405 and quasi‐100 kyr modulations of temperature and sea level changes through the early to middle Miocene as predicted by astronomical solutions, with changing dominance of the 100 and 41 kyr beat. Comparison of δ18O records with early to middle Miocene sequences from the New Jersey shelf, northeast Australian margin, Bahamas, and Maldives suggests that the dominant sea level period preserved is the 1.2 Myr obliquity cycle, with sequence boundaries associated with δ18O increases or maxima. On the New Jersey margin, higher‐order sequences reflect the quasi‐100 kyr eccentricity cycles as modulated by 405 kyr cycles. We suggest that “nesting” of stratigraphic cycles is a function of the following: (1) pervasive (though changing) Milankovitch forcing of global mean sea level change and (2) preservation that depends on sufficient sediment supply and accommodation.

The Olduvai Basin (3u S), situated just west of the East African Rift System in northern Tanzania, contains a twomillion-year record of paleoclimate and paleoenvironmental change, as well as a rich archive of vertebrate paleontology (including hominins). Milankovitch precession cycles (, 20 kyr) regulate the mean annual precipitation (250-700 mm/yr), and the , 2500 mm/yr evapotranspiration results in a negative hydrologic balance. Despite persistent aridity, extensive deposits of freshwater limestone punctuate the stratigraphic record. Between 2 and 1 Ma, Lake Olduvai occupied the basin, and its sediments are a proxy for climate-driven cycles. Three limestones (1.84, 1.80, and 1.36 Ma in age), which formed within lakemargin floodplains adjacent to the lake, were studied to determine their depositional environment using field relations, sedimentary structures, fossils, petrography, and stable-isotope and major-element geochemistry. The three limestones are similar in that they contain peloidal micrite, siliciclastic detrital grains, and rhizoconcretions. Abundant faunal remains (gastropods, ostracods, Charaphytes, and fish) indicate that ponded water was relatively fresh and alkaline.Geochemical and stable-isotope data indicate two types of groundwater-sourced carbonate-producing waters: a deepersourced fluid that was enriched in iron and manganese due to extended water-rock interactions, and a shallower groundwater that traveled through alluvial-fan deposits. Regional faults tapped the deeper groundwater, producing carbonate at spring sites, while seeps associated with basinward changes in alluvial-fan slopes drew on shallower groundwater sources. Isotope compositions indicate that fault-related waters experienced some evaporation as water moved away from the spring sites while compositions of the seep-related carbonate remained relatively constant. Pedogenic alteration and meteoric calcite cementation affected the carbonate when the spring and seep sites dried out. Secondary strontium-rich dolomite precipitated within the limestones during burial under paleo-Lake Olduvai sediments and fluids during periods of lake expansion.Integrating these data within the geological context of regional paleoclimatic and local environmental change indicates that the freshwater carbonates formed periodically when the conditions were just right, i.e., a ''Goldilocks Effect.'' Carbonates near the basin center formed from groundwater flowing under a hydraulic head from faults or fractures during the falling limbs of Milankovitch cycles when the lake was in recession. Carbonates near the basin margin formed from an increased rate of groundwater seepage that occurred only on the rising limbs of cycles. In both contexts, the continuous flow of groundwater with surface evaporation and CO 2 degassing optimizes the conditions for limestone formation in this arid environment. These results help explain the formation of freshwater limestones in the rift basin, link the carbonates to specific portions of Milankovitch cycles, and document th...

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