Paleoclimate records reveal elusive ~200-kyr eccentricity cycle for the first time

The main cycles of Earth's eccentricity as recorded in paleoclimate archives signify 2ⁿᵈ order terms, combining two g-frequencies associated with the precession of perihelia of the inner planets and Jupiter. However, many weaker cycles are present and may be documented as well. Here we report on a thus far unnoticed ~200-kyr cycle observed in rhythmically bedded marine marls and limestones of early Paleocene age in NE Spain and adjacent France. This cycle is expressed as alternating distinct and less distinct ~100-kyr CaCO₃ maxima and associated magnetic susceptibility (MS) minima, which correspond to successive minima of the short ~100-kyr eccentricity cycle.The most plausible explanation is that this ~200-kyr cycle represents a weak but real cycle in eccentricity. Due to its weakness, this cycle is only expressed as alternatingly strong and weak ~100-kyr minima in eccentricity time series, a pattern that is identical to what we observe in the proxy records and outcrop. Its origin as eccentricity cycle is complex as it is composed of 4 to 6 individual components. A simplified solution solely based on the five leading g-frequencies reveals that most of these components originate from combinations of these 5 frequencies. However, they do not correspond to 2ⁿᵈ order terms, which underlie the principal short and long eccentricity cycles, but to much weaker higher order terms. Three of them have been identified as 4ᵗʰ order terms and two as 6ᵗʰ order terms, representing combinations of 2ⁿᵈ order terms, such as the first harmonic, 2(g2 – g5), of the 405-kyr cycle and related components.It is for the first time that the combined effect of such weak components has been identified in paleoclimatic records, which is only possible because they are not overwhelmed by much stronger 2ⁿᵈ order terms in this frequency band. Finally, the typical pattern that results from interference with the much stronger ~100-kyr cycle might be used as a template for testing the accuracy of astronomical solutions in the future.