High-resolution and expanded records of early Aptian Oceanic Anoxic Event (OAE) 1a (120 Ma) were recovered from outer shelf and slope deposits from the Sierra Madre Oriental of northeast Mexico and from hemipelagic deposits at Deep Sea Drilling Project Site 398 on Vigo Seamount in the North Atlantic Ocean. OAE1a is recognized in these deposits by minor increases in organic carbon content (up to 2 wt%) and widely-recognized, characteristic d¹³C_org and d¹³C_carb variations that record significant changes in global carbon cycling. Here we present continuous dm-scale records of grain size distributions, wt% TOC and carbonate, C/N ratios, and C and N isotopes for these stratigraphically-expanded sections. Our high-resolution rock magnetic study of the same stratigraphic intervals establishes orbital timescales for OAE1a at these sites.

The highly expanded records reveal significantly more structure in isotopic and TOC variation throughout OAE1a than contemporaneous condensed deep-sea records. Our data define an initial prominent negative excursion followed by repeated and short-lived (10³ to 10⁴ yr) fluctuations and a final interval of near invariant d¹³C_carb and d¹³C_org values. Elevated TOC, a significant increase in C/N ratios from background marine values, rapid shifts in C and N isotopic compositions and sustained decoupling of d¹³C_org and d¹⁵N_org occur throughout the interval of short-lived isotopic fluctuations. Anhysteretic remanent magnetization (ARM) measurements were used as proxies for terrigenous influx during OAE1a. Spectral analyses of ARM depth series reveal dominant cycles with wavelength ratios resembling those of modern orbital periodicities, implying that orbital variations may have modulated depositional processes. These orbitally tuned geochemical proxy records indicate that the Aptian deposits from northeastern Mexican and Site 398 on Vigo Seamount hold promise for refining phase relationships between geochemical and isotopic proxies throughout OAE1a, and, in turn, constraining the mechanics of addition of light carbon to the ocean-atmosphere system.