Influence Of Obliquity Research Articles

The Lutetian global stratotype section and point at Gorrondatxe revisited: Biomagnetostratigraphic refinements and astronomical tuning

Biomagnetostratigraphic studies carried out at the Ypresian/Lutetian (Eocene) Gorrondatxe section solved major chronostratigraphic calibration discrepancies that had existed for several decades. The new calibration resulted in the definition of the Global Stratotype Section and Point (GSSP) for the Lutetian Stage at the Gorrondatxe layer containing the lowest occurrence (LO) of the calcareous nannofossil Blackites inflatus (base of Subzone CP12b). However, subsequent studies cast some doubt on the reliability of the Gorrondatxe biomagnetostratigraphic data and, consequently, the appropriateness of the criterion used for the definition of the Lutetian GSSP. In order to address the issues raised, the biomagnetostratigraphy of the Gorrondatxe section was revisited and an additional cyclostratigraphic analysis was undertaken using magnetic susceptibility data series. The present study shows that the Gorrondatxe biomagnetostratigraphy is reliable. The spectral analyses showed the dominance of precession (∼20 ky) and short (∼100 ky) eccentricity forcing on sedimentation, but the influence of obliquity (∼38 and ∼ 55 ky) cycles was also detected and modulation by long (405 ky) eccentricity cycles was deduced. The succession was tuned to the 49.122–48.084 Ma interval and the astronomical ages obtained for a significant number of biomagnetostratigraphic events compared well with those derived from other studies. One of the exceptions is the LO of B. inflatus, which seems to be older in Gorrondatxe than in the Mediterranean and Western Atlantic areas. This diachrony most likely also delayed the LO of B. inflatus in historical reference sections of the North Sea area, rendering previous claims that the Lutetian GSSP caused some historical Lutetian sections to be Ypresian in age invalid. In fact, the definition of the base of the Lutetian Stage by the LO of B. inflatus at Gorrondatxe, now astronomically age dated at 48.455 Ma, best guarantees that the original concepts of the Ypresian and Lutetian historical stratotypes are maintained. Combined with secondary biomagnetostratigraphic marker events and an astronomically tuned cyclostratigraphy, the Ypresian/Lutetian Gorrondatxe section provides the globally significant chronostratigraphic reference model expected of any GSSP.

Lunar cratering asymmetries with high lunar orbital obliquity and inclination of the Moon

Accurate estimation of cratering asymmetry on the Moon is crucial for understanding Moon evolution history. Early studies of cratering asymmetry have omitted the contributions of high lunar obliquity and inclination. Here, we include lunar obliquity and inclination as new controlling variables to derive the cratering rate spatial variation as a function of longitude and latitude. With examining the influence of lunar obliquity and inclination on the asteroids population encountered by the Moon, we then have derived general formulas of the cratering rate spatial variation based on the crater scaling law. Our formulas with addition of lunar obliquity and inclination can reproduce the lunar cratering rate asymmetry at the current Earth-Moon distance and predict the apex/ant-apex ratio and the pole/equator ratio of this lunar cratering rate to be 1.36 and 0.87, respectively. The apex/ant-apex ratio is decreasing as the obliquity and inclination increasing. Combining with the evolution of lunar obliquity and inclination, our model shows that the apex/ant-apex ratio does not monotonically decrease with Earth-Moon distance and hence the influences of obliquity and inclination are not negligible on evolution of apex/ant-apex ratio. This model is generalizable to other planets and moons, especially for different spin-orbit resonances.

The influence of obliquity in the early Holocene Asian summer monsoon

AbstractThe early Holocene climatic optimum is associated with perihelion precession and high obliquity, though most studies emphasize the former over the latter. Asian monsoon proxy records only decisively show the precessional impact. To explore the obliquity effect, four climate simulations are conducted by fixing orbital parameters of present‐day (0K), early Holocene (11K), the lowest obliquity (31K), and 11K's precession and eccentricity with 31K's obliquity (11Kp31Ko). We show that high obliquity significantly augments the precessional impact by shifting the Asian monsoon farther north than present. By contrast, the present‐day monsoon seasonality can still be identified in the simulations with low obliquity. We argue that the upper tropospheric (South Asian) and lower tropospheric (North Pacific) high‐pressure systems are affected by the subtropical atmospheric heating changes responding to obliquity. As a consequence, associated with the changes in meridional gradients of geopotential height and temperature made by the heating, midlatitude transient eddies and monsoon‐midlatitude interactions are modulated.

Obliquity forcing of low-latitude climate

Abstract. The influence of obliquity, the tilt of the Earth's rotational axis, on incoming solar radiation at low latitudes is small, yet many tropical and subtropical palaeoclimate records reveal a clear obliquity signal. Several mechanisms have been proposed to explain this signal, such as the remote influence of high-latitude glacials, the remote effect of insolation changes at mid- to high latitudes independent of glacial cyclicity, shifts in the latitudinal extent of the tropics, and changes in latitudinal insolation gradients. Using a sophisticated coupled ocean–atmosphere global climate model, EC-Earth, without dynamical ice sheets, we performed two idealized experiments of obliquity extremes. Our results show that obliquity-induced changes in tropical climate can occur without high-latitude ice sheet fluctuations. Furthermore, the tropical circulation changes are consistent with obliquity-induced changes in the cross-equatorial insolation gradient, suggesting that this gradient may be used to explain obliquity signals in low-latitude palaeoclimate records instead of the classical 65° N summer insolation curve.

HABITABLE CLIMATES: THE INFLUENCE OF ECCENTRICITY

In the outer regions of the habitable zone, the risk of transitioning into a globally frozen "snowball" state poses a threat to the habitability of planets with the capacity to host water-based life. We use a one-dimensional energy balance climate model (EBM) to examine how obliquity, spin rate, orbital eccentricity, and ocean coverage might influence the onset of such a snowball state. For an exoplanet, these parameters may be strikingly different from the values observed for Earth. Since, for constant semimajor axis, the annual mean stellar irradiation scales with (1-e^2)^(-1/2), one might expect the greatest habitable semimajor axis (for fixed atmospheric composition) to scale as (1-e^2)^(-1/4). We find that this standard ansatz provides a reasonable lower bound on the outer boundary of the habitable zone, but the influence of obliquity and ocean fraction can be profound in the context of planets on eccentric orbits. For planets with eccentricity 0.5, our EBM suggests that the greatest habitable semimajor axis can vary by more than 0.8 AU (78%!) depending on obliquity, with higher obliquity worlds generally more stable against snowball transitions. One might also expect that the long winter at an eccentric planet's apoastron would render it more susceptible to global freezing. Our models suggest that this is not a significant risk for Earth-like planets around Sun-like stars since such planets are buffered by the thermal inertia provided by oceans covering at least 10% of their surface. Since planets on eccentric orbits spend much of their year particularly far from the star, such worlds might turn out to be especially good targets for direct observations with missions such as TPF-Darwin. Nevertheless, the extreme temperature variations achieved on highly eccentric exo-Earths raise questions about the adaptability of life to marginally or transiently habitable conditions.

The influence of obliquity on europan cycloid formation

Tectonic patterns on Europa are influenced by tidal stress. An important well-recognized component is associated with the orbital eccentricity, which produces a diurnally varying stress as Jupiter's apparent position in Europa's sky oscillates in longitude. Cycloidal lineaments seem to have formed as cracks propagated in this diurnally varying stress field. Maps of theoretical cycloid patterns capture many of the characteristics of the observed distribution on Europa. However, a few details of the observed cycloid distribution have not been reproduced by previous models. Recently, it has been shown that Europa has a finite forced obliquity, so Jupiter's apparent position in Europa's sky will also oscillate in latitude. We explore this new type of diurnal effect on cycloid formation. We find that stress from obliquity may be the key to explaining several characteristics of observed cycloids such as the shape of equator-crossing cycloids and the shift in the crack patterns in the Argadnel Regio region. All of these improvements of the fit between observation and theory seem to require Jupiter crossing Europa's equatorial plane 45° to 180° after perijove passage, suggestive of complex orbital dynamics that locks the direction of Europa's pericenter with the direction of the ascending node at the time these cycloids were formed.

Orbital signatures in a late Miocene dinoflagellate record from Crete (Greece)

A high-resolution palynological study of the cyclically bedded Faneromeni section (upper Tortonian-lower Messinian) on Crete (Greece) is presented. This study aims to recognize orbitally-driven variations in the palynological record and to validate the age model based on the astronomical calibration of the sedimentary cycles. Four palynology-based environmental proxies were utilised using interpretations of fossil dinoflagellate associations based on modern ecological characteristics. Cross-spectral analysis between the proxy records and astronomical target curve, the 65°N summer insolation, yielded in most cases significant spectral power and coherence in the precession and/or obliquity frequency bands. Precession-controlled variations in the proxy records are related to lithology and indicate that maxima in continental input and minima in sea surface salinity coincide with sapropel formation. The influence of obliquity is most clearly reflected in the index of continental versus marine palynomorphs (S-D). The absence of a distinct time lag relative to obliquity indicates that the 41-kyr component in continental input is controlled by oscillations in regional Mediterranean climate rather than by glacial cyclicity. Phase relations in the different astronomical frequency bands of the spectrum, as compared with the Mediterranean Pliocene, essentially confirm the validity of the Miocene astronomical time scale. Finally, a major non-cyclic change in the palynological assemblage at 6.68 Ma indicates enhanced salinity and decreased river discharge. This shift coincides with a significant drop in sedimentation rate informally termed the “Early Messinian Sediment starvation Event”.

Convolution calculations of dose in the buildup regions for high energy photon beams obliquely incident

The central dose received from high energy photon beams that are obliquely incident on an absorber is markedly different from that absorbed when such beams are normally incident at the surface. In this work, we calculate the dose differences using a convolution of energy deposition kernels in a phantom presenting an oblique entrance surface. The dose distributions were calculated with oblique incident 1.25, 3, and 6 MeV monoenergetic and polyenergetic photon beams angled from 0 degrees to 80 degrees. In order to study the dose variations with surface obliquity, we introduced an obliquity factor. For each energy, the obliquity factor was calculated as a function of depth and field size. We found that, ignoring the electron contamination from the air and from the treatment machine head, the influence of obliquity can be described in terms of upstream and downstream contribution of the electrons set in motion by the primary photons. In actual beams, especially for large field sizes, the electron contamination becomes significant and tends to reduce the influence of surface obliquity. Results indicate that the obliquity factor is highly dependent on the beam energy and depth, and are in good agreement with our experimental results measured for 10 and 25 MV x-ray beams. In this paper a theoretical explanation of these dose variations due to oblique incidence of the beam is presented.