Earth's Precession Research Articles

Asynchronous variations of mercury accumulation since the last deglaciation in the eastern and western Nanling mountains in South China

There are differences in the response of long-term mercury (Hg) deposition to climate change, so it is necessary to clarify the influencing factors of Hg deposition in different locations. The Dahu and Daping Swamps are located in the eastern and western parts of the Nanling Mountains in South China, respectively. They are asynchronous variations of Hg deposition reflected by the two swamps since the last deglaciation. Based on the comprehensive comparative analysis of physical, chemical, and biological proxies, it is indicated that Hg deposition changes in the Dahu and Daping mainly record the changes in Hg content in the catchment input from organic matter. The differences in organic matter sources, core surrounding environment, Hg sources and atmospheric sedimentation contribution are reasons associated with the catchment environment for the observed differences in swamp Hg deposition and the response to climate. In cold/dry periods, Hg adsorption by terrestrial organic matter in Dahu Swamp were more significant, while in warm/humid periods, Hg adsorption by endogenous organic matter in Daping Swamp and wet deposition of nearby Hg sources, were the main climate-related processes for their asynchronous response. In addition, Dahu and Daping Swamps show similarities and differences in their Hg deposition variations. In both swamps, multi-millennial (∼1000−2200yr) and multi-centennial cycles (∼300–500yr) were observed and may be related to the harmonic period of the Earth's precession cycle and the solar activity cycle. This study helps to understand the differences in response between sedimentary Hg and climate, and to understand the regional and global proxies used to record climate and environmental changes.

El Niño-Southern Oscillation and East Asian Monsoon controlled Kuroshio Current evolution over the last 42 kyr

Kuroshio Current (KC) plays a significant role in global climate change by transporting high-temperature and high-salinity water from low-latitude to mid-high-latitude waters. However, the KC evolution and its main controlling factors remained unclear on the orbital timescales. Here, we research the comprehensive record of high-resolution terrigenous mass accumulation rate (MAR), grain size, SrNd isotopic compositions of clay-size fractions and clay mineral assemblages in core SCS0003 sediments from the northern slope of the South China Sea (SCS) since 42 kyr. The SrNd isotope compositions (<2 μm) and clay minerals illustrate that the smectite mainly came from Luzon Island, with a small content of smectite during the last glacial period, but an increase by about 3–4 times during the Holocene. Furthermore, the smectite MAR flux indicates that the intensity of KC intrusion of the SCS was consistent with the intensity of the open Pacific over the last 42 kyr, which was different from the opposite trend proposed by predecessors. Then it further reveals that the KC was relatively weak and stable in the last glacial period as a whole, but overall became stronger with a larger amplitude in the Holocene, with an increase in the early Holocene and a decline in the middle and late Holocene. Moreover, since 42 kyr El Niño-Southern Oscillation (ENSO) and East Asian monsoon (EAM) controlled by the insolation change in the Earth's precession were the major driving mechanisms of KC evolution, instead of ENSO as the only main controlling factor. On the orbital timescales, our results emphasize that low-latitude processes by the variation of insolation on the precession are very important in driving global climate change, not just high-latitude processes.

MJO in Different Orbital Regimes: Role of the Mean State in the MJO’s Amplitude during Boreal Winter

Abstract The Madden–Julian oscillation (MJO) exhibits pronounced seasonality, with one of the key unanswered questions being the following: what controls the maximum in MJO precipitation variance in the Southern Hemisphere during boreal winter? In this study, we examine a set of global climate model simulations in which the eccentricity and precession of Earth’s orbit are altered to change the boreal winter mean state in an attempt to reveal the processes that are responsible for the MJO’s amplitude in the boreal winter. In response to the forced insolation changes, the north–south asymmetry in sea surface temperature is amplified in boreal fall, which intensifies the Hadley circulation in boreal winter. The stronger Hadley circulation yields higher mean precipitation and stronger mean lower-tropospheric westerlies in the southern part of the Indo-Pacific warm pool. The MJO precipitation variability increases significantly where the mean precipitation and lower-tropospheric westerlies strengthen. In the column-integrated moisture budget of the simulated MJO, only surface latent heat flux feedback shows a trend that is consistent with the MJO’s amplitude, suggesting an important role for the surface latent heat flux feedback in the MJO’s amplitude during the boreal winter. An analysis of the moisture–precipitation relationship in the simulations shows that the increase in the mean precipitation lowers the convective moisture adjustment time scale, leading to the increase in precipitation variance. Our results suggest that the mean-state precipitation plays a critical role in the maintenance mechanism of the MJO.

Constraints on Earth-Moon dynamical parameters from Eocene cyclostratigraphy

The theoretical prediction of Earth-Moon dynamics in deep geologic time is hampered by limited knowledge of Earth's past tidal dissipation and dynamical ellipticity. Cyclostratigraphy can, however, provide valuable information on the geologic evolution of the Earth-Moon dynamical parameters. Here we investigate Eocene cyclostratigraphy to estimate Earth's precession rate (p), Earth-Moon distance (EMD) and length-of-day (LOD). Two highly-resolved deep sea records are selected from the Atlantic Ocean, with ages centered on 42.5 Ma (40.9–44.3 Ma; IODP Site U1410) and 54.95 Ma (54.1–55.8 Ma; ODP Site 1262). The sedimentary sequences at these sites recorded Milankovitch forcing signals with prominent precession index (Site 1262) and obliquity (Site U1410) frequencies and their amplitude modulations. At Site U1410, amplitude modulation of the main obliquity cycle was tuned to the Earth's 173-kyr orbital inclination (s3-s6) metronome, and at Site 1262, the dominant precession index amplitude modulation to the 405-kyr orbital eccentricity (g2-g5) metronome. This enabled estimation of Earth's precession rate p of 51.486 ± 0.064 arcsec/yr and 51.546 ± 0.168 arcsec/yr for 42.5 Ma and 54.95 Ma, respectively. These p estimates indicate EMD values of 383.710 ± 0.090 × 103 km and 382.625 ± 0.265 × 103 km, and LOD values of 23.679 ± 0.0115 h and 23.667 ± 0.039 h for 42.5 Ma and 54.95 Ma, respectively. These estimates deviate from theoretical predictions based on back calculation assuming present-day tidal dissipation. In particular, a significant discrepancy between the model and observations (cyclostratigraphy) occurs at 54.95 Ma (Site 1262) suggesting contributions from other effects on Earth's rotation, including changes in tidal dissipation and/or Earth's dynamical ellipticity related to mantle convection and the early Eocene global warming.

Fluvial Sedimentary Response to Late Quaternary Climate and Tectonics at the Himalayan Frontal Thrust, Central Nepal

AbstractTo investigate the subsurface structure surrounding the Main Frontal Thrust (MFT) in central Nepal, we drilled and cored sediments to depths of 45–100 m at 10 sites. Our boreholes were located along previously acquired high‐resolution seismic profiles across the MFT imaging the upper 1–2 km of the subsurface, which revealed a beveled erosional surface in the hanging wall above a broad, gentle anticline, as well as growth strata in the footwall. The boreholes exhibit interlayered clays, silts, sands, and gravels, dated with optically stimulated luminescence and radiocarbon to &lt;72.5 ± 4.3 ka, with a transition from finer to coarser sediments at ∼13.5 ± 0.1 ka. Near the fault tip, the deposits exhibit steeper dips and deformation bands. A 25‐m‐thick section of silt and clay above the south end of the buried anticline is interpreted as a temporary lacustrine depocenter formed due to uplift near the fault tip. Based on the distribution of marker beds and sediment ages, we interpret a shortening rate of 3.1–12.1 m/ka on the MFT. Three major transitions between fluvial‐lacustrine and coarse fluvial channel facies are inferred from the boreholes, and the timings of these transitions correlate with Indian monsoonal intensity variations linked to Earth's precession. We infer that a strengthened monsoon led to increased river discharge and advance of coarse bedload‐dominant braided channels, whereas a weak monsoon formed a finer‐grained channel environment. These monsoonal climate variations have affected the depositional environment and river base levels in this region, influencing the formation and apparent relative uplift of nearby river terraces.

Empirical Reconstruction of Earth‐Moon and Solar System Dynamical Parameters for the Past 2.5 Billion Years From Cyclostratigraphy

AbstractThe chaotic behavior of the Solar System prevents the prediction of the precise evolution of the planetary secular frequencies. Here we use four cyclostratigraphic sequences, ∼259 Ma Wujiaping Formation, ∼455 Ma Pingliang Formation, ∼655 Ma Datangpo Formation and ∼2,465 Ma Dales Gorge Member, and Bayesian inversion to provide powerful geological constraints to this problem. The inversion provides constraining estimates for the Solar System fundamental secular frequencies g1, g2, g3, g4, and g5, and the Earth's precession rate k from 105.26 ± 2.68 (2σ) arcseconds/year at ∼2,465 Ma to 55.86 ± 2.60 (2σ) arcseconds/year at ∼259 Ma. Stochastic monotone interpolation enables the reconstruction of Earth‐Moon distance from 2.5 billion years to the present. Calculation of tidal dissipation supports the ocean model before 1.4 Ga, but is outside the error range after ∼1.8 Ga, suggesting lower tidal dissipation in the early Proterozoic Eon, and a possible high value in the early Paleozoic.

Orbital-scale vegetation-ocean-atmosphere linkages in western Japan during the last 550 ka based on a pollen record from the IODP site U1427 in the Japan Sea

Climate and vegetation in Japan are strongly influenced by the East Asian Monsoon (EAM) and the ocean environment of the Japan Sea. This study discusses the findings from a new pollen record from Integrated Ocean Drilling Program Site U1427, situated off the northern coast of the Honshu Island in western Japan, which allows for direct comparisons with other oceanographic proxies from this area. Our findings demonstrate the orbital-scale vegetation changes in this area and their land-ocean-atmosphere linkages during the last 550 ka. During surface-water freshening events in the Japan Sea, likely associated with colder water temperatures caused by the very low eustatic sea levels and the consequent isolation from the surrounding seas during the full-glacial periods, cold-cool conifer forests became dominant under the extremely dry pseudo-continental conditions in the Japanese islands. In contrast, temperate broadleaf forests increased rapidly when the surface-freshening events ended and a weak inflow of warm water began at the onset of deglaciation, in addition to the rapid atmospheric climate warming during termination periods. During the intermediate periods between the full interglacial to the full-glacial, the Tp ratio (pollen temperature index) increased in association with the summer insolation maxima caused by the Earth's precession cycle that coincide with Chinese stalagmite negative δ18O peaks, whereas Cryptomeria pollen abundance (high-precipitation index) increased in association with the winter insolation maxima. Those orbital-scale vegetation fluctuations were attributed to the changes in seasonal temperature and precipitation patterns, where were influenced by the EAM changes that were associated with seasonal insolation changes around the Japanese archipelago. Cryptomeria forests likely developed under cool and wet conditions throughout a year with low seasonal contrasts, whereas deciduous broadleaf forests increased under condition of high seasonality with cool summers and colder, drier winters. Alternating fluctuations between the Tp ratio and Cryptomeria pollen abundance following the insolation changes were typically observed after MIS 9 and are thought to have been affected by the periodicity shift in the EAM fluctuations across the Mid-Brunhes Event.

The Viscous and Ohmic Damping of the Earth's Free Core Nutation.

The cause for the damping of the Earth's free core nutation (FCN) and the free inner core nutation eigenmodes has been a matter of debate since the earliest reliable estimations from nutation observations were made available. Numerical studies are difficult given the extreme values of some of the parameters associated with the Earth's fluid outer core, where important energy dissipation mechanisms can take place. We present a fully 3D numerical model for the FCN capable of describing accurately viscous and Ohmic dissipation processes taking place in the bulk of the fluid core as well as in the boundary layers. We find an asymptotic regime, appropriate for Earth's parameters, where viscous and Ohmic processes contribute to the total damping, with the dissipation taking place almost exclusively in the boundary layers. By matching the observed nutational damping, we infer an enhanced effective viscosity matching and validating methods from previous studies. We suggest that turbulence caused by the Earth's precession can be a source for the enhanced viscosity.

Reply detection of a 27.5-My cycle in extinctions of non-marine tetrapods in light of a similar cycle in marine extinctions and coordinated geologic events

ABSTRACT Our circular-spectral and Fourier analyses of the ages of the 10 recognized non-marine tetrapod extinction events over the last 300 My revealed a significant spectral peak at 27.5 My. Omerbashich, using his Gauss-Vaniçek method of spectral analysis, fails to find a significant 27.5 My cycle in the same data. He claims to find predominant short (< 1 My) Earth precession cycles in the data set, where the smallest interval between extinction events is 8 My. In response to Omerbashich, we performed a new analysis of non-marine extinctions using a best-fit method that again displays the high spectral peak at ~27.5 My. A similar cycle has been widely reported in the related record of the ages of extinction events of marine species, although earlier Omerbashich (2006), using the same Gauss-Vaniçek method, was not able to detect that cycle. Furthermore, using a new compilation of the ages of 89 major geologic events over the last 260 My (including the extinction events), we also find evidence for the 27.5 My cycle pervading the geologic record. We conclude, therefore, that the evidence suggests that the 27.5 My cycle that we detected is physically meaningful. In our paper (Rampino et al., 2020) we reported evidence, using circular-spectral and Fourier-analysis methods, for a significant underlying 27.5 My periodicity in the ten episodes of extinctions of non-marine tetrapod families over the last 300 My. We compiled the extinction data from an exhaustive search of the relevant geological literature on non-marine tetrapod occurrences, and we believe that we have detected the major non-marine extinction events. Four of the ten non-marine events lacked published estimated magnitude data, but we suggested, based on the magnitudes of the other distinct extinction events, that they most likely represented ≥ 10% extinction of tetrapod families (Rampino et al., 2020). It is important to note, however, that we did not make use of the percent-extinction estimates in any of our statistical analyses; we treated the ages as Dirac delta functions (spikes), so that, counter to the methods of Omerbashich (2021), the relative amplitudes are irrelevant in our study.

Palaeosalinity and palaeoclimatic geochemical proxies (elements Ti, Mg, Al) vary with Milankovitch cyclicity (1.3 to 2.0 Ma), OGCP cores, Palaeolake Olduvai, Tanzania

The analysis of geochemical palaeoclimate and palaeosalinity proxy elements Ti, Mg, and Al, derived from X-ray fluorescence (XRF) scans of Olduvai Beds I and II from Olduvai Gorge Coring Project (OGCP) borehole Cores 2A and 3A, provides a record of cyclic variation between ~1.3 Ma and ~2.0 Ma. The boreholes were drilled into the depocentre of the Olduvai Basin between Fifth and FLK Faults, where Palaeolake Olduvai was most persistent and deepest. During most of Bed I the lake was particularly deep and probably meromictic, preserving high TOC contents and commonly preserving fine lamination due to lack of bioturbation. Accretion rates were also high during Bed I, when rates of basinal subsidence were maximal due to crustal stretching, associated with basaltic volcanism, towards the end of bimodal Ngorongoro volcanism. Basaltic magma effusive activity is manifested as tuffs, scoriaceous layers and the Bed I Basalt complex lava flows. A magnesium anomaly is recorded in the claystone geochemistry at this time and deposition of dolomite and limestone beds are restricted to this syn-volcanic phase of basin history. During Bed I deposition, accretion rates (0.23 mm/yr) were high enough to permit recognition of cycles with an average periodicity of 22.3 kyr corresponding to the Earth's precession. Only the high values during the Mg anomaly are adequate for the application of the palaeosalinity proxy element ratio Mg/Al. But Ti counts provide a cyclic record in both Bed I and Bed II. During Bed II deposition, the accretion rate was much slower (0.058 mm/yr) and cyclicity averaged 40.4 kyr, corresponding to Earth's orbital obliquity. The Bed II interval corresponds to MIS Stages 40 to 64. Coincidence of precessional and obliquity minima at 1.8 Ma explains the superdrought that affected the basin at the time of emplacement of Tuff IF, when the lake was dried out. The aridity of the sequence containing Tuff IA is also associated with a precessional minimum. The cyclic record suggests that three Bed I Basalt flows were extruded at ~1.94 Ma during a time span lasting between 6 kyr and 15 kyr.

The response of tropical precipitation to Earth's precession: the role of energy fluxes and vertical stability

Abstract. The changes in Earth's precession have an impact on the tropical precipitation. This has been attributed to the changes in seasonal solar radiation at the top of the atmosphere. The primary mechanism that has been proposed is the change in thermal gradient between the two hemispheres. This may be adequate to understand the zonal mean changes, but cannot explain the variations between land and oceans. We have used a simple model of the intertropical convergence zone (ITCZ) to unravel how precipitation changes with precession. Our model attributes the changes in precipitation to the changes in energy fluxes and vertical stability. We include the horizontal advection terms in this model, which were neglected in the earlier studies. The final response of the land and oceans is a result of complex feedbacks triggered by the initial changes in the insolation. We find that the changes in precipitation over the land are mainly driven by changes in insolation, but over the oceans, precipitation changes on account of changes in surface fluxes and vertical stability. Hence insolation can be a trigger for changes in precipitation on orbital timescales, but surface energy and vertical stability play an important role too. The African monsoon intensifies during a precession minimum (higher summer insolation). This intensification is mainly due to the changes in vertical stability. The precipitation over the Bay of Bengal decreases for minimum precession. This is on account of a remote response to the enhanced convective heating to the west of the Bay of Bengal. This weakens the surface winds and thus leads to a decrease in the surface latent heat fluxes and hence the precipitation.

Testing Late Cretaceous astronomical solutions in a 15 million year astrochronologic record from North America

Astronomically-forced insolation cycles (“Milankovitch cycles”) serve as a primary control on climate, and when preserved in the stratigraphic record, they provide a high-resolution in situ chronometer. These celestial rhythms derive from the fundamental frequencies of the Solar System, and Earth's precession constant. The fundamental frequencies of Earth and Mars presently define a secular resonance of 2:1 (2.4 Myr to 1.2 Myr), and are hypothesized to have undergone a transition between 50–100 Ma, due to the chaotic behavior of the Solar System. In this study, a 15-Myr long cyclostratigraphic record from the mid-latitude Western Interior Seaway (WIS; spanning 82–97 Ma) is assembled to quantify eccentricity amplitude modulation (AM) and obliquity AM, which could reveal the 2:1 resonance when present. Ten high-precision radioisotopic dates from the study interval permit a rigorous anchoring of the floating astrochronologies for comparison with theoretical astronomical models. The analysis is complimented by the construction of an astronomically tuned δ13Corg composite for the early Cenomanian-early Campanian, which allows a new assessment of linkages between million-year scale astronomical forcing and perturbations of the Late Cretaceous carbon cycle. Evaluation of the La2004 model reveals a switch of eccentricity modulation from 2.4-Myr cycles to a 1.2-Myr cycle from 90 to 86.5 Ma, followed by a return to a 2.4 Myr cycle in younger strata, compatible with observations from the WIS composite. These two resonance transitions are absent in the La2010d and La2011 models. Among 11 solutions calculated by Zeebe (2017), some show resonance transitions during this period, but none of them match the geological record with respect to the observed long eccentricity modulation. The results of this study also suggest that positive carbon isotope excursions are closely related to obliquity power minima, but this interpretation will require further analysis for full confirmation.

Simulating the Occurrence of the Last Sapropel Event (S1): Mediterranean Basin Ocean Dynamics Simulations Using Nd Isotopic Composition Modeling

AbstractSapropels are sediments rich in black, pelagic organic matter which occur mainly in the Eastern Mediterranean, documenting anoxic environments and high biological productivity. The quasiperiodicity of deposition of sapropels ‐over millions of years‐ relates to the Earth's precession cycle, which directly enhances the African monsoon, ultimately increasing freshwater input from the Nile. The last sapropel event, S1, occurred about 10,000 years ago (Early Holocene), when the Mediterranean region was warmer and wetter than today. Several modeling studies reflect the impact of this climate and a stronger Nile influx on Mediterranean oceanic circulation, but the regional models used lacked the spatial resolution necessary to simulate winter intermediate and deep convection. Here, we investigate recently occurring changes in the convective areas in the Eastern Mediterranean, using a regional ocean–atmosphere coupled climate model of high spatial resolution, essential to the simulation of a realistic Mediterranean circulation for present‐day conditions. We focused on the thermohaline circulation and the simulation of neodymium isotopic composition to compare our modeling results to modern data and paleo‐proxies. A sensitivity experiment shows a radical response of the Mediterranean to enhanced Nile discharge, creating the appropriate conditions for sapropel formation. We thus demonstrate that increased discharge of the Nile River can trigger the shutdown of Eastern Mediterranean convection and create conditions favorable to the development of anoxic events.

Proving Heliocentrism and Measuring the Astronomical Unit in a Laboratory Astronomy Class Via the Aberration of Starlight

The objective reality of the Earth's motion about the Sun was finally proven observationally by Bradley (1727) when he correctly explained the ~20'' annual, elliptical motions of stars as being due to aberration of starlight caused by the motion of the Earth. This effect can be detected today with ordinary astrophotographic equipment by measuring the coordinates of the center of star trails, which reveal the apparent position of the Celestial Poles, which vary due to the aberration of starlight, in addition to the effects of the nutation and precession of Earth's Poles. Despite my aspirations, I have not found the time to even begin the project of measuring the aberration. I nonetheless feel that it is a rich project in observational astronomy and fundamental physics, and a pleasingly didactic exercise appropriate for upper-division undergraduate and graduate astronomy and physics laboratory classes, whose complexity and length may be justified by the novelty of proving heliocentrism with nothing but a small telescope and camera. I therefore publish this Research Note to disseminate the idea in hopes that I might receive word in a few years of a triumphant astronomy class's proof of heliocentrism, along with their best estimates of the astronomical unit and precession.

Phase Changes and Seasonal Warming in Early Instrumental Temperature Records

Phase analyses of the annual cycle of monthly temperature time series that date back to the eighteenth century show trending behavior that has been difficult to interpret. Negative trends in the estimated phase have been identified with precession of Earth’s axis of rotation, but the implied later onset of seasons is at odds with recent satellite measurements and with the phenological record. Positive trends in the phase and the occurrence of trends of both signs in temperature time series from geographically nearby locations have remained mysterious. This paper shows that there is a mathematical equivalence between trends in phases and seasonally differing warming trends, in particular more intense warming in winters than in summers. Using temperature time series from 16 Northern Hemispheric locations reaching back to the eighteenth century and a statistical model that can estimate the seasonal warming trends, the authors reject the hypothesis that the timing of the seasons in these locations is jointly driven by precession.

Seasonal Changes in Central England Temperatures

Summary The aim of the paper is to assess how climate change is reflected in the variation of the seasonal patterns of the monthly central England temperature time series between 1772 and 2013. In particular, we model changes in the amplitude and phase of the seasonal cycle. Starting from the seminal work of Thomson, various studies have documented a shift in the phase of the annual cycle, implying an earlier onset of the spring season at various European locations. A significant reduction in the amplitude of the seasonal cycle is also documented. The literature so far has concentrated on the measurement of this phenomenon by various methods, among which complex demodulation and wavelet decompositions are prominent. We offer new insight by considering a model that allows for seasonally varying deterministic and stochastic trends, as well as seasonally varying auto-correlation and residual variances. The model can be summarized as containing a permanent and a transitory component, where global warming is captured in the permanent component, on which the seasons load differentially. The phase of the seasonal cycle, in contrast, seems to be following the trend that was identified by Thomson with Earth's precession in a stable manner. We identify the reported fluctuations as transitory.

Earth science: An extended yardstick for climate variability.

Decoded and precisely dated information encrypted in stalagmites from a cave in China reveal past climatic changes and provide insight into the complex interactions in today's climate system. See Letter p.640 Prior records of the Asian monsoon have revealed cyclic variations over hundreds of thousands of years, probably driven by variations in insolation caused by the precession of Earth's orbit. Hai Cheng and colleagues now provide a speleothem record from Chinese cave samples that extends earlier records to 640,000 years ago, close to the maximum age possible with uranium/thorium dating. This spectacular record confirms that the characteristic '100,000-year' ice age cycle corresponds to an integral number (four or five) of precession cycles, and that insolation influences millennial-scale variations in monsoon strength.

The Asian monsoon over the past 640,000 years and ice age terminations.

Oxygen isotope records from Chinese caves characterize changes in both the Asian monsoon and global climate. Here, using our new speleothem data, we extend the Chinese record to cover the full uranium/thorium dating range, that is, the past 640,000 years. The record's length and temporal precision allow us to test the idea that insolation changes caused by the Earth's precession drove the terminations of each of the last seven ice ages as well as the millennia-long intervals of reduced monsoon rainfall associated with each of the terminations. On the basis of our record's timing, the terminations are separated by four or five precession cycles, supporting the idea that the '100,000-year' ice age cycle is an average of discrete numbers of precession cycles. Furthermore, the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands, and is nearly in anti-phase with summer boreal insolation. These observations indicate that insolation, in part, sets the pace of the occurrence of millennial-scale events, including those associated with terminations and 'unfinished terminations'.

Tropically-driven climate shifts in southwestern Europe during MIS 19, a low eccentricity interglacial

The relative roles of high- versus low-latitude forcing of millennial-scale climate variability are still not well understood. Here we present terrestrial–marine climate profiles from the southwestern Iberian margin, a region particularly affected by precession, that show millennial climate oscillations related to a nonlinear response to the Earth's precession cycle during Marine Isotope Stage (MIS) 19. MIS 19 has been considered the best analogue to our present interglacial from an astronomical point of view due to the reduced eccentricity centred at 785 ka. In our records, seven millennial-scale forest contractions punctuated MIS 19 superimposed to two orbitally-driven Mediterranean forest expansions. In contrast to our present interglacial, we evidence for the first time low latitude-driven 5000-yr cycles of drying and cooling in the western Mediterranean region, along with warmth in the subtropical gyre related to the fourth harmonic of precession. These cycles indicate repeated intensification of North Atlantic meridional moisture transport that along with decrease in boreal summer insolation triggered ice growth and may have contributed to the glacial inception, at ∼774 ka. The freshwater fluxes during MIS 19ab amplified the cooling events in the North Atlantic promoting further cooling and leading to MIS 18 glaciation. The discrepancy between the dominant cyclicity observed during MIS 1, 2500-yr, and that of MIS 19, 5000-yr, challenges the similar duration of the Holocene and MIS 19c interglacials under natural boundary conditions.

Coupled measurements of δ18O and δD of hydration water and salinity of fluid inclusions in gypsum from the Messinian Yesares Member, Sorbas Basin (SE Spain)

We studied one cycle (Cycle 6) of gypsum–marl deposition from the Messinian Yesares Member in Sorbas Basin, Spain. The objective was to reconstruct the changing environment of deposition and its relation to astronomically-forced climate change. The δ18O and δD of gypsum hydration water (CaSO4 • 2H2O) and salinity of fluid inclusions were measured in the same samples to test if they record the composition of the mother fluid from which gypsum was precipitated. Water isotopes are highly correlated with fluid inclusion salinity suggesting the hydration water has not exchanged after formation. The relatively low water isotope values and fluid inclusion salinities indicate a significant influence of meteoric water, whereas δ34S, δ18OSO4 and 87Sr/86Sr support a dominant marine origin for the gypsum deposits. The discrepancy between water and elemental isotope signatures can be reconciled if meteoric water dissolved previously deposited marine sulfates supplying calcium and sulfate ions to the basin which maintained gypsum saturation. This recycling process accounts for the marine δ34S, δ18OSO4 and 87Sr/86Sr signatures, whereas the low δ18O and δD values of gypsum hydration water and fluid inclusion salinities reflect the influence of freshwater.The cyclic deposition of gypsum and marl in the Yesares Member has previously been interpreted to reflect changing climate related to Earth's precession cycle. We demonstrate that the δ18O, δD and salinity of the parent brine increased from low values at the base of the cycle to a maximum in the massive gypsum palisade, and decreased again to lower values in the supercones at the top of the cycle. This pattern, together with changes in mineralogy (calcite–dolomite–gypsum), is consistent with a precession-driven change in climate with wettest conditions (summer insolation maxima) associated with the base of the calcium carbonate marls and driest conditions (summer insolation minima) during formation of the gypsum palisade.