We report here on a new paleomagnetic (directions and intensities) and coupled K/Ar and 40Ar/39Ar analysis of 35 different flows, emplaced in the Chaîne des Puys during the 75 to 10 kyr interval, ...which contains the Mono Lake and Laschamp excursions. There is a remarkable agreement between the new set of absolute volcanic intensities and published sedimentary (GLOPIS-75) and cosmogenic (10Be and 36Cl) records. The Laschamp and Mono Lake excursions are clearly revealed by a very significant intensity drop at 41.2±1.6 ka and 34.2±1.2 ka respectively. The duration of the Laschamp excursion is ∼1500 yr and about 640 yr when the drop of paleointensity or the directional change are considered respectively. The intensity drop at the Mono Lake is twice as short. In the ∼7 ka interval separating the two excursions, the field intensity recovers to almost non-transitional values. The rate of decrease of the field intensity during these excursions attains 18 nT/yr for the Laschamp and even greater value (33 nT/yr) for the Mono Lake. This figure is, for the Laschamp excursion, similar to the present field intensity decrease in the last two centuries so that one may wonder whether such a high rate of change may be characteristic of an impending geomagnetic event (reversal or excursion). We suggest that the name Auckland excursion should be used for the present-day called Mono Lake.
•New lava flows recording the intensity low of the Mono Lake.•New flows recording the Laschamp excursion with a mean age of 41.3±0.6 ka (2σ).•∼1500 yr (intensity), 640 yr (directions) duration for Laschamp excursion.•Full intensity recover between the well distinct Laschamp and Mono Lake excursions.•Rate of decrease for the Laschamp excursion similar to that observed since 1840.
Dansgaard‐Oeschger (D‐O) cycles are the most dramatic, frequent, and wide‐reaching abrupt climate changes in the geologic record. On Greenland, D‐O cycles are characterized by an abrupt warming of 10 ...± 5°C from a cold stadial to a warm interstadial phase, followed by gradual cooling before a rapid return to stadial conditions. The mechanisms responsible for these millennial cycles are not fully understood but are widely thought to involve abrupt changes in Atlantic Meridional Overturning Circulation due to freshwater perturbations. Here we present a new, high‐resolution multiproxy marine sediment core monitoring changes in the warm Atlantic inflow to the Nordic seas as well as in local sea ice cover and influx of ice‐rafted debris. In contrast to previous studies, the freshwater input is found to be coincident with warm interstadials on Greenland and has a Fennoscandian rather than Laurentide source. Furthermore, the data suggest a different thermohaline structure for the Nordic seas during cold stadials in which relatively warm Atlantic water circulates beneath a fresh surface layer and the presence of sea ice is inferred from benthic oxygen isotopes. This implies a delicate balance between the warm subsurface Atlantic water and fresh surface layer, with the possibility of abrupt changes in sea ice cover, and suggests a novel mechanism for the abrupt D‐O events observed in Greenland ice cores.
Key Points
High‐resolution Nordic Seas core showing DO‐events synchronous to Greenland
Abrupt changes in Nordic Seas sea ice related to warm subsurface Atlantic water
A novel mechanism explaining DO‐cycles without ad‐hoc freshwater forcing
Accurate reconstruction of past ocean temperatures is of critical importance to paleoclimatology. Carbonate clumped isotope thermometry (“Δ47”) is a relatively recent technique based on the strong ...relationship between calcification temperature and the statistical excess of 13C–18O bonds in carbonates. Its application to foraminifera holds great scientific potential, particularly because Δ47 paleotemperature reconstructions do not require assumptions regarding the 18O composition of seawater. However there are still relatively few published observations investigating the potential influence of parameters such as salinity or foraminiferal size and species. We present a new calibration data set based on 234 replicate analyses of 9 planktonic and 2 benthic species of foraminifera collected from recent core-top sediments, with calcification temperatures ranging from −2 to 25 °C. We observe a strong relationship between Δ47 values and independent, oxygen-18 estimates of calcification temperatures:Δ47 = 41.63 × 103/T2 + 0.2056
The formal precision of this regression (± 0.7–1.0 °C at 95 % confidence level) is much smaller than typical analytical errors. Our observations confirm the absence of significant species-specific biases or salinity effects. We also investigate potential foraminifer size effects between 200 and >560 μm in 6 species, and conclude that all size fractions from a given core-top location and species display statistically undistinguishable Δ47 values. These findings provide a robust foundation for future inter-laboratories comparisons and paleoceanographic applications.
Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved ...epibenthic foraminiferal δ13C record, we show that the influence of North Atlantic Deep Water (NADW) was strong at the onset of the last interglacial period and was then interrupted by several prominent centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer and fresher than at present.
Within the precession band, an interhemispheric antiphase pattern in the tropical hydroclimate is supported by many paleorecords, and optimally explained by the forcing of precessional insolation ...change. However, scenarios within the western equatorial Pacific (WEP), which plays the role of the ascending center of atmospheric convection, remain poorly determined. In this study, a marine sediment core from the Halmahera Sea, East Indonesia, was analyzed with high‐resolution XRF scanning, quantitative discrete XRF, and ICP‐AES/MS measurements. The terrigenous fractions in this core are constrained by their trace elemental characteristics to be locally sourced from Halmahera Island, and hence reflect variations in the local riverine runoff and precipitation. On this basis, a continuous record of precipitation changes of the western equatorial Pacific was reconstructed with multidecadal resolution over the last ∼240 ka, using an age model established by the correlation between an adjusted ice volume model and benthic δ18O constrained by 14C dating. The records of terrigenous input show a dominant ∼23 kyr periodicity with a 90°∼100° phase lag to the boreal summer (i.e., in‐phase with the boreal autumn) insolation change. This pattern can be explained by the variability in the convective activity over the WEP, which might be primarily controlled by precessional changes in the El Niño and Southern Oscillation (ENSO) system. A dynamic linkage is implied between the precessional variations in the convective activity in the WEP and the East Asian and Australia‐Indonesian summer monsoons (EASM and AISM), in the sense of their distinct stable phase relationship to precession.
Key Points:
The age model is established with reliability on precession band
The provenance of sediment is determined to be mainly locally derived
The terrigenous fraction varies in‐phase with boreal autumn insolation
A sub-surface dwelling planktonic foraminifer, Pulleniatina obliquiloculata, in particular its shell's geochemical properties such as stable oxygen isotopic composition and Mg/Ca ratios, are widely ...used to reconstruct past changes in the thermocline of tropical Indo-Pacific. However, the depth range that P. obliquiloculata population generally dwells or calcifies has been shown to significantly vary, depending on the regions and the seasons. In this study, δ18O and Mg/Ca of core-top P. obliquiloculata from the tropical Indo-Pacific are analyzed by means of comparison with modern hydrographic data. We apply a δ18O-based approach to decipher the varying apparent calcification depth (ACD) of this species for the tropical Indo-Pacific, and find that P. obliquiloculata's ACD varies between 50 and 150 m regionally, in connection with local oceanographic settings. At given ACD, exponential relationships between temperature and P. obliquiloculata Mg/Ca are determined as Mg/Ca = 0.245 ± 0.041 exp. (0.088 ± 0.009 T) for these core-top samples. The sensitivity of P. obliquiloculata Mg/Ca to temperature is in good accordance with most previous findings, particularly the recent work using core-top samples from the western tropical Pacific. Taken together, our results not only improve the understanding of the calcification conditions of P. obliquiloculata, but also make it possible to better estimate the sub-surface temperature of the Indo-Pacific Warm Pool (IPWP) using Late-Quaternary sedimentary materials.
•The average ACD of P. obliquiloculata ranges 50–100 m water depth in tropical Indo-Pacific•We show how ACD of P. obliquiloculata varies with regional oceanographic settings•New Mg/Ca-T relationship of P. obliquiloculata is defined for tropical Indo-Pacific
Fast and nondestructive X‐ray fluorescence (XRF) core scanning provides high‐resolution element data that are widely used in paleoclimate studies. However, various matrix and specimen effects prevent ...the use of semiquantitative raw XRF core‐scanning intensities for robust paleoenvironmental interpretations. We present here a case study of a 50.8 m‐long piston Core MD12‐3432 retrieved from the northern South China Sea. The absorption effect of interstitial water is identified as the major source of deviations between XRF core‐scanning intensities and measured element concentrations. The existing two calibration methods, i.e., normalized median‐scaled calibration (NMS) and multivariate log‐ratio calibration (MLC), are tested with this sequence after the application of water absorption correction. The results indicate that an improvement is still required to appropriately correct the influence of downcore changes in interstitial water content in the long sediment core. Consequently, we implement a new polynomial water content correction in NMS and MLC methods, referred as NPS and P_MLC calibrations. Results calibrated by these two improved methods indicate that the influence of downcore water content changes is now appropriately corrected. We therefore recommend either of the two methods to be applied for robust paleoenvironmental interpretations of major elements measured by XRF‐scanning in long sediment sequences with significant downcore interstitial water content changes.
Key Points:
Changes in interstitial water is the main factor to be corrected for XRF intensities in long cores
A polynomial correction of interstitial water effect is proposed and applied to two existing XRF calibration methods
The two improved calibrations proposed here are suited for correcting varying water contents in long sediment cores
The circulation of the deep Atlantic Ocean during the height of the last ice age appears to have been quite different from today. We review observations implying that Atlantic meridional overturning ...circulation during the Last Glacial Maximum was neither extremely sluggish nor an enhanced version of present-day circulation. The distribution of the decay products of uranium in sediments is consistent with a residence time for deep waters in the Atlantic only slightly greater than today. However, evidence from multiple water-mass tracers supports a different distribution of deep-water properties, including density, which is dynamically linked to circulation.
Ocean variability at decadal time-scales remains poorly described partly because of the scarcity of high temporal resolution marine records. Here, we present a reconstruction of Sea Surface ...Temperatures (SSTs) over the past two millennia at unprecedented temporal resolution (2 to 5 years), from a marine core located off North Iceland. Alkenone paleothermometry was used to infer SST variability, and tephrochronology to build the age model. Spectral analyses of the SST signal indicate intermittent 20–25 year oscillations, with periods of strong and weak power, that are likely reflecting the ocean response to wind forcing, presumably the North Atlantic Oscillation (NAO). Warmer SSTs and paleo-magnetic proxy data, between 1000 and 1350 year A.D., overlapping the Medieval Warm Period (MWP), suggest enhanced heat transport across the Denmark Strait by the North Icelandic Irminger Current (NIIC). This is in contrast with the subsequent period, which includes the Little Ice Age (LIA), showing continuous cooling towards the 20th century. Reduced NIIC flow through the Denmark Strait likely resulting from higher freshwater and sea ice export from the Arctic would account for the observed colder conditions.
We present sea surface, upper thermocline, and benthic δ18O data, as well as temperature and paleoproductivity proxy data, from the International Marine Global Change Study Program (IMAGES) Core ...MD06‐3067 (6°31′N, 126°30′E, 1575 m water depth), located in the western equatorial Pacific Ocean within the flow path of the Mindanao Current. Our records reveal considerable glacial‐interglacial and suborbital variability in the Mindanao Dome upwelling over the last 160 kyr. Dome activity generally intensified during glacial intervals resulting in cooler thermocline waters, whereas it substantially declined during interglacials, in particular in the early Holocene and early marine oxygen isotope stage (MIS) 5e, when upwelling waters did not reach the thermocline. During MIS 3 and MIS 2, enhanced surface productivity together with remarkably low SST and low upper ocean thermal contrast provide evidence for episodic glacial upwelling to the surface, whereas transient surface warming marks periodic collapses of the Mindanao Dome upwelling during Heinrich events. We attribute the high variability during MIS 3 and MIS 2 to changes in the El Niño Southern Oscillation state that affected boreal winter monsoonal winds and upper ocean circulation. Glacial upwelling intensified when a strong cyclonic gyre became established, whereas El Niño–like conditions during Heinrich events tended to suppress the cyclonic circulation, reducing Ekman transport. Thus, our findings demonstrate that variations in the Mindanao Dome upwelling are closely linked to the position and intensity of the tropical convection and also reflect far‐field influences from the high latitudes.
PDF
