Paleomagnetic and Rock Magnetic Stratigraphy of the Mid- Cretaceous

意大利翁布里亚阿尔比亚科特萨剖面的古地磁研究确定了白垩纪正常极性超时内高温赤铁矿携带的几个特征反向剩余磁化(ChRM)区间。倒置的间隔对应于在翁布里亚全境发现的两个红色带(色素赤铁矿)，这可能反映了海底在沉积过程中的氧化状态。反转的Chrm可以表示获得的18m.y的再磁化。在反极性chron 33R(或更高版本)中沉积后。如果是再磁化，这个过程是非常有趣的，因为它可以产生类似于极性间隔的再磁化模式。或者，反向ChRM可能记录了在阿尔卑斯中期(107-104 Ma)海底强烈氧化事件期间获得的反向地磁场极性间隔。如果正确，其中几个间隔具有足够的持续时间(100KYR)，可以用于磁性地层学研究，并可能提供白垩纪安静带洋壳的标志。如果是主要的，这些间隔也表明了地磁反转频率模型的修改。最近，在瑞士的Albian Breggia剖面也发现了两个红色的含赤铁矿的沉积带。红色带可能不仅仅具有区域意义；它们可能标志着与白垩纪全球缺氧事件类似(但在氧化方面相反)的古海洋条件的快速变化。古地磁数据表明，Breggia沉积物中存在反向ChRM，因此我们可以排除只影响部分测量剖面的循环流体的再磁化情况。然而，重新激活先前存在的矿物相是目前正在考虑的另一种再磁化机制。然而，在地层学上很窄的间隔内，在相隔很好的地点提供反向ChRM的资金，进一步支持了初级磁化模型。我们建议将我们的研究扩展到意大利和深海(ODP)的参考剖面，旨在通过岩石磁学和旋回地层学(层理和地球化学)测量来表征阿尔卑斯中期沉积物的古气候特征。首先，我们希望确定氧化间隔的性质、持续时间和程度。其次，我们希望利用这些数据作为一种手段，批判性地检验在阿尔比亚(和其他)远洋沉积物中观察到的剩余磁化成分的锁定机制。

Paleomagnetic study of the Albian Contessa section (Umbria, Italy) has defined several intervals of reversed characteristic remanent magnetizations (ChRM) carried by high unblocking- temperature hematite within the Cretaceous Normal Polarity Superchron. The reversed intervals correspond with 2 reddish zones (pigmentary hematite) found throughout Umbria which may reflect the oxidation state of the seafloor during deposition. The reversed ChRM may represent a remagnetization acquired 18 m.y. after deposition in reversed polarity chron 33R (or later). If a remagnetization, the process is of great interest since it can produce remagnetization patterns which resemble polarity intervals. Alternatively, the reversed ChRM may record intervals of reversed geomagnetic field polarity acquired during intense seafloor oxidation episodes during the mid-Albian (107-104 Ma). If correct, several of the intervals are of sufficient duration (100 kyr) to be of use in magnetostratigraphic studies and might provide markers on oceanic crust of the Cretaceous Quiet Zone. If primary, the intervals also suggest a modification of models of geomagnetic reversal frequency. Recently, 2 zones of reddish, hematite-bearing sediments were also identified in the Albian Breggia section (Switzerland). The reddish zones may have more than a regional significance; they may mark rapid changes in paleoceanographic conditions analogous to (but opposite in oxidation) to the Cretaceous global anoxic events. Paleomagnetic data suggest the presence of reversed ChRMs from the Breggia sediments from which we can exclude a remagnetization scenario calling upon circulating fluids affecting only a portion of the measured sections. Reactivation of a preexisting mineral phase, however, is another remagnetization mechanism which is presently being considered. Nevertheless, the funding of reversed ChRMs in a stratigraphically narrow interval at well-separated sites gives further support to a primary magnetization model. We propose an extension of our studies to a reference section in Italy and the deep sea (ODP) aimed at characterizing the paleoclimate signature of the mid-Albian sediments through rock- magnetic and cyclostratigraphic (bedding and geochemical) measurements. First, we hope to determine the nature, duration and extent of the oxidized intervals. Second, we hope to use these data as a means to examine critically the lock-in mechanism of the remanent components of magnetization observed in the Albian (and other) pelagic sediments.