Magnetostratigraphy Of Sediment Core ARC5-ICE4 From The Lomonosov Ridge,Arctic Ocean

The Arctic Ocean is one of the most sensitive regions to global climate change and plays an important role in regulating the global atmosphere and ocean circulation.Its sediments bear important information about the paleoceanographic and paleoclimatic evolutions.Therefore,the Arctic Ocean sediments are of great significance to the studies of the development and evolution of the Earth system,especially climate and environment in geological history.However,microbiological fossils are always rare or even absent in the sediments(especially in the deep-sea sediments),which has made the traditional biostratigraphy and oxygen isotope stratigraphy difficult to take effect,and thus the establishment of a reliable chronological framework is full of difficulties and challenges.Magnetostratigraphy has been playing an important role in the establishment of the chronological framework of Cenozoic sedimentary strata,and it is also one of the main ways to construct the first-order chronological frameworks of long marine sediment cores.Due to the relatively homogeneous and fine-grained sediments with high magnetization values,magnetostratigraphy has once became the main dating method.However,since 1980 s,a series of other dating results(such as amino acid epimerization,calcareous nannofossil stratigraphy,Mn cycle stratigraphy,Optically Stimulated Luminescence,etc.)have questioned the early million-year magnetostratigraphic chronology,so that a considerable number of researchers reinterpreted the polarity anomalies in the upper of these cores as the polarity excursions in the Brunhes Chron rather than the geomagnetic signals in the Matuyama Chron.This thus has resulted in a long-time debate between the “geomagnetic reversal” and “excursion” theories.Recently,a few studies have attributed these polarity anomalies to the titanomaghemite “self-reversal” caused by low-temperature oxidation of the primary(titano)magnetite.It is obvious that the interpretation and applicability of the deep-sea magnetostratigraphy in the Arctic Ocean are highly controversial,which therefore seriously restricts the development of relevant paleoceanographic and paleoenvironmental studies.In view of this,rock magnetic analyses were conducted on the sediment core ARC5-ICE4 recovered from the Lomonosov Ridge,Arctic Ocean,during the 5th Chinese National Arctic Research Expedition.Combining these results with the existing data,the properties of magnetic minerals in the studied sediments were further clarified.Meanwhile,orientated samples were also collected from an adjacent core(ARC8-LR01)and preliminary paleomagnetic measurements were also conducted to make an inter-borehole comparison and verification.Based on the above analyses,the relative paleointensity and environmental magnetism of core ARC5-ICE4 were systematically investigated,and then the constructed polarity sequence was interpreted more reasonably.The following conclusions were obtained:1.The magnetic components in the studied sediments are dominated by magnetite,and part of these magnetic particles was subjected to slight surface oxidation,with a small quantity of hematite.2.The identified magnetic minerals exhibit relatively homogeneous pseudosingle domain(PSD)characteristics as a whole,and the variation in magnetic content along depths is also restrained.,which basically meets the prerequisite for "magnetic uniformity" of sediment for relative paleointensity(RPI)research;3.The negative polarities spanning the 1.15-2.04 m interval(R1)are not resulted from “self-reversal”,but from the authentic records of the paleomagnetic field.These facts,coupled with the relative paleointensity(RPI)model and cyclicity of S-ratio value,correspond R1 to the Matuyama Chron(?2.58-0.78 Ma)4.The polarity transition at 1.15 m and 2.04 m of the core is initially defined as the B/M and M/G boundaries,with corresponding ages of 0.78 Ma and 2.58 Ma,respectively.It is speculated that the bottom boundary age is less than 3.58 Ma(or3.04 Ma),and there may be significant sedimentary discontinuities and deficiencies in the Quaternary system;5.The above magnetic stratigraphic chronological patterns of sedimentation rate agreeing well with the mid-Pleistocene transition(MPT)and intensification of Northern Hemisphere glaciation(i NHG),which authenticates further its fidelity.In summary,this study confirms that even if the "self-reversal" phenomenon of titanomaghemite exists in the deep-sea sediments of the Arctic Ocean,its scope of influence is still local;Simultaneously,it also suggests that magnetostratigraphy may have different adaptability in different regions of the deep ocean in the Arctic Ocean,and further comprehensive chronostratigraphy research is highly necessary to be carried out.