Distinguishing detrital versus chemical remanent magnetization in sediments: Toward a better understanding of relative paleointensity records

沉积物提供了详细和连续的地磁场行为记录。除其他外，这些时间序列提供了过去相对场强变化的估计，这些估计被用来校准高层大气中产生的核素产生的气候指标。当从沉积物中提取和比较地磁场信息时，最重要的假设是记录机制保持不变。通常的假设是，剩磁是通过作用于磁性颗粒的外场的扭矩获得的，这被称为沉积(碎屑)或沉积后剩磁。然而，成岩作用可以在沉积过程中或沉积后沉淀亚铁磁性铁氧化物和/或硫化物，从而产生二次化学剩余磁化。沉积残磁和化学残磁之间的记录机制背后的物理基础是根本不同的，但用现有的方法对后者的识别仍然具有挑战性，因此它的潜在影响往往被忽视--特别是关于相对古强度记录。根据再沉积实验的初步结果，我们发现无滞后剩磁参数的各向异性随相对古强度的变化而变化，这意味着我们可能已经找到了一种在基于磁各向异性的古强度估算中减小散射的方法。另一方面，化学剩磁与沉积剩磁具有截然不同的各向异性特征。我们展示了一个来自百慕大隆起(ODP1063站点)的沉积物的例子，其中包含化学沉淀云母岩的层位。之前对该地点不同工作组的研究得出结论，碎屑磁铁矿是唯一的剩磁载体。利用磁滞剩余磁化技术，通过实验对云母进行了鉴定。我们希望使用滞后的剩余磁化实验来更好地理解古强度记录过程在化学过程和沉积过程之间的不同，以及当它们同时作用时，最终目标是产生更可靠的相对古强度记录。

Sediments provide detailed and continuous records of geomagnetic field behavior. Amongst other things, these time series provide estimates for relative field intensity variations in the past, which are used and to calibrate climate proxies stemming from nuclide production in the upper atmosphere. An overriding assumption when extracting and comparing geomagnetic field information from sediments is that the recording mechanism remained constant. The common hypothesis is that magnetic remanence is acquired via torque from the external field acting on a magnetic particle, which is termed a depositional (detrital) or post-depositional remanent magnetization. However, diagenetic processes can precipitate ferrimagnetic iron oxides and/or sulphides during or after deposition, thereby producing a secondary, chemical remanent magnetization. The physics behind the recording mechanisms between depositional and chemical remanences are fundamentally different, yet the identification of the latter remains challenging with existing methods, and thus it's potential influence is often ignored--especially concerning relative paleointensity records. With preliminary results from redeposition experiments we show that anisotropy of anhysteretic remanent magnetization parameters vary with relative paleointensity, which means we may have found a means to reduce the scatter in paleointensity estimates based on magnetic anisotropy. On the other hand, a chemical remanent magnetization has a much different anisotropy signature than a depositional remanence. We show an example of sediments from the Bermuda Rise (ODP site 1063) that contains horizons of the chemical precipitate greigite. Previous studies from different working groups on this site conclude detrital magnetite is the sole remanence carrier. The identification of greigite was made possible through experiments using anhysteretic remanent magnetization techniques. We would like to use anhysteretic remanent magnetization-based experiments to better understand how the paleointensity recording process differs between chemical and depositional processes, as well as when they act simultaneously, with the ultimate goal of producing more robust relative paleointensity records.