Marine continental shelf sediments with high deposition rates may provide useful archives of rapid geomagnetic secular variation as long as the primary magnetization is not altered substantially by diagenesis. To quantify the effects of sulfate (SO₄^2-) reduction, which is a dominant early diagenetic process in such sediments, on paleomagnetic recording, we analyzed four 6-m long sediment cores from the eastern Mediterranean shelf. Two cores did not reach the methanogenic zone and are characterized by continuous organoclastic sulfate reduction (OSR), while the other two have a distinctive shallow sulfate-methane transition zone (SMTZ). Age models based on 28 radiocarbon ages indicate steady deposition rates with spatially varying age spans, which suggest that different parts of the shelf stopped accumulating sediments at different times during the Holocene. The upper sediment column in all cores is dominated by detrital titanomagnetite and biogenic magnetite. OSR-affected sediments record continuous (titano) magnetite dissolution, which resulted in steady magnetic susceptibility and remanence decreases. For cores that reach the methanogenic zone, similar behavior is observed at or above the STMZ, but magnetic properties stabilize at greater depths. Paleomagnetic directions in these sediments are more coherent, with better agreement with geomagnetic models than sediments affected by OSR. We suggest that methane-rich sediments with a shallow SMTZ and high sedimentation rates can better preserve primary paleomagnetic signals than OSR-dominated sediments due to a lack of dissolved sulfide in the main methanogenic zone, and that a susceptibility decline with depth should be a warning sign for paleomagnetic studies.

中文翻译：

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大陆架沉积物中硫酸盐还原途径的古地磁印记：有机碎屑与甲烷的厌氧氧化


具有高沉积速率的海洋大陆架沉积物可以提供快速地磁长期变化的有用档案，只要初级磁化作用没有被成岩作用实质性改变。为了量化硫酸盐 （SO ₄ ^2- ） 还原（此类沉积物中主要的早期成岩过程）对古地磁记录的影响，我们分析了来自东地中海大陆架的四个 6 m 长的沉积物岩芯。两个岩心未到达产甲烷带，其特征是连续的有机碎屑硫酸盐还原 （OSR），而另外两个岩心具有独特的浅层硫酸盐-甲烷过渡带 （SMTZ）。基于 28 个放射性碳年龄的年龄模型表明，在空间上年龄跨度不同的情况下，大陆架的不同部分在不同的时间停止了沉积物的积累。所有岩心中的上部沉积物柱以碎屑钛磁铁矿和生物磁铁矿为主。受 OSR 影响的沉积物记录连续的（钛合金）磁铁矿溶解，这导致稳定的磁化率和剩磁降低。对于到达产甲烷区的岩心，在 STMZ 或更高处观察到类似的行为，但磁性在更深的地方稳定下来。这些沉积物中的古地磁方向更加连贯，与受 OSR 影响的沉积物相比，与地磁模型的一致性更高。我们认为，由于主要产甲烷带缺乏溶解的硫化物，具有浅 SMTZ 和高沉降速率的富含甲烷的沉积物比 OSR 为主的沉积物更能保留初级古地磁信号，并且磁化率随深度下降应该是古地磁研究的警告信号。