On the Bloch-Kato conjecture for some four-dimensional symplectic Galois representations

The Bloch-Kato conjecture predicts a far-reaching connection between orders of vanishing of $L$-functions and the ranks of Selmer groups of $p$-adic Galois representations. In this article, we consider the four-dimensional, symplectic Galois representations arising from automorphic representations $\pi$ of $\mathrm{GSp}_4(\mathbb A_{\mathbb Q})$ with trivial central character and with the lowest cohomological archimedean weight. Under mild technical conditions, we prove that the Selmer group vanishes when the central value $L(\pi,\mathrm{spin},1/2)$ is nonzero. In the spirit of bipartite Euler systems, we bound the Selmer group by using level-raising congruences to construct ramified Galois cohomology classes. The relation to $L$-values comes via the $\mathrm{GSpin}_3\hookrightarrow \mathrm{GSpin}_5$ periods on a compact inner form of $\mathrm{GSp}_4$. We also prove a result towards the rank-one case: if the $\pi$-isotypic part of the Abel-Jacobi image of any of Kudla's one-cycles on the Siegel threefold is nonzero, it generates the full Selmer group. These cycles are linear combinations of embedded quaternionic Shimura curves, and under the conjectural arithmetic Rallis inner product formula, their heights are related to $L'(\pi,\mathrm{spin},1/2)$.